# Study of diffuse H II regions potentially forming part of the gas streams around Sgr A*

Study of diffuse H II regions potentially forming part of the gas streams around Sgr A* Abstract We present a study of diffuse extended ionized gas towards three clouds located in the Galactic Centre (GC). One line of sight (LOS) is towards the 20 km s−1 cloud (LOS−0.11) in the Sgr A region, another LOS is towards the 50 km s−1 cloud (LOS−0.02), also in Sgr A, while the third is towards the Sgr B2 cloud (LOS+0.693). The emission from the ionized gas is detected from Hnα and Hmβ radio recombination lines (RRLs). Henα and Hemβ RRL emission is detected with the same n and m as those from the hydrogen RRLs only towards LOS+0.693. RRLs probe gas with positive and negative velocities towards the two Sgr A sources. The Hmβ to Hnα ratios reveal that the ionized gas is emitted under local thermodynamic equilibrium conditions in these regions. We find a He to H mass fraction of 0.29±0.01 consistent with the typical GC value, supporting the idea that massive stars have increased the He abundance compared to its primordial value. Physical properties are derived for the studied sources. We propose that the negative velocity component of both Sgr A sources is part of gas streams considered previously to model the GC cloud kinematics. Associated massive stars with what are presumably the closest H ii regions to LOS−0.11 (positive velocity gas), LOS−0.02, and LOS+0.693 could be the main sources of ultraviolet photons ionizing the gas. The negative velocity components of both Sgr A sources might be ionized by the same massive stars, but only if they are in the same gas stream. ISM: clouds, H ii regions, Galaxy: centre 1 INTRODUCTION The proximity of the Galactic Centre (GC), at a distance of about 7.86 kpc (Boehle et al. 2016), offers a unique opportunity to look at a galactic nucleus in great detail. Several studies have been carried out to establish the physical properties and the kinematics of ionized gas towards the main compact H ii regions located at the centre of the Galaxy (Ho et al. 1985; Mehringer et al. 1993; Zhao et al. 1993; Mills et al. 2011). Sgr A West, located around the supermassive black hole Sgr A*, is a spiral-shaped region of ionized gas whose emission is thermal in nature (Ekers et al. 1983). Sgr A East is a non-thermal source surrounding Sgr A West in projection (Ekers et al. 1983). There is also a group of four H ii regions, known collectively as G−0.02 −0.07, made up of the regions denoted as A, B, C, and D (see Fig. 1, upper panel). G−0.02 −0.07 is located at a projected distance of ∼6 pc from Sgr A*. These H ii regions likely reside within the 50 km s−1 cloud1 (Goss et al. 1985; Mills et al. 2011), one of the massive clouds in the Sgr A complex. Sgr A East may be impacting the 50 km s−1 cloud at its west side (Serabyn, Lacy & Achtermann 1992). Massive O stars are thought to be ionizing the A–D regions (Lau et al. 2014). Using line to continuum ratios, Goss et al. (1985) found electron temperatures in the range of ∼5000–7000 K for the four compact H II regions. Figure 1. View largeDownload slide VLA radio-continuum maps at 24.5 GHz towards the three LOSs observed by us (see Section 2.1). The three LOSs are shown as black circles with the size of the GBT beam of 48 arcsec at 13.09 GHz. Upper panel: the region A partly falls inside LOS−0.02. The regions B, C, and D are also seen in the field. Middle panel: LOS−0.11 overlaps with part of the non-thermal source Sgr A-E (Lu et al. 2003). The region G appears to be the closest H ii region to LOS−0.11 (Ho et al. 1985). Bottom panel: LOS+0.693 lies close to the H ii region L located northeast of Sgr B2N. Figure 1. View largeDownload slide VLA radio-continuum maps at 24.5 GHz towards the three LOSs observed by us (see Section 2.1). The three LOSs are shown as black circles with the size of the GBT beam of 48 arcsec at 13.09 GHz. Upper panel: the region A partly falls inside LOS−0.02. The regions B, C, and D are also seen in the field. Middle panel: LOS−0.11 overlaps with part of the non-thermal source Sgr A-E (Lu et al. 2003). The region G appears to be the closest H ii region to LOS−0.11 (Ho et al. 1985). Bottom panel: LOS+0.693 lies close to the H ii region L located northeast of Sgr B2N. Another H ii region labelled as G (see Fig. 1, middle panel), located at ∼13 pc in projection from Sgr A*, is thought to be excited by one O9 or five B0 stars (Ho et al. 1985). The region G appears to be embedded in the 20 km s−1 cloud (Armstrong, Jackson & Ho 1989), another massive cloud in the Sgr A complex. Sgr A–E is considered a non-thermal source (Lu, Wang & Lang 2003), which lies close to the region G (see Fig. 1). Armstrong et al. (1989) found an electron temperature of ∼7500 K for the region G. Zhao et al. (1993) studied five H ii regions (identified as H1 through to H5) located between Sgr A West and the Arched Filaments H ii complex containing a group of curved ridges showing velocities from 15 to −70 km s−1 (Lang, Goss & Morris 2001). The H1–H5 sources show gas velocities from −20 to −60 km s−1, which seem to be associated with a −30 km s−1 cloud (Zhao et al. 1993). However, negative velocities of the ionized gas are not only observed towards the H1–H5 regions and the Arched Filaments H ii complex, as previously thought, but also towards many other regions of the Sgr A complex. In fact, a GC large-scale map obtained by Royster & Yusef-Zadeh (2014) shows ionized gas towards the Sgr A complex with negative velocities reaching up to ∼−130 km s−1. A recent position-velocity map of the C II emission (Langer et al. 2017), which is considered as a good tracer of the ionized gas, shows a similar distribution as in the map obtained by Royster & Yusef-Zadeh (2014). Clouds of diffuse ionized gas in Sgr A with velocities from ∼−130 to +130 km s−1 are shown on the channel maps of the C II emission obtained by García (2015). On the other hand, the Sgr B2 complex lies at a projected distance of ∼120 pc from the GC. This complex contains many dozens of compact and ultracompact H ii regions (Gaume et al. 1995; De Pree et al. 2005). Many of these H ii regions are associated with the Sgr B2 north (N), main (M), and south (S) hot cores where star formation is taking place (Gordon et al. 1993). The ionized gas in the Sgr B2 complex shows velocities predominantly in the range of 50–70 km s−1 (Mehringer et al. 1993). There is a H ii region labelled as L (Mehringer et al. 1993) that is located at a projected distance of ∼1.6 pc from Sgr B2N (see Fig. 1, bottom panel). The region L has an electron temperature of ∼6500 K (Mehringer et al. 1993) and it is believed to be excited by one O5.5 star (Gaume et al. 1995). The 20 and 50 km s−1 clouds are considered as part of a set of clouds moving on stable x2 orbits around the GC in a 100×60 pc elliptical and twisted ring (Molinari et al. 2011). In this scenario, both clouds are located in the front region of the ring while its background gas, which is around both clouds as seen in projection, show velocities from ∼0 to −60 km s−1 (Molinari et al. 2011). Kruijssen, Dale & Longmore (2015) also modelled the gas kinematics studied by Molinari et al. (2011), reproducing the kinematics of molecular gas using an open gas stream divided into four gas streams orbiting the GC. The back side of the open stream is composed of streams 3 and 4, while streams 1 and 2 are two ends of the open stream located at its front side (Kruijssen et al. 2015). The 20 and 50 km s−1 clouds are contained in the gas stream 1. A recent study (Langer et al. 2017) revealed that the ionized gas velocities of the Sgr A and Sgr B2 clouds are better explained by the gas streams proposed by Kruijssen et al. (2015) rather than by the elliptical ring proposed by Molinari et al. (2011). Henshaw et al. (2016) found that two spiral arms or gas streams reproduce the molecular gas distribution of several GC clouds. Since no known physical model explains the spiral arms (Henshaw et al. 2016), open streams might be the most likely structure. In this paper, we focus on studying the physical properties and kinematics of the diffuse ionized gas of selected GC regions. Using radio recombination lines (RRLs) observed with the Green Bank Telescope (GBT) of NRAO,2 we find that RRLs show positive and negative velocities towards two lines of sight (LOS) in the Sgr A complex, one towards the 50 km s−1 cloud (LOS−0.02) and another towards the 20 km s−1 cloud (LOS−0.11). We also study the ionized gas along one LOS in the Sgr B2 complex (LOS+0.693) for comparison purposes. Fig. 1 shows the observed positions of the three LOS, where other GC sources are indicated. As indicated in Fig. 1LOS−0.02 covers part of the emission arising from the H ii region A. The region G appears to be the closest thermal H ii region to LOS−0.11 (Ho et al. 1985) since Sgr A-E is considered a non-thermal source in nature (Lu et al. 2003; Yusef-Zadeh et al. 2005). LOS+0.693 lies close to the H ii region L (see Fig. 1, bottom panel). This paper is organized as follows. In Section 2, we present the observations and data used in this work. We present the main results in Section 3, focusing on the line identification of RRLs and Gaussian fits in Section 3.1, the local thermodynamic equilibrium (LTE) of the ionized gas in Section 3.2, helium to hydrogen ratio in Section 3.3, and electron densities and the number of Lyman continuum photons in Section 3.4. We discuss whether the RRL emission detected with the GBT is extended and diffuse in Section 4.1, the kinematics of the ionized gas in Section 4.2, and the sources of gas ionization in Section 4.3. Finally, the conclusions of this work are presented in Section 5. 2 OBSERVATIONS AND DATA REDUCTION The observations were carried out with the NRAO 100-m GBT in 2009 July–October. We used the Ku-band receiver connected to the spectrometer that provided four 200 MHz spectral windows in two polarizations. This configuration provides a spectral resolution of 24.4 kHz or 0.6 km s−1. Spectra were calibrated using a noise tube and the line intensities, affected by 20 per cent uncertainties, are given in T$$_\mathrm{A}^*$$ scale. The position-switched mode was used during the observations. As mentioned, the studied LOSs are shown in Fig. 1. The angular resolution is 45 arcsec at 14.19 GHz, which corresponds to ∼1.7 pc at the distance of the GC. We used the reference positions selected and verified by Martín et al. (2008), which were originally based on large scale CS maps (Bally, Stark & Wilson 1987). The three observed LOS positions and their reference positions are indicated in Table 1. Table 1. Observed positions and their references. Source  Position  Reference    RA(J2000)  Dec.(J2000)  RA(J2000)  Dec.(J2000)  LOS−0.02  17h45m51.0s  −28°59΄06.0΄  17h46m00.1s  −29°16΄47.2΄  LOS−0.11  17h45m39.0s  −29°04΄05.0΄  17h46m00.1s  −29°16΄47.2΄  LOS+0.693  17h47m22.0s  −28°21΄27.0΄  17h46m23.0s  −28°16΄37.3΄  Source  Position  Reference    RA(J2000)  Dec.(J2000)  RA(J2000)  Dec.(J2000)  LOS−0.02  17h45m51.0s  −28°59΄06.0΄  17h46m00.1s  −29°16΄47.2΄  LOS−0.11  17h45m39.0s  −29°04΄05.0΄  17h46m00.1s  −29°16΄47.2΄  LOS+0.693  17h47m22.0s  −28°21΄27.0΄  17h46m23.0s  −28°16΄37.3΄  View Large Using the gbtidl package, 3 we inspected all scans of the SDFITS files, and the baseline subtraction and average were applied to the calibrated spectra. Then the data were imported into the madcuba package4 for further processing. The spectra were smoothed to a velocity resolution of ∼5 km s−1 appropriate for the RRL widths, Δ$$v$$r, of ∼30 km s−1 observed in the GC (Mehringer et al. 1993). 2.1 Archival VLA data To find out whether the emission detected with the GBT is affected by emission arising from compact H ii regions (see discussion in Section 4.1), we have used VLA data at 24.5 GHz available in the NRAO archive.5 The VLA data reduction and imaging were done using the casa package6 (version 4.7.0). The observations were carried out in 2012 using the DnC configuration. We have build continuum maps, shown in Fig. 1, and also a H64α cube for the LOS−0.02 region as this information will be required in Section 4.1. The continuum maps and cube were obtained using the clean task of casa. The spatial resolution of the maps and cube is 2.52×2.47 arcsec2. The cube has a rms noise of ∼4 mJy beam−1 per channel, while the continuum maps of both Sgr A regions and the LOS+0.693 region have rms noises of ∼2 and ∼20 mJy beam−1, respectively. 3 RESULTS 3.1 Line identification and gaussian fits To identify hydrogen (H) and helium (He) RRLs, we have used a catalogue included in the madcubaij package, which contains the frequencies of the RRLs estimated according to the Dirac theory described by Towle, Feldman & Watson (1996). The RRLs detected in LOS−0.11, LOS−0.02, and LOS+0.693 are shown in Fig. 2, 3, and 4–5, respectively. We have detected emission from Hnα lines with n = 79–75 and Hmβ lines with m = 99–96, 94 towards LOS−0.11 and LOS−0.02. Hydrogen RRLs with the same n and m are also detected towards LOS+0.693 but, in this case, we have also detected the H95β, whose frequency is not in the bandwidth of our observations towards either of the two Sgr A sources. All these RRLs are detected with a significance higher than 3σ. The strongest RRLs are observed in LOS+0.693, whereas the weakest lines are detected in LOS−0.11. We have detected the emission from Henα lines, with n = 79–75 and Hemβ lines with m = 99–94, only towards LOS+0.693 (see Fig. 5). As shown in Fig. 2 and 3, the RRLs in both of the of Sgr A sources reveal two velocity components, while the RRLs in LOS+0.693 show only a single velocity component. Figure 2. View largeDownload slide Hydrogen RRLs observed towards LOS−0.11. The dashed red lines show the velocities of 20 and −30 km s−1. The spectra are shown with a spectral resolution of ∼5 km s−1. Figure 2. View largeDownload slide Hydrogen RRLs observed towards LOS−0.11. The dashed red lines show the velocities of 20 and −30 km s−1. The spectra are shown with a spectral resolution of ∼5 km s−1. Figure 3. View largeDownload slide Hydrogen RRLs observed towards LOS−0.02. The dashed red lines show the velocities of 50 and −40 km s−1. The spectra are shown with a spectral resolution of ∼5 km s−1. Figure 3. View largeDownload slide Hydrogen RRLs observed towards LOS−0.02. The dashed red lines show the velocities of 50 and −40 km s−1. The spectra are shown with a spectral resolution of ∼5 km s−1. Figure 4. View largeDownload slide Hydrogen RRL observed towards LOS+0.693. The dashed red line shows the velocity of 70 km s−1. The spectra are shown with a spectral resolution of ∼5 km s−1. Figure 4. View largeDownload slide Hydrogen RRL observed towards LOS+0.693. The dashed red line shows the velocity of 70 km s−1. The spectra are shown with a spectral resolution of ∼5 km s−1. Figure 5. View largeDownload slide Helium RRLs observed towards LOS+0.693. The dashed red line shows the velocity of 70 km s−1. The spectra are shown with a spectral resolution of ∼5 km s−1. Figure 5. View largeDownload slide Helium RRLs observed towards LOS+0.693. The dashed red line shows the velocity of 70 km s−1. The spectra are shown with a spectral resolution of ∼5 km s−1. Gaussian fits to the RRLs are used to derive the peak intensity (T$$_\mathrm{A}^*$$), central line velocity ($$v$$r), full width at half-maximum (Δ$$v$$r), the integrated line intensity ($$\int T_\mathrm{A}^* \text{d}v_\mathrm{r}$$), and their respective uncertainties. The frequencies of the RRLs and the derived parameters for each source are listed in Tables 2 –4. The RRLs found in both sources of Sgr A are fitted with two Gaussian lines. The two velocity components are labelled as +20 and −30 km s−1 in LOS−0.11 and as +50 and −40 km s−1 in LOS−0.02 in Tables 7, 9, and 10. As mentioned, Henα lines are detected only in LOS+0.693, and the parameters derived using Gaussian fits are given in Table 5, where upper limits for the T$$_\mathrm{A}^*$$ and $$\int T_\mathrm{A}^* \text{d}v_\mathrm{r}$$ of the He94β line are also listed. For both sources of Sgr A, we have estimated 3σ upper limits for T$$_\mathrm{A}^*$$ of the He lines shown in Table 6 because we will study the He to H ratio in Section 3.3. In Table 6, there are no upper limits for the He95β line as it was not observed in either of the Sgr A sources. Table 2. Hydrogen RRL parameters derived for LOS−0.11 using Gaussian fits with two velocity components. RRL  ν  T$$^*_\mathrm{A}$$  $$v$$r  Δ$$v$$r  $$\int T_\mathrm{A}^* \text{d}v_\mathrm{r}$$    (GHz)  (mK)  (km s−1)  (km s−1)  (× 102 mK km s−1)  H79α  13.09  19.4 ± 1.6  22.9 ± 1.3  30.2 ± 2.9  6.2 ± 0.8      12.6 ± 1.3  −26.5 ± 2.5  49.7 ± 6.5  6.7 ± 1.2  H78α  13.60  17.9 ± 0.9  21.9 ± 1.3  38.2 ± 2.8  7.3 ± 0.7      11.0 ± 0.8  −32.3 ± 2.4  48.6 ± 5.7  5.7 ± 0.8  H77α  14.12  22.9 ± 3.5  21.8 ± 0.8  27.3 ± 2.6  6.6 ± 1.3      9.8 ± 1.1  −17.2 ± 9.1  80 ± 17  8.4 ± 2.2  H76α  14.69  19.0 ± 1.1  22.9 ± 1.2  29.9 ± 2.7  6.1 ± 0.7      13.1 ± 1.0  −28.7 ± 1.9  51.3 ± 4.8  7.2 ± 0.9  H75α  15.28  20.0 ± 1.0  22.5 ± 0.9  30.0 ± 2.0  6.4 ± 0.6      10.4 ± 0.8  −32.5 ± 2.0  51.6 ± 5.3  5.7 ± 0.8  H99β  13.15  5.9 ± 1.1  17.3 ± 3.5  25.8 ± 7.1  1.6 ± 0.6      4.7 ± 0.5  −30.0 ± 6.0  50 ± 13  2.5 ± 0.8  H98β  13.56  9.0 ± 0.9  20.2 ± 1.7  25.1 ± 4.2  2.4 ± 0.5      3.3 ± 0.6  −30.0 ± 5.2  50 ± 14  1.8 ± 0.6  H97β  13.98  8.1 ± 1.0  17.4 ± 1.6  37.0 ± 3.8  3.2 ± 0.6      3.0 ± 0.5  −30.0 ± 8.7  50 ± 19  1.6 ± 0.7  H96β  14.41  7.6 ± 0.9  16.9 ± 2.4  29.0 ± 8.3  2.3 ± 0.8      3.7 ± 0.6  −30.0 ± 7.3  50 ± 16  1.9 ± 0.7  H94β  15.34  5.7 ± 0.6  22.4 ± 2.6  35.8 ± 6.1  2.1 ± 0.5      4.0 ± 1.0  −30.0 ± 3.7  50.0 ± 9.2  2.2 ± 0.8  RRL  ν  T$$^*_\mathrm{A}$$  $$v$$r  Δ$$v$$r  $$\int T_\mathrm{A}^* \text{d}v_\mathrm{r}$$    (GHz)  (mK)  (km s−1)  (km s−1)  (× 102 mK km s−1)  H79α  13.09  19.4 ± 1.6  22.9 ± 1.3  30.2 ± 2.9  6.2 ± 0.8      12.6 ± 1.3  −26.5 ± 2.5  49.7 ± 6.5  6.7 ± 1.2  H78α  13.60  17.9 ± 0.9  21.9 ± 1.3  38.2 ± 2.8  7.3 ± 0.7      11.0 ± 0.8  −32.3 ± 2.4  48.6 ± 5.7  5.7 ± 0.8  H77α  14.12  22.9 ± 3.5  21.8 ± 0.8  27.3 ± 2.6  6.6 ± 1.3      9.8 ± 1.1  −17.2 ± 9.1  80 ± 17  8.4 ± 2.2  H76α  14.69  19.0 ± 1.1  22.9 ± 1.2  29.9 ± 2.7  6.1 ± 0.7      13.1 ± 1.0  −28.7 ± 1.9  51.3 ± 4.8  7.2 ± 0.9  H75α  15.28  20.0 ± 1.0  22.5 ± 0.9  30.0 ± 2.0  6.4 ± 0.6      10.4 ± 0.8  −32.5 ± 2.0  51.6 ± 5.3  5.7 ± 0.8  H99β  13.15  5.9 ± 1.1  17.3 ± 3.5  25.8 ± 7.1  1.6 ± 0.6      4.7 ± 0.5  −30.0 ± 6.0  50 ± 13  2.5 ± 0.8  H98β  13.56  9.0 ± 0.9  20.2 ± 1.7  25.1 ± 4.2  2.4 ± 0.5      3.3 ± 0.6  −30.0 ± 5.2  50 ± 14  1.8 ± 0.6  H97β  13.98  8.1 ± 1.0  17.4 ± 1.6  37.0 ± 3.8  3.2 ± 0.6      3.0 ± 0.5  −30.0 ± 8.7  50 ± 19  1.6 ± 0.7  H96β  14.41  7.6 ± 0.9  16.9 ± 2.4  29.0 ± 8.3  2.3 ± 0.8      3.7 ± 0.6  −30.0 ± 7.3  50 ± 16  1.9 ± 0.7  H94β  15.34  5.7 ± 0.6  22.4 ± 2.6  35.8 ± 6.1  2.1 ± 0.5      4.0 ± 1.0  −30.0 ± 3.7  50.0 ± 9.2  2.2 ± 0.8  View Large Table 3. Hydrogen RRL parameters derived for LOS−0.02 using Gaussian fits with two velocity components. RRL  ν  T$$^*_\mathrm{A}$$  $$v$$r  Δ$$v$$r  $$\int T_\mathrm{A}^* \text{d}v_\mathrm{r}$$    (GHz)  (mK)  (km s−1)  (km s−1)  (× 102 mK km s−1)  H79α  13.09  92.8 ± 3.5  46.1 ± 0.5  27.6 ± 1.1  27.2 ± 1.7      45.6 ± 2.3  −39.2 ± 1.4  60.6 ± 3.4  29.4 ± 2.3  H78α  13.60  83.0 ± 2.9  46.6 ± 0.5  29.7 ± 1.1  26.3 ± 1.4      42.9 ± 2.0  −41.0 ± 1.3  61.9 ± 3.1  28.2 ± 2.0  H77α  14.12  72.8 ± 1.7  45.6 ± 0.4  31.8 ± 0.9  24.6 ± 1.0      28.1 ± 1.2  −39.8 ± 1.3  61.8 ± 3.3  18.5 ± 1.3  H76α  14.69  73.6 ± 2.1  46.5 ± 0.4  29.5 ± 1.0  23.1 ± 1.1      30.8 ± 1.5  −41.6 ± 1.4  63.3 ± 3.2  20.8 ± 1.5  H75α  15.28  57.4 ± 2.1  48.1 ± 0.6  32.1 ± 1.3  19.6 ± 1.2      34.6 ± 1.6  −39.8 ± 1.2  57.0 ± 2.9  21.0 ± 1.5  H99β  13.15  23.1 ± 2.2  48.1 ± 1.2  24.4 ± 2.8  6.0 ± 0.9      9.6 ± 1.5  −40.0 ± 4.1  60.0 ± 9.6  6.2 ± 1.4  H98β  13.56  24.4 ± 1.1  45.6 ± 0.6  31.4 ± 1.5  8.2 ± 0.6      11.1 ± 0.8  −40.3 ± 1.9  59.6 ± 4.4  7.1 ± 0.8  H97β  13.98  20.2 ± 0.9  46.0 ± 0.6  28.9 ± 1.4  6.2 ± 0.4      7.9 ± 0.7  −37.9 ± 2.0  53.6 ± 4.7  4.5 ± 0.6  H96β  14.41  20.3 ± 1.0  48.3 ± 0.7  30.5 ± 1.6  6.6 ± 0.5      8.9 ± 0.9  −31.0 ± 2.8  56.2 ± 7.1  5.3 ± 0.6  H94β  15.34  14.6 ± 1.3  51.4 ± 1.4  35.4 ± 3.4  5.5 ± 0.8      8.3 ± 1.0  −40.2 ± 2.8  60.0 ± 6.6  5.3 ± 0.9  RRL  ν  T$$^*_\mathrm{A}$$  $$v$$r  Δ$$v$$r  $$\int T_\mathrm{A}^* \text{d}v_\mathrm{r}$$    (GHz)  (mK)  (km s−1)  (km s−1)  (× 102 mK km s−1)  H79α  13.09  92.8 ± 3.5  46.1 ± 0.5  27.6 ± 1.1  27.2 ± 1.7      45.6 ± 2.3  −39.2 ± 1.4  60.6 ± 3.4  29.4 ± 2.3  H78α  13.60  83.0 ± 2.9  46.6 ± 0.5  29.7 ± 1.1  26.3 ± 1.4      42.9 ± 2.0  −41.0 ± 1.3  61.9 ± 3.1  28.2 ± 2.0  H77α  14.12  72.8 ± 1.7  45.6 ± 0.4  31.8 ± 0.9  24.6 ± 1.0      28.1 ± 1.2  −39.8 ± 1.3  61.8 ± 3.3  18.5 ± 1.3  H76α  14.69  73.6 ± 2.1  46.5 ± 0.4  29.5 ± 1.0  23.1 ± 1.1      30.8 ± 1.5  −41.6 ± 1.4  63.3 ± 3.2  20.8 ± 1.5  H75α  15.28  57.4 ± 2.1  48.1 ± 0.6  32.1 ± 1.3  19.6 ± 1.2      34.6 ± 1.6  −39.8 ± 1.2  57.0 ± 2.9  21.0 ± 1.5  H99β  13.15  23.1 ± 2.2  48.1 ± 1.2  24.4 ± 2.8  6.0 ± 0.9      9.6 ± 1.5  −40.0 ± 4.1  60.0 ± 9.6  6.2 ± 1.4  H98β  13.56  24.4 ± 1.1  45.6 ± 0.6  31.4 ± 1.5  8.2 ± 0.6      11.1 ± 0.8  −40.3 ± 1.9  59.6 ± 4.4  7.1 ± 0.8  H97β  13.98  20.2 ± 0.9  46.0 ± 0.6  28.9 ± 1.4  6.2 ± 0.4      7.9 ± 0.7  −37.9 ± 2.0  53.6 ± 4.7  4.5 ± 0.6  H96β  14.41  20.3 ± 1.0  48.3 ± 0.7  30.5 ± 1.6  6.6 ± 0.5      8.9 ± 0.9  −31.0 ± 2.8  56.2 ± 7.1  5.3 ± 0.6  H94β  15.34  14.6 ± 1.3  51.4 ± 1.4  35.4 ± 3.4  5.5 ± 0.8      8.3 ± 1.0  −40.2 ± 2.8  60.0 ± 6.6  5.3 ± 0.9  View Large Table 4. Hydrogen RRL parameters derived or LOS+0.693 using Gaussian fits. RRL  ν  T$$^*_\mathrm{A}$$  $$v$$r  Δ$$v$$r  $$\int T_\mathrm{A}^* \text{d}v_\mathrm{r}$$    (GHz)  (mK)  (km s−1)  (km s−1)  (× 103 mK km s−1)  H79α  13.09  441.6 ± 3.2  71.4 ± 0.1  31.0 ± 0.3  14.6 ± 0.2  H78α  13.60  401.4 ± 3.1  71.8 ± 0.1  32.0 ± 0.3  13.7 ± 0.2  H77α  14.12  382.0 ± 4.8  71.8 ± 0.2  31.0 ± 0.4  12.6 ± 0.2  H76α  14.69  383.3 ± 6.5  71.7 ± 0.2  30.7 ± 0.6  12.5 ± 0.3  H75α  15.28  350.4 ± 2.5  72.3 ± 0.1  30.0 ± 0.3  11.2 ± 0.1  H99β  13.15  93.2 ± 3.1  70.2 ± 0.5  37.0 ± 1.3  3.7 ± 0.2  H98β  13.56  91.1 ± 1.0  71.8 ± 0.3  33.0 ± 0.4  3.2 ± 0.1  H97β  13.98  88.4 ± 3.7  71.3 ± 0.4  32.2 ± 1.0  3.0 ± 0.1  H96β  14.41  84.5 ± 1.8  73.5 ± 0.4  37.4 ± 0.8  3.4 ± 0.1  H95β  14.87  91.9 ± 2.8  72.0 ± 0.4  30.5 ± 1.0  3.0 ± 0.1  H94β  15.34  78.5 ± 1.0  72.6 ± 0.2  32.0 ± 0.5  2.7 ± 0.1  RRL  ν  T$$^*_\mathrm{A}$$  $$v$$r  Δ$$v$$r  $$\int T_\mathrm{A}^* \text{d}v_\mathrm{r}$$    (GHz)  (mK)  (km s−1)  (km s−1)  (× 103 mK km s−1)  H79α  13.09  441.6 ± 3.2  71.4 ± 0.1  31.0 ± 0.3  14.6 ± 0.2  H78α  13.60  401.4 ± 3.1  71.8 ± 0.1  32.0 ± 0.3  13.7 ± 0.2  H77α  14.12  382.0 ± 4.8  71.8 ± 0.2  31.0 ± 0.4  12.6 ± 0.2  H76α  14.69  383.3 ± 6.5  71.7 ± 0.2  30.7 ± 0.6  12.5 ± 0.3  H75α  15.28  350.4 ± 2.5  72.3 ± 0.1  30.0 ± 0.3  11.2 ± 0.1  H99β  13.15  93.2 ± 3.1  70.2 ± 0.5  37.0 ± 1.3  3.7 ± 0.2  H98β  13.56  91.1 ± 1.0  71.8 ± 0.3  33.0 ± 0.4  3.2 ± 0.1  H97β  13.98  88.4 ± 3.7  71.3 ± 0.4  32.2 ± 1.0  3.0 ± 0.1  H96β  14.41  84.5 ± 1.8  73.5 ± 0.4  37.4 ± 0.8  3.4 ± 0.1  H95β  14.87  91.9 ± 2.8  72.0 ± 0.4  30.5 ± 1.0  3.0 ± 0.1  H94β  15.34  78.5 ± 1.0  72.6 ± 0.2  32.0 ± 0.5  2.7 ± 0.1  View Large Table 5. Helium RRL parameters derived for LOS+0.693 using Gaussian fits. RRL  ν  T$$^*_\mathrm{A}$$  $$v$$r  Δ$$v$$r  $$\int T_\mathrm{A}^* dv_\mathrm{r}$$    (GHz)  (mK)  (km s−1)  (km s−1)  (× 102 mK km s−1)  He79α  13.09  31.8 ± 2.0  71.5 ± 0.7  26.6 ± 1.6  9.0 ± 0.8  He78α  13.60  35.8 ± 2.9  73.5 ± 0.9  26.0 ± 2.0  9.9 ± 1.2  He77α  14.13  28.2 ± 3.9  73.0 ± 1.6  26.8 ± 3.7  8.0 ± 1.7  He76α  14.70  23.4 ± 1.8  72.8 ± 0.8  25.2 ± 1.9  6.3 ± 0.7  He75α  15.29  24.3 ± 5.4  72.3 ± 2.3  23.3 ± 5.4  6.0 ± 2.0  He99β  13.15  7.1 ± 3.5  65.7 ± 5.6  30 ± 13  2.3 ± 1.6  He98β  13.56  9.5 ± 2.1  70.8 ± 1.9  22.2 ± 4.6  2.2 ± 0.7  He97β  13.98  8.0 ± 2.7  66.6 ± 4.5  27 ± 11  2.3 ± 1.3  He96β  14.42  7.7 ± 1.2  70.2 ± 1.3  21.7 ± 3.1  1.8 ± 0.4  He95β  14.87  7.8 ± 1.3  71.8 ± 1.1  17.4 ± 2.5  1.4 ± 0.3  He94β  15.35  <6.5a  –  –  <1.7b  RRL  ν  T$$^*_\mathrm{A}$$  $$v$$r  Δ$$v$$r  $$\int T_\mathrm{A}^* dv_\mathrm{r}$$    (GHz)  (mK)  (km s−1)  (km s−1)  (× 102 mK km s−1)  He79α  13.09  31.8 ± 2.0  71.5 ± 0.7  26.6 ± 1.6  9.0 ± 0.8  He78α  13.60  35.8 ± 2.9  73.5 ± 0.9  26.0 ± 2.0  9.9 ± 1.2  He77α  14.13  28.2 ± 3.9  73.0 ± 1.6  26.8 ± 3.7  8.0 ± 1.7  He76α  14.70  23.4 ± 1.8  72.8 ± 0.8  25.2 ± 1.9  6.3 ± 0.7  He75α  15.29  24.3 ± 5.4  72.3 ± 2.3  23.3 ± 5.4  6.0 ± 2.0  He99β  13.15  7.1 ± 3.5  65.7 ± 5.6  30 ± 13  2.3 ± 1.6  He98β  13.56  9.5 ± 2.1  70.8 ± 1.9  22.2 ± 4.6  2.2 ± 0.7  He97β  13.98  8.0 ± 2.7  66.6 ± 4.5  27 ± 11  2.3 ± 1.3  He96β  14.42  7.7 ± 1.2  70.2 ± 1.3  21.7 ± 3.1  1.8 ± 0.4  He95β  14.87  7.8 ± 1.3  71.8 ± 1.1  17.4 ± 2.5  1.4 ± 0.3  He94β  15.35  <6.5a  –  –  <1.7b  a3σ upper limit on the line intensity. b3σ upper limit on the velocity-integrated line intensity. View Large Table 6. 3σ upper limits on the He line intensities for LOS−0.11 and LOS−0.02.     LOS−0.11  LOS−0.02  RRL  ν  T$$^*_\mathrm{A}$$    (GHz)  (mK)  He79α  13.09  <5  <11  He78α  13.60  <3  <9  He77α  14.13  <11  <5  He76α  14.70  <3  <6  He75α  15.29  <3  <6  He99β  13.15  <3  <8  He98β  13.56  <3  <3  He97β  13.98  <3  <3  He96β  14.42  <3  <3  He94β  15.35  <3  <4      LOS−0.11  LOS−0.02  RRL  ν  T$$^*_\mathrm{A}$$    (GHz)  (mK)  He79α  13.09  <5  <11  He78α  13.60  <3  <9  He77α  14.13  <11  <5  He76α  14.70  <3  <6  He75α  15.29  <3  <6  He99β  13.15  <3  <8  He98β  13.56  <3  <3  He97β  13.98  <3  <3  He96β  14.42  <3  <3  He94β  15.35  <3  <4  View Large Table 7. Hmβ/Hnα ratios for the three GC LOSs.     LOS−0.11  LOS−0.02  LOS+0.693  Ratio  Modela  $$v_\mathrm{r}^{\mathrm{{\it b}}}$$  $$\frac{I_{\mathrm{H}m\beta }}{I_{\mathrm{H}n\alpha }}$$  $$v_\mathrm{r}^\mathrm{(b)}$$  $$\frac{I_{\mathrm{H}m\beta }}{I_{\mathrm{H}n\alpha }}{}^\mathrm{{\it c}}$$  $$v_\mathrm{r}^{\mathrm{{\it b}}}$$  $$\frac{I_{\mathrm{H}m\beta }}{I_{\mathrm{H}n\alpha }}{}^\mathrm{{\it c}}$$    (per cent)  (km s−1)  (per cent)  (km s−1)  (per cent)  (km s−1)  (per cent)  H99β/H79α  27.3  +20  25.8 ± 9.8  +50  22.1 ± 3.7  +70  25.2 ± 1.3      −30  37 ± 13  −40  20.9 ± 5.2      H98β/H78α  27.0  +20  33.0 ± 7.6  +50  31.1 ± 2.7  +70  23.4 ± 0.5      −30  31 ± 11  −40  25.0 ± 3.2      H96β/H77α  27.7  +20  35 ± 13  +50  26.8 ± 2.3  +70  26.7 ± 1.0      −30  23 ± 11  −40  28.8 ± 3.9      H96β/H76α  26.6  +20  39 ± 13  +50  28.6 ± 2.5  +70  26.8 ± 1.1      −30  27 ± 11  −40  25.7 ± 3.5      H94β/H75α  26.6  +20  33.3 ± 7.7  +50  28.2 ± 4.2  +70  23.9 ± 0.6      −30  38 ± 14  −40  25.2 ± 4.6          LOS−0.11  LOS−0.02  LOS+0.693  Ratio  Modela  $$v_\mathrm{r}^{\mathrm{{\it b}}}$$  $$\frac{I_{\mathrm{H}m\beta }}{I_{\mathrm{H}n\alpha }}$$  $$v_\mathrm{r}^\mathrm{(b)}$$  $$\frac{I_{\mathrm{H}m\beta }}{I_{\mathrm{H}n\alpha }}{}^\mathrm{{\it c}}$$  $$v_\mathrm{r}^{\mathrm{{\it b}}}$$  $$\frac{I_{\mathrm{H}m\beta }}{I_{\mathrm{H}n\alpha }}{}^\mathrm{{\it c}}$$    (per cent)  (km s−1)  (per cent)  (km s−1)  (per cent)  (km s−1)  (per cent)  H99β/H79α  27.3  +20  25.8 ± 9.8  +50  22.1 ± 3.7  +70  25.2 ± 1.3      −30  37 ± 13  −40  20.9 ± 5.2      H98β/H78α  27.0  +20  33.0 ± 7.6  +50  31.1 ± 2.7  +70  23.4 ± 0.5      −30  31 ± 11  −40  25.0 ± 3.2      H96β/H77α  27.7  +20  35 ± 13  +50  26.8 ± 2.3  +70  26.7 ± 1.0      −30  23 ± 11  −40  28.8 ± 3.9      H96β/H76α  26.6  +20  39 ± 13  +50  28.6 ± 2.5  +70  26.8 ± 1.1      −30  27 ± 11  −40  25.7 ± 3.5      H94β/H75α  26.6  +20  33.3 ± 7.7  +50  28.2 ± 4.2  +70  23.9 ± 0.6      −30  38 ± 14  −40  25.2 ± 4.6      aEstimated values assuming LTE conditions and optically thin radio continuum emission (see the text). bThe velocity components identified in LOS−0.11 and LOS−0.02 are labelled as +20 and −30 km s−1, and +50, and −40 km s−1, respectively. Only one velocity component labelled as +70 km s−1 is identified in LOS+0.693. cValues that do not match the expected LTE ratio are in bold print. View Large Table 8. Hemβ/Henα ratios for LOS+0.693. Ratio  Modela  $$\frac{I_{\mathrm{He}m\beta }}{I_{\mathrm{He}n\alpha }}$$    ( per cent)  ( per cent)  He99β/He79α  27.3  25 ± 18  He98β/He78α  27.0  22.7 ± 7.8  He96β/He77α  27.7  22.1 ± 6.7  He96β/He76α  26.6  28.3 ± 7.0  He94β/He75α  26.6  <25.4  Ratio  Modela  $$\frac{I_{\mathrm{He}m\beta }}{I_{\mathrm{He}n\alpha }}$$    ( per cent)  ( per cent)  He99β/He79α  27.3  25 ± 18  He98β/He78α  27.0  22.7 ± 7.8  He96β/He77α  27.7  22.1 ± 6.7  He96β/He76α  26.6  28.3 ± 7.0  He94β/He75α  26.6  <25.4  aEstimated values assuming LTE conditions and optically thin radio continuum emission (see the text). View Large Table 9. Helium-to-hydrogen line intensity ratios for the three GC sources.   LOS−0.11  LOS−0.02  LOS+0.693  Ratio  v$$_\mathrm{r}^{a}$$  $$\frac{I_{\mathrm{He}n\alpha }}{I_{\mathrm{H}n\alpha }}$$  v$$_\mathrm{r}^{a}$$  $$\frac{I_{\mathrm{He}n\alpha }}{I_{\mathrm{H}n\alpha }}$$  v$$_\mathrm{r}^{a}$$  $$\frac{I_{\mathrm{He}n\alpha }}{I_{\mathrm{H}n\alpha }}$$    (km s−1)  ( per cent)  (km s−1)  ( per cent)  (km s−1)  ( per cent)  He79α/H79α  +20  <20  +50  <10  +70  7.2 ± 0.5    −30  <40  −40  <20      He78α/H78α  +20  <10  +50  <10  +70  9.0 ± 0.7    −30  <20  −40  <20      He77α/H77α  +20  <50  +50  <10  +70  7.4 ± 1.0    −30  <100  −40  <20      He76α/H76α  +20  <20  +50  <10  +70  6.1 ± 0.5    −30  <30  −40  <20      He75α/H75α  +20  <20  +50  <10  +70  6.9 ± 1.5    −30  <30  −40  <20      He99β/H99β  +20  <50  +50  <40  +70  7.6 ± 3.8    −30  <70  −40  <90      He98β/H98β  +20  <30  +50  <10  +70  10.5 ± 2.3    −30  <80  −40  <30      He97β/H97β  +20  <40  +50  <10  +70  9.0 ± 3.1    −30  <100  −40  <30      He96β/H96β  +20  <40  +50  <10  +70  9.1 ± 1.4    −30  <80  −40  <30      He95β/H95βb  +20  –  +50  –  +70  8.5 ± 1.4    −30  –  −40  –      He94β/H94β  +20  <60  +50  <30  +70  <8.3    −30  <90  −40  <50        LOS−0.11  LOS−0.02  LOS+0.693  Ratio  v$$_\mathrm{r}^{a}$$  $$\frac{I_{\mathrm{He}n\alpha }}{I_{\mathrm{H}n\alpha }}$$  v$$_\mathrm{r}^{a}$$  $$\frac{I_{\mathrm{He}n\alpha }}{I_{\mathrm{H}n\alpha }}$$  v$$_\mathrm{r}^{a}$$  $$\frac{I_{\mathrm{He}n\alpha }}{I_{\mathrm{H}n\alpha }}$$    (km s−1)  ( per cent)  (km s−1)  ( per cent)  (km s−1)  ( per cent)  He79α/H79α  +20  <20  +50  <10  +70  7.2 ± 0.5    −30  <40  −40  <20      He78α/H78α  +20  <10  +50  <10  +70  9.0 ± 0.7    −30  <20  −40  <20      He77α/H77α  +20  <50  +50  <10  +70  7.4 ± 1.0    −30  <100  −40  <20      He76α/H76α  +20  <20  +50  <10  +70  6.1 ± 0.5    −30  <30  −40  <20      He75α/H75α  +20  <20  +50  <10  +70  6.9 ± 1.5    −30  <30  −40  <20      He99β/H99β  +20  <50  +50  <40  +70  7.6 ± 3.8    −30  <70  −40  <90      He98β/H98β  +20  <30  +50  <10  +70  10.5 ± 2.3    −30  <80  −40  <30      He97β/H97β  +20  <40  +50  <10  +70  9.0 ± 3.1    −30  <100  −40  <30      He96β/H96β  +20  <40  +50  <10  +70  9.1 ± 1.4    −30  <80  −40  <30      He95β/H95βb  +20  –  +50  –  +70  8.5 ± 1.4    −30  –  −40  –      He94β/H94β  +20  <60  +50  <30  +70  <8.3    −30  <90  −40  <50      aThe velocity components identified in LOS−0.11 and LOS−0.02 are labelled as +20 and −30 km s−1, and +50 and −40 km s−1, respectively. Only one velocity component labelled as +70 km s−1 is identified in LOS+0.693. bThe He95β/H95β line intensity ratios for both sources of Sgr A were not measured because the He95β RRL was not observed. View Large Table 10. Physical properties derived for the three GC sources. Source  RRL  v$$_\mathrm{r}^{a}$$  Sc  ne  log (NLyc)      (km s−1)  (mJy)  (cm−3)  (ph. s−1)  LOS−0.11  H77α  +20  122 ± 23  71 ± 7  47.14 ± 0.08      −30  153 ± 40  80 ± 10  47.24 ± 0.10    H96β  +20  42 ± 14  43 ± 7  46.70 ± 0.12      −30  35 ± 13  39 ± 7  46.62 ± 0.14  LOS−0.02  H77α  +50  451 ± 18  137 ± 3  47.71 ± 0.02      −40  339 ± 25  119 ± 4  47.58 ± 0.03    H96β  +50  118 ± 9  72 ± 3  47.15 ± 0.03      −40  96 ± 11  65 ± 4  47.06 ± 0.05  LOS+0.693  H77α  +70  2311 ± 45  310 ± 3  48.42 ± 0.01    H96β  +70  603 ± 19  163 ± 3  47.86 ± 0.01  Source  RRL  v$$_\mathrm{r}^{a}$$  Sc  ne  log (NLyc)      (km s−1)  (mJy)  (cm−3)  (ph. s−1)  LOS−0.11  H77α  +20  122 ± 23  71 ± 7  47.14 ± 0.08      −30  153 ± 40  80 ± 10  47.24 ± 0.10    H96β  +20  42 ± 14  43 ± 7  46.70 ± 0.12      −30  35 ± 13  39 ± 7  46.62 ± 0.14  LOS−0.02  H77α  +50  451 ± 18  137 ± 3  47.71 ± 0.02      −40  339 ± 25  119 ± 4  47.58 ± 0.03    H96β  +50  118 ± 9  72 ± 3  47.15 ± 0.03      −40  96 ± 11  65 ± 4  47.06 ± 0.05  LOS+0.693  H77α  +70  2311 ± 45  310 ± 3  48.42 ± 0.01    H96β  +70  603 ± 19  163 ± 3  47.86 ± 0.01  aThe velocity components identified in LOS−0.11 and LOS−0.02 are labelled as +20 and −30 km s−1, and +50 and −40 km s−1, respectively. Only one velocity component labelled as +70 km s−1 is identified in LOS+0.693. View Large 3.2 LTE conditions In order to check whether LTE conditions apply in the three GC sources, we have derived the Hmβ to Hnα integrated line intensity ratios (hereafter Hmβ to Hnα ratios), using Hnα and Hmβ lines that were observed simultaneously to avoid uncertainties related to pointing and flux calibration. We show the value of these ratios in Table 7 for the different velocity components and the three GC sources. In this table, we also list the Hmβ to Hnα ratios estimated assuming LTE conditions and optically thin radio continuum emission. We also show the Hemβ to Henα integrated line intensity ratios for LOS+0.693 in Table 8, where the expected LTE values are also listed. As seen in Table 7, the three GC sources have Hmβ to Hnα ratios that are consistent, within their uncertainties, with those predicted in LTE, but there are values (in bold print) in this table that do not match the expected LTE ratio. In LOS−0.02, the measured H99β to H79α and H98β to H78α (positive velocity component) ratios are inconsistent with the LTE values likely due to uncertainty in the baseline correction of the H99β and H98β lines. The same reason may explain why the H99β, H98β, and H94β lines, in LOS+0.693, show intensities lower than those expected in LTE. On the other hand, the measured Hemβ to Henα ratios (see Table 8) are consistent within their uncertainties with the values expected in LTE. In summary, the Hmβ to Hnα ratios derived for the three GC sources and the Hemβ to Henα ratios derived for LOS+0.693 show that the ionized gas in the studied sources can be reasonably assumed to be emitted under LTE conditions. 3.3 Helium to hydrogen ratio As mentioned above, helium RRLs have only been detected towards LOS+0.693. We have derived the He-to-H line intensity ratios (see Table 9) for those RRL transitions where the same principal quantum number has been detected for the two elements. Otherwise, we provide upper limits assuming that the line intensity of the non-detected RRLs is lower than 3σ. We note that the Henα RRLs are located at ≈122 km s−1 with respect to Hnα RRLs, as expected by the difference of their rest frequencies (Towle et al. 1996). Thus, the derived ratios are not affected by possible effects of calibration since both spectral lines are observed simultaneously at close frequencies. We find an average He-to-H intensity ratio of 7.3±0.2 per cent or 4He mass fraction Y = 0.29±0.01 for LOS+0.693. This ratio is consistent with those found in interferometry studies (Roelfsema et al. 1987; Mehringer et al. 1993) that trace more compact regions (≲0.7 pc) than our diffuse regions (∼1.7 pc). The most stringent upper limits on the He−to−H ratio derived for LOS−0.02 and LOS−0.11 are consistent with the He-to-H number ratio of <10 per cent found in GC H ii regions (Roelfsema et al. 1987). The estimation of 0.29±0.01 helium abundance by mass differs by 14 per cent from that of 0.25 as predicted by big bang nucleosynthesis (Coc et al. 2012; Tsivilev et al. 2013). This finding suggests, as expected (Wilson & Rood 1994; Gordon & Sorochenko 2009), that high-mass stars in the GC have enriched the ISM with helium-4, in a past intense burst of star formation in this region, thus increasing its abundance compared to the primordial value. 3.4 Electron densities and number of Lyman ionizing photons In this section, we derive the average electron density ne of the ionized gas following the equation as in Mezger & Henderson (1967), where ne is given by   \begin{eqnarray} \begin{array}{rl}\left(\frac{n_\mathrm{e}}{\mathrm{cm^{-3}}}\right) =&6.351 \cdot 10^2 u_1 a^{0.5} \left(\frac{T_\mathrm{e}}{10^4 \mathrm{\ K}}\right)^{0.175} \left(\frac{\nu }{\mathrm{GHz}}\right)^{0.05} \\ &\left(\frac{S_\mathrm{c}}{\mathrm{Jy}}\right)^{0.5} \left(\frac{D}{\mathrm{kpc}}\right)^{-0.5} \left(\frac{\Theta }{\mathrm{arcmin}}\right)^{-1.5} \mathrm{,} \end{array} \end{eqnarray} (1)where Te is the electron temperature, ν is the frequency, Sc is the continuum flux, D is the distance to the GC [7.86 kpc, Boehle et al. (2016)], and Θ is the source size (which is assumed to be equal to the telescope beam size corresponding to ≈1.7 pc at the GC distance). The parameter a accounts for the deviation between the exact equation for the optical depth for free–free emission and its approximation (Mezger & Henderson 1967). For our study, we have used an average value of a equal to 0.98. We have assumed that our three GC sources have spherical geometry, and in this case the model conversion factor u1 is equal to 0.775 (Mezger & Henderson 1967). The Sc is derived from the H77α and H96β RRL emission assuming an optically thin regime and the average Te found for compact H ii regions of the GC, i.e. Te ≈ 6300 K (Goss et al. 1985). Sc values derived from the H77α and H96β RRLs are similar, within their uncertainties, to those obtained from the other detected Hnα and Hmβ lines, respectively. For this reason Table 10 lists only the Sc values derived from the H77α and H96β lines. For the three GC sources the estimated values of ne are given in Table 10. Using the formula given in Rohlfs & Wilson (1999) (see equation 13.2) we have also calculated the number of Lyman ionizing photons, NLyc, as follows:   \begin{eqnarray} N_\mathrm{Lyc}=\frac{4}{3}\pi \left(\frac{\Theta }{2}\right)^3 n_\mathrm{e} n_\mathrm{p} \alpha ^{(2)} \mathrm{,} \end{eqnarray} (2)where np is the proton density (which is equal to ne under LTE conditions, see Section 3.2), and α(2) is the recombination coefficient (Spitzer 2004). The derived values of NLyc for the three GC sources are shown in Table 10. 4 DISCUSSION 4.1 Extended and diffuse RRL emission towards the three GC LOS As previously mentioned, the only compact H ii region which falls inside the GBT beam of our observations is towards LOS−0.02. This suggests that our GBT observations trace extended RRL emission towards LOS−0.11 and LOS+0.693. In the case of LOS+0.693, this idea is also supported by the extended H69α emission map of Sgr B2 shown in Fig. 6. This figure is obtained using the HOPS data (Purcell et al. 2012). The HOPS data has a spatial resolution of 2.4 arcmin at the frequency (19.59 GHz) of the H69α line, which is a factor ∼3 worse than the average spatial resolution of our observations. Unfortunately, the HOPS data has a rms noise of ∼40 mK, which is not enough to obtain H69α line emission maps for regions where both Sgr A sources were observed. Figure 6. View largeDownload slide H69α integrated line emission of Sgr B2 obtained using HOPS data (Purcell et al. 2012). The range of velocity integration is from 20 to 80 km s−1. LOS+0.693 is indicated with a red circle with the size of the Half-Power Beam Width of the GBT observations (48 arcsec at 13.09 GHz). Figure 6. View largeDownload slide H69α integrated line emission of Sgr B2 obtained using HOPS data (Purcell et al. 2012). The range of velocity integration is from 20 to 80 km s−1. LOS+0.693 is indicated with a red circle with the size of the Half-Power Beam Width of the GBT observations (48 arcsec at 13.09 GHz). In order to figure out whether the emission detected by the GBT towards LOS−0.02 is arising exclusively from the compact A region or not, we have compared the RRL emission measured using the VLA with that of our GBT observations. For this, we have first determined the spectral index α of the region A. This region shows a α of 0.06±0.04 derived considering the Sc of 590±30 mJy that we have measured at 24.5 GHz (using the VLA map shown in Fig. 1) and the value of 570±20 mJy derived at 14.7 GHz by Goss et al. (1985). Thus, T$$^*_\mathrm{A}$$ ∝ ν1.16 assuming that the free−free and RRL emission is optically thin. We have measured the H64α peak line intensity of 67±22 mJy by integrating the channel map at the peak intensity of 47 km s−1 (obtained using the data cube described in Section 2.1) over the Half-Power Beam Width of the GBT. By using the previous relation, we have extrapolated the H64α peak line intensity to that expected for the H76α line, finding a T$$^*_\mathrm{A}$$ of 39±12 mJy at 14.7 GHz, which is similar to that of 40.2±1.2 mJy as measured by the GBT. This suggests that part of the region A inside the GBT beam may contribute significantly to the RRL emission detected in LOS−0.02. The T$$^*_\mathrm{A}$$ of 39±12 mJy found for the H76α line is a factor ∼2 lower than that of 114 mJy as measured by Goss et al. (1985) for the entire region A, which agrees with the fact that only half of the region A falls inside the GBT beam size towards LOS−0.02. Despite this finding, we believe that it is unlikely that the GBT data traces extended RRL emission towards LOS+0.693 and LOS−0.11 and that it does not trace extended RRL emission towards LOS−0.02. In fact, this is supported in Fig. 7 (upper panel) where we show the C i, C iiI and H79α spectra of LOS−0.11 and LOS−0.02. It can be seen that both the positive and negative velocity components of the extended ionized gas traced by the C ii emission (García 2015) are also well traced by the H79α line emission. Therefore, in this paper, we consider that the GBT traces extended ionized gas in the three LOSs. Figure 7. View largeDownload slide Upper panels: C i (black), C ii (red), and H79α (green) spectra observed towards LOS−0.11 and LOS−0.02. Bottom panels: C i (black) and C ii (red) spectra observed towards the H1 source and Sgr A*. The line intensity of several spectra is multiplied by a factor for comparison purposes. C ii spectra is affected by absorption features (indicated by dashed lines) associated with the 3 kpc, 4.5 kpc, and local spiral arms (Oka et al. 1998). The C i and C ii spectra are obtained with a spatial resolution of 46 arcsec similar to that of the H79α spectra. Figure 7. View largeDownload slide Upper panels: C i (black), C ii (red), and H79α (green) spectra observed towards LOS−0.11 and LOS−0.02. Bottom panels: C i (black) and C ii (red) spectra observed towards the H1 source and Sgr A*. The line intensity of several spectra is multiplied by a factor for comparison purposes. C ii spectra is affected by absorption features (indicated by dashed lines) associated with the 3 kpc, 4.5 kpc, and local spiral arms (Oka et al. 1998). The C i and C ii spectra are obtained with a spatial resolution of 46 arcsec similar to that of the H79α spectra. The studied ionized gas is also diffuse because we have found ne of ∼40–310 cm−3, which are much lower than those of 3600–5100 cm−3 found in compact H ii regions of the GC (Mills et al. 2011). The ne of ∼40–120 cm−3 found for the negative velocity gas of LOS−0.11 and LOS−0.02 are consistent with those of ∼100–130 cm−3 found for diffuse ionized gas of the Arched Filaments H ii complex (Langer et al. 2017). C ii emission traces a negative velocity component not only in both Sgr A sources but also in the H1 source and Sgr A* (see the bottom panel of Fig. 7). We also note in Fig. 7 that the C i emission does not trace the negative velocity component in LOS−0.11 and LOS−0.02 but it does partially in the H1 source and Sgr A*. 4.2 Kinematics of the ionized gas Our RRLs show diffuse ionized gas with negative and positive velocities in LOS−0.11 and LOS−0.02. In Fig. 8, we show the velocities of both Sgr A sources on a position-velocity diagram for the C ii emission. Four gas streams from the model of Kruijssen et al. (2015) are also shown in this figure. The $$v$$r of both velocity components of LOS−0.11 are consistent with the velocities of streams 1 and 4, while the $$v$$r of both velocity components of LOS−0.02 agree with the velocities of streams 1 and 2. This suggests that along the two LOSs the GBT traces diffuse and extended ionized gas that are part of the gas streams orbiting the GC. The positions of LOS−0.11 and LOS−0.02 (see Fig. 8, upper panel) also support that the positive velocity gas in both sources is part of stream 1. The velocities and positions of the regions H1–H5 (see Fig. 8) show that these sources are likely associated with stream 2. If this hypothesis is correct, then the negative velocity gas of LOS−0.02 could coexist with the H1–H5 sources. Our hypothesis is in agreement with the finding of Langer et al. (2017) that the kinematics of the ionized gas in the Sgr A and Sgr B2 complexes, as traced by the C ii emission, is well explained by the gas streams proposed by Kruijssen et al. (2015). Figure 8. View largeDownload slide Upper panel: four streams used to model the GC gas kinematics (Kruijssen et al. 2015). The NH3(1,1) emission map obtained using HOPS data (Purcell et al. 2012) is shown in grey-scale. The map is integrated over the velocity range −100–+100 km s−1. Filled triangles indicate positions of LOS−0.11 and LOS−0.02, while filled squares show the positions of H1–H5 sources. The black cross shows the position of Sgr A*. Bottom panel: The four streams are drawn on the position-velocity C ii map obtained using HIFI data (García et al. 2016). This map covers 0.1° in latitude centred at 0°. Central line velocities of LOS−0.11 and LOS−0.02 are shown with filled triangles, while the central line velocities of H1–H5 sources (Zhao et al. 1993) are indicated with filled squares. Velocity error bars overlap with filled triangles. Figure 8. View largeDownload slide Upper panel: four streams used to model the GC gas kinematics (Kruijssen et al. 2015). The NH3(1,1) emission map obtained using HOPS data (Purcell et al. 2012) is shown in grey-scale. The map is integrated over the velocity range −100–+100 km s−1. Filled triangles indicate positions of LOS−0.11 and LOS−0.02, while filled squares show the positions of H1–H5 sources. The black cross shows the position of Sgr A*. Bottom panel: The four streams are drawn on the position-velocity C ii map obtained using HIFI data (García et al. 2016). This map covers 0.1° in latitude centred at 0°. Central line velocities of LOS−0.11 and LOS−0.02 are shown with filled triangles, while the central line velocities of H1–H5 sources (Zhao et al. 1993) are indicated with filled squares. Velocity error bars overlap with filled triangles. 4.3 Sources of ionization 4.3.1 Positive velocity gas in the three GC LOSs The ionized gas studied in LOS−0.11 is at a projected distance of ∼3.8 pc from the region G (see Fig. 1), whose massive stars are thought to be the closest compact source of ionization to the positive velocity LOS−0.11 gas (Ho et al. 1985). On the other hand, since the region L lies close to LOS+0.693 (see Fig. 1, bottom panel), it is expected that the main ionization source of the LOS+0.693 gas could be the massive stars responsible for the ionization of the region L. As can be seen in the upper panel of Fig. 1, part of the emission arising in the region A falls within the GBT beam towards LOS−0.02. Thus, we expect that the gas with positive velocities in this GC source is mainly ionized by the massive stars in the H ii region A. To test whether the H ii region A could be the main source of ionization of the positive velocity gas in LOS−0.02, we estimate the number of photons inside the GBT beam, NΩ, and that ∼50 per cent of the region A falls inside the GBT beam (see Fig. 1). Considering the location of the GBT beam centre, then the compact H ii region A would actually be displaced from the telescope beam centre. For this geometry, we can estimate an upper limit to NΩ following the expression given by Rodríguez-Fernández & Martín-Pintado (2005):   \begin{eqnarray} N_{\Omega }=\frac{N_\mathrm{Lyc}}{4\pi r^2} \Omega D^2 \mathrm{,} \end{eqnarray} (3)where r is the radius of the H ii region. We derive the upper limit of 1050.95 photons s−1 for the value of NΩ by using the NLyc value provided by Mills et al. (2011) for the H ii region A, Ω = 45 arcsec and r = 1.7 pc in equation (3). It seems that the positive velocity gas of LOS−0.02 is mainly ionized by the massive stars in the H ii region A because the NLyc values of LOS−0.02, given in Table 10, are consistent with the upper limit of 1050.95 photons s−1. 4.3.2 Negative velocity gas in LOS−0.11 and LOS−0.02 The ionized gas components with negative velocities found towards both sources of Sgr A raises the question of the source of ionization. The top–down view shown in fig. 6 of Kruijssen et al. (2015) gives us information about the distances between Sgr A* and the four streams considered in their kinematical model. In this scenario Sgr A* is located between both the 20 and 50 km s−1 clouds and their background gas streams 3 and 4, at a projected distance of ∼60 pc from these features. If the negative velocity LOS−0.02 gas is part of the gas stream 2, as discussed in Section 4.2, then it may be ionized by the photons arising in massive O6–O7 stars, which also ionize the presumably closest UC–H ii regions, i.e. H1–H5 (Zhao et al. 1993), located at least ∼12 pc away from the negative velocity gas observed towards LOS−0.02 (see Fig. 8, upper panel). Of course, other ionizing sources apart from those proposed may exist in the environment of the negative velocity LOS−0.11 gas. On the other hand, the negative velocity LOS−0.02 gas is likely part of the stream 4, as discussed in Section 4.2. So far there are no compact H ii regions or massive stars whose velocities and positions are consistent with those of the gas stream 4 around LOS−0.11, hence the identification of ionizing sources of the negative velocity LOS−0.11 gas remains unclear. A possibility is that the negative velocity LOS−0.11 gas is actually part of stream 2, despite the difference in their velocites (see Fig. 8, bottom panel), thus being also ionized by the massive stars inside H1–H5 sources as for LOS−0.02. Considering the gas stream model proposed by Kruijssen et al. (2015), the massive young stars orbiting Sgr A* can be ruled out as ionizing sources of the negative velocity LOS−0.11 and LOS−0.02 gas since in this scenario Sgr A* is ∼60 pc away from gas streams 2 and 4 along the two LOSs. 5 CONCLUSIONS Using the GBT telescope, we have detected extended and diffuse ionized emission towards three GC LOSs. The main conclusions of the present work are as follows: We found that the ionized gas observed towards the three GC sources is emitted under LTE conditions based on the Hmβ-to-Hnα integrated line intensity ratios. We found a 4He mass fraction Y of 0.29±0.01 that supports the hypothesis that high-mass stars in the GC have enriched the helium-4 abundance in the ISM as compared to the primordial value. For LOS−0.11, LOS−0.02, and LOS+0.693, we have derived ne and NLyc values. The studied gas is characterized by ne of ∼40–310 cm−3. The ionized gas detected towards regions of the 20 and 50 km s−1 clouds is likely associated, following the Kruijssen et al. (2015) model, with gas stream 1 orbiting the GC, while the ionized gas moving with negative velocities in LOS−0.02 and LOS−0.11 is likely associated with the gas streams 2 and 4, respectively, located in projection ∼12 pc above stream 1. The LOS−0.02 gas at positive velocities is mainly ionized by ultraviolet (UV) photons produced in the massive stars also ionizing the H ii region A. The massive stars inside the H ii regions L and G are considered the closest sources of gas ionization of LOS+0.693 and LOS−0.11 (positive velocity component), respectively. We propose that the gas with negative velocities observed towards LOS−0.02 may be ionized by UV photons originating in the massive stars of the presumably closest H ii regions H1–H5. The negative velocity gas observed towards LOS−0.11 is likely associated with gas stream 4. We were not able to propose any possible ionizing sources of the negative velocity LOS−0.11 gas because, so far, there are no compact H ii regions or massive stars having both velocities and positions similar to those expected for gas stream 4 around LOS−0.11. However, if the negative velocity components of both Sgr A sources are part of the stream 2, then the massive stars in the H1–H5 regions could be the main sources of UV photons ionizing the gas with negative velocities of both Sgr A sources. We compared C i spectra with our H79α spectra, finding that C i emission does not trace the negative velocity component of either of the Sgr A sources. This indicates that this diffuse gas component is fully ionized. ACKNOWLEDGEMENTS We thank the anonymous referee for comments, which helped to improve this paper. AB-R acknowledges support from a DGAPA postdoctoral grant (year 2015) to UNAM. JM-P acknowledges partial support by the MINECO under grants ESP2015−65597 −C4−1 and ESP2017− and Comunidad de Madrid grant number S2013/ICE−2822 SpaceTec−CM. Footnotes 1 The name is given by its local standard of rest radial velocities. 2 The National Radio Astronomy Observatory is a facility of the National Science Foundation, operated under a cooperative agreement by Associated Universities, Inc. 3 gbtidl is an NRAO data reduction package, written in the IDL language for the reduction of GBT data. 4 This package have been developed at the Centro de Astrobiología. More information about this package in http://cab.inta-csic.es/madcuba/Portada.html. 5 https://archive.nrao.edu/ 6 http://casa.nrao.edu/ REFERENCES Armstrong D. A., Jackson J. M., Ho P. T. P., 1989, in Morris M., ed. IAU Symp. 136, The Center of the Galaxy . Kluwer, Dordrecht, p. 389 Google Scholar CrossRef Search ADS   Bally J., Stark A. A., Wilson R. W., 1987, ApJS , 65, 13 https://doi.org/10.1086/191217 CrossRef Search ADS   Boehle A. et al.  , 2016, ApJ , 830, 17 https://doi.org/10.3847/0004-637X/830/1/17 CrossRef Search ADS   Coc A., Goriely S., Xu Y., Saimpert M., Vangioni E., 2012, ApJ , 744, 158 https://doi.org/10.1088/0004-637X/744/2/158 CrossRef Search ADS   De Pree C. G., Wilder D. J., Deblasio J., Mercer A. J., Davis L. E., 2005, ApJ , 624, L101 https://doi.org/10.1086/430738 CrossRef Search ADS   Ekers R. D., van Gorkom J. H., Schwarz U. J., Goss W. M., 1983, A&A , 122, 143 García P., 2015, PhD thesis , Univ, Cologne García P., Simon R., Stutzki J., Güsten R., Requena-Torres M. A., Higgins R., 2016, A&A , 588, A131 CrossRef Search ADS   Gaume R. A., Claussen M. J., De Pree C. G., Goss W. M., Mehringer D. M., 1995, ApJ , 449, 663 https://doi.org/10.1086/176087 CrossRef Search ADS   Gordon M. A, Sorochenko R. L., 2009, Radio Recombination Lines: Their Physics and Astronomical Applications . Springer, New York Google Scholar CrossRef Search ADS   Gordon M. A., Berkermann U., Mezger P. G., Zylka R., Haslam C. G. T., Kreysa E., Sievers A., Lemke R., 1993, A&A , 280, 208 Goss W. M., Schwarz U. J., van Gorkom J. H., Ekers R. D., 1985, MNRAS , 215, 69 https://doi.org/10.1093/mnras/215.1.69P CrossRef Search ADS   Henshaw J. D. et al.  , 2016, MNRAS , 457, 2675 https://doi.org/10.1093/mnras/stw121 CrossRef Search ADS   Ho P. T. P., Jackson J. M., Barrett A. H., Armstrong J. T., 1985, ApJ , 288, 17 https://doi.org/10.1086/162823 CrossRef Search ADS   Kruijssen J. M. D., Dale J. E., Longmore S. N., 2015, MNRAS , 447, 1059 https://doi.org/10.1093/mnras/stu2526 CrossRef Search ADS   Lang C. C., Goss W. M., Morris M., 2001, AJ , 121, 2681 https://doi.org/10.1086/320373 CrossRef Search ADS   Langer W. D., Velusamy T., Morris M. R., Goldsmith P. F., Pineda J. L., 2017, A&A , 599, A136 CrossRef Search ADS   Lau R. M., Herter T. L., Morris M. R., Adams J. D., 2014, ApJ , 794, 108 https://doi.org/10.1088/0004-637X/794/2/108 CrossRef Search ADS   Lu F. J., Wang Q. D., Lang C. C., 2003, AJ , 126, 319 https://doi.org/10.1086/375754 CrossRef Search ADS   Martín S., Requena-Torres M. A., Martín-Pintado J., Mauersberger R., 2008, ApJ , 678, 245 https://doi.org/10.1086/533409 CrossRef Search ADS   Mehringer D. M., Palmer P., Goss W. M., Yuzef-Zadeh F., 1993, ApJ , 412, 684 https://doi.org/10.1086/172954 CrossRef Search ADS   Mezger P. G., Henderson A. P., 1967, ApJ , 147, 471 https://doi.org/10.1086/149030 CrossRef Search ADS   Mills E., Morris M. R., Lang C. C., Dong H., Wang Q. D., Cotera A., Stolovy S. R., 2011, ApJ , 735, 84 https://doi.org/10.1088/0004-637X/735/2/84 CrossRef Search ADS   Molinari A. et al.  , 2011, ApJL , 735, L33 https://doi.org/10.1088/2041-8205/735/2/L33 CrossRef Search ADS   Oka T., Hasegawa T., Sato F., Tsuboi M., Miyazaki A., 1998, ApJS , 118, 455 https://doi.org/10.1086/313138 CrossRef Search ADS   Purcell C. R. et al.  , 2012, MNRAS , 426, 3 https://doi.org/10.1111/j.1365-2966.2012.21800.x CrossRef Search ADS   Rodríguez-Fernández N. J., Martín-Pintado J., 2005, A&A , 429, 923 CrossRef Search ADS   Roelfsema P. R., Goss W. M., Whiteoak J. B., Gardner F. F., Pankonin V., 1987, A&A , 175, 219 Rohlfs K., Wilson T. L., 1999, Tools of Radio Astronomy . Springer-Verlag, Heidelberg Royster M. J., Yusef-Zadeh F., 2014, in Sjouwerman L., Ott J., Lang C., eds, Proc. IAU Symp. 303, The Galactic Center: Feeding and Feedback in a Normal Galactic Nucleus . Kluwer, Dordrecht, p. 92 Serabyn E., Lacy J. H., Achtermann J. M., 1992, ApJ , 395, 166 https://doi.org/10.1086/171640 CrossRef Search ADS   Spitzer L., 2004, Physical processes in the Intertellar Medium , Wiley-VCH, p. 107 Towle J. P., Feldman P. A., Watson J. K. G., 1996, ApJS , 107, 747 https://doi.org/10.1086/192380 CrossRef Search ADS   Tsivilev A. P., Parfenov S. Yu., Sobolev A. M., Krasnov V. V., 2013, Astron. Lett., Springer , 39, 737 https://doi.org/10.1134/S106377371310006X CrossRef Search ADS   Wilson T. L., Rood R. T., 1994, ARA&A , 32, 191 CrossRef Search ADS   Yusef-Zadeh F., Wardle M., Muno M., Law C., Pound M., 2005, Adv. Space Res. , 35, 1074 https://doi.org/10.1016/j.asr.2005.02.057 CrossRef Search ADS   Zhao J.-H., Desai K., Goss W. M., Yusef-Zadeh F., 1993, ApJ , 418, 235 https://doi.org/10.1086/173385 CrossRef Search ADS   © 2018 The Author(s) Published by Oxford University Press on behalf of the Royal Astronomical Society http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png Monthly Notices of the Royal Astronomical Society Oxford University Press

# Study of diffuse H II regions potentially forming part of the gas streams around Sgr A*

11 pages

/lp/ou_press/study-of-diffuse-h-ii-regions-potentially-forming-part-of-the-gas-fkspseXHHl
Publisher
The Royal Astronomical Society
ISSN
0035-8711
eISSN
1365-2966
D.O.I.
10.1093/mnras/sty391
Publisher site
See Article on Publisher Site

### Abstract

Abstract We present a study of diffuse extended ionized gas towards three clouds located in the Galactic Centre (GC). One line of sight (LOS) is towards the 20 km s−1 cloud (LOS−0.11) in the Sgr A region, another LOS is towards the 50 km s−1 cloud (LOS−0.02), also in Sgr A, while the third is towards the Sgr B2 cloud (LOS+0.693). The emission from the ionized gas is detected from Hnα and Hmβ radio recombination lines (RRLs). Henα and Hemβ RRL emission is detected with the same n and m as those from the hydrogen RRLs only towards LOS+0.693. RRLs probe gas with positive and negative velocities towards the two Sgr A sources. The Hmβ to Hnα ratios reveal that the ionized gas is emitted under local thermodynamic equilibrium conditions in these regions. We find a He to H mass fraction of 0.29±0.01 consistent with the typical GC value, supporting the idea that massive stars have increased the He abundance compared to its primordial value. Physical properties are derived for the studied sources. We propose that the negative velocity component of both Sgr A sources is part of gas streams considered previously to model the GC cloud kinematics. Associated massive stars with what are presumably the closest H ii regions to LOS−0.11 (positive velocity gas), LOS−0.02, and LOS+0.693 could be the main sources of ultraviolet photons ionizing the gas. The negative velocity components of both Sgr A sources might be ionized by the same massive stars, but only if they are in the same gas stream. ISM: clouds, H ii regions, Galaxy: centre 1 INTRODUCTION The proximity of the Galactic Centre (GC), at a distance of about 7.86 kpc (Boehle et al. 2016), offers a unique opportunity to look at a galactic nucleus in great detail. Several studies have been carried out to establish the physical properties and the kinematics of ionized gas towards the main compact H ii regions located at the centre of the Galaxy (Ho et al. 1985; Mehringer et al. 1993; Zhao et al. 1993; Mills et al. 2011). Sgr A West, located around the supermassive black hole Sgr A*, is a spiral-shaped region of ionized gas whose emission is thermal in nature (Ekers et al. 1983). Sgr A East is a non-thermal source surrounding Sgr A West in projection (Ekers et al. 1983). There is also a group of four H ii regions, known collectively as G−0.02 −0.07, made up of the regions denoted as A, B, C, and D (see Fig. 1, upper panel). G−0.02 −0.07 is located at a projected distance of ∼6 pc from Sgr A*. These H ii regions likely reside within the 50 km s−1 cloud1 (Goss et al. 1985; Mills et al. 2011), one of the massive clouds in the Sgr A complex. Sgr A East may be impacting the 50 km s−1 cloud at its west side (Serabyn, Lacy & Achtermann 1992). Massive O stars are thought to be ionizing the A–D regions (Lau et al. 2014). Using line to continuum ratios, Goss et al. (1985) found electron temperatures in the range of ∼5000–7000 K for the four compact H II regions. Figure 1. View largeDownload slide VLA radio-continuum maps at 24.5 GHz towards the three LOSs observed by us (see Section 2.1). The three LOSs are shown as black circles with the size of the GBT beam of 48 arcsec at 13.09 GHz. Upper panel: the region A partly falls inside LOS−0.02. The regions B, C, and D are also seen in the field. Middle panel: LOS−0.11 overlaps with part of the non-thermal source Sgr A-E (Lu et al. 2003). The region G appears to be the closest H ii region to LOS−0.11 (Ho et al. 1985). Bottom panel: LOS+0.693 lies close to the H ii region L located northeast of Sgr B2N. Figure 1. View largeDownload slide VLA radio-continuum maps at 24.5 GHz towards the three LOSs observed by us (see Section 2.1). The three LOSs are shown as black circles with the size of the GBT beam of 48 arcsec at 13.09 GHz. Upper panel: the region A partly falls inside LOS−0.02. The regions B, C, and D are also seen in the field. Middle panel: LOS−0.11 overlaps with part of the non-thermal source Sgr A-E (Lu et al. 2003). The region G appears to be the closest H ii region to LOS−0.11 (Ho et al. 1985). Bottom panel: LOS+0.693 lies close to the H ii region L located northeast of Sgr B2N. Another H ii region labelled as G (see Fig. 1, middle panel), located at ∼13 pc in projection from Sgr A*, is thought to be excited by one O9 or five B0 stars (Ho et al. 1985). The region G appears to be embedded in the 20 km s−1 cloud (Armstrong, Jackson & Ho 1989), another massive cloud in the Sgr A complex. Sgr A–E is considered a non-thermal source (Lu, Wang & Lang 2003), which lies close to the region G (see Fig. 1). Armstrong et al. (1989) found an electron temperature of ∼7500 K for the region G. Zhao et al. (1993) studied five H ii regions (identified as H1 through to H5) located between Sgr A West and the Arched Filaments H ii complex containing a group of curved ridges showing velocities from 15 to −70 km s−1 (Lang, Goss & Morris 2001). The H1–H5 sources show gas velocities from −20 to −60 km s−1, which seem to be associated with a −30 km s−1 cloud (Zhao et al. 1993). However, negative velocities of the ionized gas are not only observed towards the H1–H5 regions and the Arched Filaments H ii complex, as previously thought, but also towards many other regions of the Sgr A complex. In fact, a GC large-scale map obtained by Royster & Yusef-Zadeh (2014) shows ionized gas towards the Sgr A complex with negative velocities reaching up to ∼−130 km s−1. A recent position-velocity map of the C II emission (Langer et al. 2017), which is considered as a good tracer of the ionized gas, shows a similar distribution as in the map obtained by Royster & Yusef-Zadeh (2014). Clouds of diffuse ionized gas in Sgr A with velocities from ∼−130 to +130 km s−1 are shown on the channel maps of the C II emission obtained by García (2015). On the other hand, the Sgr B2 complex lies at a projected distance of ∼120 pc from the GC. This complex contains many dozens of compact and ultracompact H ii regions (Gaume et al. 1995; De Pree et al. 2005). Many of these H ii regions are associated with the Sgr B2 north (N), main (M), and south (S) hot cores where star formation is taking place (Gordon et al. 1993). The ionized gas in the Sgr B2 complex shows velocities predominantly in the range of 50–70 km s−1 (Mehringer et al. 1993). There is a H ii region labelled as L (Mehringer et al. 1993) that is located at a projected distance of ∼1.6 pc from Sgr B2N (see Fig. 1, bottom panel). The region L has an electron temperature of ∼6500 K (Mehringer et al. 1993) and it is believed to be excited by one O5.5 star (Gaume et al. 1995). The 20 and 50 km s−1 clouds are considered as part of a set of clouds moving on stable x2 orbits around the GC in a 100×60 pc elliptical and twisted ring (Molinari et al. 2011). In this scenario, both clouds are located in the front region of the ring while its background gas, which is around both clouds as seen in projection, show velocities from ∼0 to −60 km s−1 (Molinari et al. 2011). Kruijssen, Dale & Longmore (2015) also modelled the gas kinematics studied by Molinari et al. (2011), reproducing the kinematics of molecular gas using an open gas stream divided into four gas streams orbiting the GC. The back side of the open stream is composed of streams 3 and 4, while streams 1 and 2 are two ends of the open stream located at its front side (Kruijssen et al. 2015). The 20 and 50 km s−1 clouds are contained in the gas stream 1. A recent study (Langer et al. 2017) revealed that the ionized gas velocities of the Sgr A and Sgr B2 clouds are better explained by the gas streams proposed by Kruijssen et al. (2015) rather than by the elliptical ring proposed by Molinari et al. (2011). Henshaw et al. (2016) found that two spiral arms or gas streams reproduce the molecular gas distribution of several GC clouds. Since no known physical model explains the spiral arms (Henshaw et al. 2016), open streams might be the most likely structure. In this paper, we focus on studying the physical properties and kinematics of the diffuse ionized gas of selected GC regions. Using radio recombination lines (RRLs) observed with the Green Bank Telescope (GBT) of NRAO,2 we find that RRLs show positive and negative velocities towards two lines of sight (LOS) in the Sgr A complex, one towards the 50 km s−1 cloud (LOS−0.02) and another towards the 20 km s−1 cloud (LOS−0.11). We also study the ionized gas along one LOS in the Sgr B2 complex (LOS+0.693) for comparison purposes. Fig. 1 shows the observed positions of the three LOS, where other GC sources are indicated. As indicated in Fig. 1LOS−0.02 covers part of the emission arising from the H ii region A. The region G appears to be the closest thermal H ii region to LOS−0.11 (Ho et al. 1985) since Sgr A-E is considered a non-thermal source in nature (Lu et al. 2003; Yusef-Zadeh et al. 2005). LOS+0.693 lies close to the H ii region L (see Fig. 1, bottom panel). This paper is organized as follows. In Section 2, we present the observations and data used in this work. We present the main results in Section 3, focusing on the line identification of RRLs and Gaussian fits in Section 3.1, the local thermodynamic equilibrium (LTE) of the ionized gas in Section 3.2, helium to hydrogen ratio in Section 3.3, and electron densities and the number of Lyman continuum photons in Section 3.4. We discuss whether the RRL emission detected with the GBT is extended and diffuse in Section 4.1, the kinematics of the ionized gas in Section 4.2, and the sources of gas ionization in Section 4.3. Finally, the conclusions of this work are presented in Section 5. 2 OBSERVATIONS AND DATA REDUCTION The observations were carried out with the NRAO 100-m GBT in 2009 July–October. We used the Ku-band receiver connected to the spectrometer that provided four 200 MHz spectral windows in two polarizations. This configuration provides a spectral resolution of 24.4 kHz or 0.6 km s−1. Spectra were calibrated using a noise tube and the line intensities, affected by 20 per cent uncertainties, are given in T$$_\mathrm{A}^*$$ scale. The position-switched mode was used during the observations. As mentioned, the studied LOSs are shown in Fig. 1. The angular resolution is 45 arcsec at 14.19 GHz, which corresponds to ∼1.7 pc at the distance of the GC. We used the reference positions selected and verified by Martín et al. (2008), which were originally based on large scale CS maps (Bally, Stark & Wilson 1987). The three observed LOS positions and their reference positions are indicated in Table 1. Table 1. Observed positions and their references. Source  Position  Reference    RA(J2000)  Dec.(J2000)  RA(J2000)  Dec.(J2000)  LOS−0.02  17h45m51.0s  −28°59΄06.0΄  17h46m00.1s  −29°16΄47.2΄  LOS−0.11  17h45m39.0s  −29°04΄05.0΄  17h46m00.1s  −29°16΄47.2΄  LOS+0.693  17h47m22.0s  −28°21΄27.0΄  17h46m23.0s  −28°16΄37.3΄  Source  Position  Reference    RA(J2000)  Dec.(J2000)  RA(J2000)  Dec.(J2000)  LOS−0.02  17h45m51.0s  −28°59΄06.0΄  17h46m00.1s  −29°16΄47.2΄  LOS−0.11  17h45m39.0s  −29°04΄05.0΄  17h46m00.1s  −29°16΄47.2΄  LOS+0.693  17h47m22.0s  −28°21΄27.0΄  17h46m23.0s  −28°16΄37.3΄  View Large Using the gbtidl package, 3 we inspected all scans of the SDFITS files, and the baseline subtraction and average were applied to the calibrated spectra. Then the data were imported into the madcuba package4 for further processing. The spectra were smoothed to a velocity resolution of ∼5 km s−1 appropriate for the RRL widths, Δ$$v$$r, of ∼30 km s−1 observed in the GC (Mehringer et al. 1993). 2.1 Archival VLA data To find out whether the emission detected with the GBT is affected by emission arising from compact H ii regions (see discussion in Section 4.1), we have used VLA data at 24.5 GHz available in the NRAO archive.5 The VLA data reduction and imaging were done using the casa package6 (version 4.7.0). The observations were carried out in 2012 using the DnC configuration. We have build continuum maps, shown in Fig. 1, and also a H64α cube for the LOS−0.02 region as this information will be required in Section 4.1. The continuum maps and cube were obtained using the clean task of casa. The spatial resolution of the maps and cube is 2.52×2.47 arcsec2. The cube has a rms noise of ∼4 mJy beam−1 per channel, while the continuum maps of both Sgr A regions and the LOS+0.693 region have rms noises of ∼2 and ∼20 mJy beam−1, respectively. 3 RESULTS 3.1 Line identification and gaussian fits To identify hydrogen (H) and helium (He) RRLs, we have used a catalogue included in the madcubaij package, which contains the frequencies of the RRLs estimated according to the Dirac theory described by Towle, Feldman & Watson (1996). The RRLs detected in LOS−0.11, LOS−0.02, and LOS+0.693 are shown in Fig. 2, 3, and 4–5, respectively. We have detected emission from Hnα lines with n = 79–75 and Hmβ lines with m = 99–96, 94 towards LOS−0.11 and LOS−0.02. Hydrogen RRLs with the same n and m are also detected towards LOS+0.693 but, in this case, we have also detected the H95β, whose frequency is not in the bandwidth of our observations towards either of the two Sgr A sources. All these RRLs are detected with a significance higher than 3σ. The strongest RRLs are observed in LOS+0.693, whereas the weakest lines are detected in LOS−0.11. We have detected the emission from Henα lines, with n = 79–75 and Hemβ lines with m = 99–94, only towards LOS+0.693 (see Fig. 5). As shown in Fig. 2 and 3, the RRLs in both of the of Sgr A sources reveal two velocity components, while the RRLs in LOS+0.693 show only a single velocity component. Figure 2. View largeDownload slide Hydrogen RRLs observed towards LOS−0.11. The dashed red lines show the velocities of 20 and −30 km s−1. The spectra are shown with a spectral resolution of ∼5 km s−1. Figure 2. View largeDownload slide Hydrogen RRLs observed towards LOS−0.11. The dashed red lines show the velocities of 20 and −30 km s−1. The spectra are shown with a spectral resolution of ∼5 km s−1. Figure 3. View largeDownload slide Hydrogen RRLs observed towards LOS−0.02. The dashed red lines show the velocities of 50 and −40 km s−1. The spectra are shown with a spectral resolution of ∼5 km s−1. Figure 3. View largeDownload slide Hydrogen RRLs observed towards LOS−0.02. The dashed red lines show the velocities of 50 and −40 km s−1. The spectra are shown with a spectral resolution of ∼5 km s−1. Figure 4. View largeDownload slide Hydrogen RRL observed towards LOS+0.693. The dashed red line shows the velocity of 70 km s−1. The spectra are shown with a spectral resolution of ∼5 km s−1. Figure 4. View largeDownload slide Hydrogen RRL observed towards LOS+0.693. The dashed red line shows the velocity of 70 km s−1. The spectra are shown with a spectral resolution of ∼5 km s−1. Figure 5. View largeDownload slide Helium RRLs observed towards LOS+0.693. The dashed red line shows the velocity of 70 km s−1. The spectra are shown with a spectral resolution of ∼5 km s−1. Figure 5. View largeDownload slide Helium RRLs observed towards LOS+0.693. The dashed red line shows the velocity of 70 km s−1. The spectra are shown with a spectral resolution of ∼5 km s−1. Gaussian fits to the RRLs are used to derive the peak intensity (T$$_\mathrm{A}^*$$), central line velocity ($$v$$r), full width at half-maximum (Δ$$v$$r), the integrated line intensity ($$\int T_\mathrm{A}^* \text{d}v_\mathrm{r}$$), and their respective uncertainties. The frequencies of the RRLs and the derived parameters for each source are listed in Tables 2 –4. The RRLs found in both sources of Sgr A are fitted with two Gaussian lines. The two velocity components are labelled as +20 and −30 km s−1 in LOS−0.11 and as +50 and −40 km s−1 in LOS−0.02 in Tables 7, 9, and 10. As mentioned, Henα lines are detected only in LOS+0.693, and the parameters derived using Gaussian fits are given in Table 5, where upper limits for the T$$_\mathrm{A}^*$$ and $$\int T_\mathrm{A}^* \text{d}v_\mathrm{r}$$ of the He94β line are also listed. For both sources of Sgr A, we have estimated 3σ upper limits for T$$_\mathrm{A}^*$$ of the He lines shown in Table 6 because we will study the He to H ratio in Section 3.3. In Table 6, there are no upper limits for the He95β line as it was not observed in either of the Sgr A sources. Table 2. Hydrogen RRL parameters derived for LOS−0.11 using Gaussian fits with two velocity components. RRL  ν  T$$^*_\mathrm{A}$$  $$v$$r  Δ$$v$$r  $$\int T_\mathrm{A}^* \text{d}v_\mathrm{r}$$    (GHz)  (mK)  (km s−1)  (km s−1)  (× 102 mK km s−1)  H79α  13.09  19.4 ± 1.6  22.9 ± 1.3  30.2 ± 2.9  6.2 ± 0.8      12.6 ± 1.3  −26.5 ± 2.5  49.7 ± 6.5  6.7 ± 1.2  H78α  13.60  17.9 ± 0.9  21.9 ± 1.3  38.2 ± 2.8  7.3 ± 0.7      11.0 ± 0.8  −32.3 ± 2.4  48.6 ± 5.7  5.7 ± 0.8  H77α  14.12  22.9 ± 3.5  21.8 ± 0.8  27.3 ± 2.6  6.6 ± 1.3      9.8 ± 1.1  −17.2 ± 9.1  80 ± 17  8.4 ± 2.2  H76α  14.69  19.0 ± 1.1  22.9 ± 1.2  29.9 ± 2.7  6.1 ± 0.7      13.1 ± 1.0  −28.7 ± 1.9  51.3 ± 4.8  7.2 ± 0.9  H75α  15.28  20.0 ± 1.0  22.5 ± 0.9  30.0 ± 2.0  6.4 ± 0.6      10.4 ± 0.8  −32.5 ± 2.0  51.6 ± 5.3  5.7 ± 0.8  H99β  13.15  5.9 ± 1.1  17.3 ± 3.5  25.8 ± 7.1  1.6 ± 0.6      4.7 ± 0.5  −30.0 ± 6.0  50 ± 13  2.5 ± 0.8  H98β  13.56  9.0 ± 0.9  20.2 ± 1.7  25.1 ± 4.2  2.4 ± 0.5      3.3 ± 0.6  −30.0 ± 5.2  50 ± 14  1.8 ± 0.6  H97β  13.98  8.1 ± 1.0  17.4 ± 1.6  37.0 ± 3.8  3.2 ± 0.6      3.0 ± 0.5  −30.0 ± 8.7  50 ± 19  1.6 ± 0.7  H96β  14.41  7.6 ± 0.9  16.9 ± 2.4  29.0 ± 8.3  2.3 ± 0.8      3.7 ± 0.6  −30.0 ± 7.3  50 ± 16  1.9 ± 0.7  H94β  15.34  5.7 ± 0.6  22.4 ± 2.6  35.8 ± 6.1  2.1 ± 0.5      4.0 ± 1.0  −30.0 ± 3.7  50.0 ± 9.2  2.2 ± 0.8  RRL  ν  T$$^*_\mathrm{A}$$  $$v$$r  Δ$$v$$r  $$\int T_\mathrm{A}^* \text{d}v_\mathrm{r}$$    (GHz)  (mK)  (km s−1)  (km s−1)  (× 102 mK km s−1)  H79α  13.09  19.4 ± 1.6  22.9 ± 1.3  30.2 ± 2.9  6.2 ± 0.8      12.6 ± 1.3  −26.5 ± 2.5  49.7 ± 6.5  6.7 ± 1.2  H78α  13.60  17.9 ± 0.9  21.9 ± 1.3  38.2 ± 2.8  7.3 ± 0.7      11.0 ± 0.8  −32.3 ± 2.4  48.6 ± 5.7  5.7 ± 0.8  H77α  14.12  22.9 ± 3.5  21.8 ± 0.8  27.3 ± 2.6  6.6 ± 1.3      9.8 ± 1.1  −17.2 ± 9.1  80 ± 17  8.4 ± 2.2  H76α  14.69  19.0 ± 1.1  22.9 ± 1.2  29.9 ± 2.7  6.1 ± 0.7      13.1 ± 1.0  −28.7 ± 1.9  51.3 ± 4.8  7.2 ± 0.9  H75α  15.28  20.0 ± 1.0  22.5 ± 0.9  30.0 ± 2.0  6.4 ± 0.6      10.4 ± 0.8  −32.5 ± 2.0  51.6 ± 5.3  5.7 ± 0.8  H99β  13.15  5.9 ± 1.1  17.3 ± 3.5  25.8 ± 7.1  1.6 ± 0.6      4.7 ± 0.5  −30.0 ± 6.0  50 ± 13  2.5 ± 0.8  H98β  13.56  9.0 ± 0.9  20.2 ± 1.7  25.1 ± 4.2  2.4 ± 0.5      3.3 ± 0.6  −30.0 ± 5.2  50 ± 14  1.8 ± 0.6  H97β  13.98  8.1 ± 1.0  17.4 ± 1.6  37.0 ± 3.8  3.2 ± 0.6      3.0 ± 0.5  −30.0 ± 8.7  50 ± 19  1.6 ± 0.7  H96β  14.41  7.6 ± 0.9  16.9 ± 2.4  29.0 ± 8.3  2.3 ± 0.8      3.7 ± 0.6  −30.0 ± 7.3  50 ± 16  1.9 ± 0.7  H94β  15.34  5.7 ± 0.6  22.4 ± 2.6  35.8 ± 6.1  2.1 ± 0.5      4.0 ± 1.0  −30.0 ± 3.7  50.0 ± 9.2  2.2 ± 0.8  View Large Table 3. Hydrogen RRL parameters derived for LOS−0.02 using Gaussian fits with two velocity components. RRL  ν  T$$^*_\mathrm{A}$$  $$v$$r  Δ$$v$$r  $$\int T_\mathrm{A}^* \text{d}v_\mathrm{r}$$    (GHz)  (mK)  (km s−1)  (km s−1)  (× 102 mK km s−1)  H79α  13.09  92.8 ± 3.5  46.1 ± 0.5  27.6 ± 1.1  27.2 ± 1.7      45.6 ± 2.3  −39.2 ± 1.4  60.6 ± 3.4  29.4 ± 2.3  H78α  13.60  83.0 ± 2.9  46.6 ± 0.5  29.7 ± 1.1  26.3 ± 1.4      42.9 ± 2.0  −41.0 ± 1.3  61.9 ± 3.1  28.2 ± 2.0  H77α  14.12  72.8 ± 1.7  45.6 ± 0.4  31.8 ± 0.9  24.6 ± 1.0      28.1 ± 1.2  −39.8 ± 1.3  61.8 ± 3.3  18.5 ± 1.3  H76α  14.69  73.6 ± 2.1  46.5 ± 0.4  29.5 ± 1.0  23.1 ± 1.1      30.8 ± 1.5  −41.6 ± 1.4  63.3 ± 3.2  20.8 ± 1.5  H75α  15.28  57.4 ± 2.1  48.1 ± 0.6  32.1 ± 1.3  19.6 ± 1.2      34.6 ± 1.6  −39.8 ± 1.2  57.0 ± 2.9  21.0 ± 1.5  H99β  13.15  23.1 ± 2.2  48.1 ± 1.2  24.4 ± 2.8  6.0 ± 0.9      9.6 ± 1.5  −40.0 ± 4.1  60.0 ± 9.6  6.2 ± 1.4  H98β  13.56  24.4 ± 1.1  45.6 ± 0.6  31.4 ± 1.5  8.2 ± 0.6      11.1 ± 0.8  −40.3 ± 1.9  59.6 ± 4.4  7.1 ± 0.8  H97β  13.98  20.2 ± 0.9  46.0 ± 0.6  28.9 ± 1.4  6.2 ± 0.4      7.9 ± 0.7  −37.9 ± 2.0  53.6 ± 4.7  4.5 ± 0.6  H96β  14.41  20.3 ± 1.0  48.3 ± 0.7  30.5 ± 1.6  6.6 ± 0.5      8.9 ± 0.9  −31.0 ± 2.8  56.2 ± 7.1  5.3 ± 0.6  H94β  15.34  14.6 ± 1.3  51.4 ± 1.4  35.4 ± 3.4  5.5 ± 0.8      8.3 ± 1.0  −40.2 ± 2.8  60.0 ± 6.6  5.3 ± 0.9  RRL  ν  T$$^*_\mathrm{A}$$  $$v$$r  Δ$$v$$r  $$\int T_\mathrm{A}^* \text{d}v_\mathrm{r}$$    (GHz)  (mK)  (km s−1)  (km s−1)  (× 102 mK km s−1)  H79α  13.09  92.8 ± 3.5  46.1 ± 0.5  27.6 ± 1.1  27.2 ± 1.7      45.6 ± 2.3  −39.2 ± 1.4  60.6 ± 3.4  29.4 ± 2.3  H78α  13.60  83.0 ± 2.9  46.6 ± 0.5  29.7 ± 1.1  26.3 ± 1.4      42.9 ± 2.0  −41.0 ± 1.3  61.9 ± 3.1  28.2 ± 2.0  H77α  14.12  72.8 ± 1.7  45.6 ± 0.4  31.8 ± 0.9  24.6 ± 1.0      28.1 ± 1.2  −39.8 ± 1.3  61.8 ± 3.3  18.5 ± 1.3  H76α  14.69  73.6 ± 2.1  46.5 ± 0.4  29.5 ± 1.0  23.1 ± 1.1      30.8 ± 1.5  −41.6 ± 1.4  63.3 ± 3.2  20.8 ± 1.5  H75α  15.28  57.4 ± 2.1  48.1 ± 0.6  32.1 ± 1.3  19.6 ± 1.2      34.6 ± 1.6  −39.8 ± 1.2  57.0 ± 2.9  21.0 ± 1.5  H99β  13.15  23.1 ± 2.2  48.1 ± 1.2  24.4 ± 2.8  6.0 ± 0.9      9.6 ± 1.5  −40.0 ± 4.1  60.0 ± 9.6  6.2 ± 1.4  H98β  13.56  24.4 ± 1.1  45.6 ± 0.6  31.4 ± 1.5  8.2 ± 0.6      11.1 ± 0.8  −40.3 ± 1.9  59.6 ± 4.4  7.1 ± 0.8  H97β  13.98  20.2 ± 0.9  46.0 ± 0.6  28.9 ± 1.4  6.2 ± 0.4      7.9 ± 0.7  −37.9 ± 2.0  53.6 ± 4.7  4.5 ± 0.6  H96β  14.41  20.3 ± 1.0  48.3 ± 0.7  30.5 ± 1.6  6.6 ± 0.5      8.9 ± 0.9  −31.0 ± 2.8  56.2 ± 7.1  5.3 ± 0.6  H94β  15.34  14.6 ± 1.3  51.4 ± 1.4  35.4 ± 3.4  5.5 ± 0.8      8.3 ± 1.0  −40.2 ± 2.8  60.0 ± 6.6  5.3 ± 0.9  View Large Table 4. Hydrogen RRL parameters derived or LOS+0.693 using Gaussian fits. RRL  ν  T$$^*_\mathrm{A}$$  $$v$$r  Δ$$v$$r  $$\int T_\mathrm{A}^* \text{d}v_\mathrm{r}$$    (GHz)  (mK)  (km s−1)  (km s−1)  (× 103 mK km s−1)  H79α  13.09  441.6 ± 3.2  71.4 ± 0.1  31.0 ± 0.3  14.6 ± 0.2  H78α  13.60  401.4 ± 3.1  71.8 ± 0.1  32.0 ± 0.3  13.7 ± 0.2  H77α  14.12  382.0 ± 4.8  71.8 ± 0.2  31.0 ± 0.4  12.6 ± 0.2  H76α  14.69  383.3 ± 6.5  71.7 ± 0.2  30.7 ± 0.6  12.5 ± 0.3  H75α  15.28  350.4 ± 2.5  72.3 ± 0.1  30.0 ± 0.3  11.2 ± 0.1  H99β  13.15  93.2 ± 3.1  70.2 ± 0.5  37.0 ± 1.3  3.7 ± 0.2  H98β  13.56  91.1 ± 1.0  71.8 ± 0.3  33.0 ± 0.4  3.2 ± 0.1  H97β  13.98  88.4 ± 3.7  71.3 ± 0.4  32.2 ± 1.0  3.0 ± 0.1  H96β  14.41  84.5 ± 1.8  73.5 ± 0.4  37.4 ± 0.8  3.4 ± 0.1  H95β  14.87  91.9 ± 2.8  72.0 ± 0.4  30.5 ± 1.0  3.0 ± 0.1  H94β  15.34  78.5 ± 1.0  72.6 ± 0.2  32.0 ± 0.5  2.7 ± 0.1  RRL  ν  T$$^*_\mathrm{A}$$  $$v$$r  Δ$$v$$r  $$\int T_\mathrm{A}^* \text{d}v_\mathrm{r}$$    (GHz)  (mK)  (km s−1)  (km s−1)  (× 103 mK km s−1)  H79α  13.09  441.6 ± 3.2  71.4 ± 0.1  31.0 ± 0.3  14.6 ± 0.2  H78α  13.60  401.4 ± 3.1  71.8 ± 0.1  32.0 ± 0.3  13.7 ± 0.2  H77α  14.12  382.0 ± 4.8  71.8 ± 0.2  31.0 ± 0.4  12.6 ± 0.2  H76α  14.69  383.3 ± 6.5  71.7 ± 0.2  30.7 ± 0.6  12.5 ± 0.3  H75α  15.28  350.4 ± 2.5  72.3 ± 0.1  30.0 ± 0.3  11.2 ± 0.1  H99β  13.15  93.2 ± 3.1  70.2 ± 0.5  37.0 ± 1.3  3.7 ± 0.2  H98β  13.56  91.1 ± 1.0  71.8 ± 0.3  33.0 ± 0.4  3.2 ± 0.1  H97β  13.98  88.4 ± 3.7  71.3 ± 0.4  32.2 ± 1.0  3.0 ± 0.1  H96β  14.41  84.5 ± 1.8  73.5 ± 0.4  37.4 ± 0.8  3.4 ± 0.1  H95β  14.87  91.9 ± 2.8  72.0 ± 0.4  30.5 ± 1.0  3.0 ± 0.1  H94β  15.34  78.5 ± 1.0  72.6 ± 0.2  32.0 ± 0.5  2.7 ± 0.1  View Large Table 5. Helium RRL parameters derived for LOS+0.693 using Gaussian fits. RRL  ν  T$$^*_\mathrm{A}$$  $$v$$r  Δ$$v$$r  $$\int T_\mathrm{A}^* dv_\mathrm{r}$$    (GHz)  (mK)  (km s−1)  (km s−1)  (× 102 mK km s−1)  He79α  13.09  31.8 ± 2.0  71.5 ± 0.7  26.6 ± 1.6  9.0 ± 0.8  He78α  13.60  35.8 ± 2.9  73.5 ± 0.9  26.0 ± 2.0  9.9 ± 1.2  He77α  14.13  28.2 ± 3.9  73.0 ± 1.6  26.8 ± 3.7  8.0 ± 1.7  He76α  14.70  23.4 ± 1.8  72.8 ± 0.8  25.2 ± 1.9  6.3 ± 0.7  He75α  15.29  24.3 ± 5.4  72.3 ± 2.3  23.3 ± 5.4  6.0 ± 2.0  He99β  13.15  7.1 ± 3.5  65.7 ± 5.6  30 ± 13  2.3 ± 1.6  He98β  13.56  9.5 ± 2.1  70.8 ± 1.9  22.2 ± 4.6  2.2 ± 0.7  He97β  13.98  8.0 ± 2.7  66.6 ± 4.5  27 ± 11  2.3 ± 1.3  He96β  14.42  7.7 ± 1.2  70.2 ± 1.3  21.7 ± 3.1  1.8 ± 0.4  He95β  14.87  7.8 ± 1.3  71.8 ± 1.1  17.4 ± 2.5  1.4 ± 0.3  He94β  15.35  <6.5a  –  –  <1.7b  RRL  ν  T$$^*_\mathrm{A}$$  $$v$$r  Δ$$v$$r  $$\int T_\mathrm{A}^* dv_\mathrm{r}$$    (GHz)  (mK)  (km s−1)  (km s−1)  (× 102 mK km s−1)  He79α  13.09  31.8 ± 2.0  71.5 ± 0.7  26.6 ± 1.6  9.0 ± 0.8  He78α  13.60  35.8 ± 2.9  73.5 ± 0.9  26.0 ± 2.0  9.9 ± 1.2  He77α  14.13  28.2 ± 3.9  73.0 ± 1.6  26.8 ± 3.7  8.0 ± 1.7  He76α  14.70  23.4 ± 1.8  72.8 ± 0.8  25.2 ± 1.9  6.3 ± 0.7  He75α  15.29  24.3 ± 5.4  72.3 ± 2.3  23.3 ± 5.4  6.0 ± 2.0  He99β  13.15  7.1 ± 3.5  65.7 ± 5.6  30 ± 13  2.3 ± 1.6  He98β  13.56  9.5 ± 2.1  70.8 ± 1.9  22.2 ± 4.6  2.2 ± 0.7  He97β  13.98  8.0 ± 2.7  66.6 ± 4.5  27 ± 11  2.3 ± 1.3  He96β  14.42  7.7 ± 1.2  70.2 ± 1.3  21.7 ± 3.1  1.8 ± 0.4  He95β  14.87  7.8 ± 1.3  71.8 ± 1.1  17.4 ± 2.5  1.4 ± 0.3  He94β  15.35  <6.5a  –  –  <1.7b  a3σ upper limit on the line intensity. b3σ upper limit on the velocity-integrated line intensity. View Large Table 6. 3σ upper limits on the He line intensities for LOS−0.11 and LOS−0.02.     LOS−0.11  LOS−0.02  RRL  ν  T$$^*_\mathrm{A}$$    (GHz)  (mK)  He79α  13.09  <5  <11  He78α  13.60  <3  <9  He77α  14.13  <11  <5  He76α  14.70  <3  <6  He75α  15.29  <3  <6  He99β  13.15  <3  <8  He98β  13.56  <3  <3  He97β  13.98  <3  <3  He96β  14.42  <3  <3  He94β  15.35  <3  <4      LOS−0.11  LOS−0.02  RRL  ν  T$$^*_\mathrm{A}$$    (GHz)  (mK)  He79α  13.09  <5  <11  He78α  13.60  <3  <9  He77α  14.13  <11  <5  He76α  14.70  <3  <6  He75α  15.29  <3  <6  He99β  13.15  <3  <8  He98β  13.56  <3  <3  He97β  13.98  <3  <3  He96β  14.42  <3  <3  He94β  15.35  <3  <4  View Large Table 7. Hmβ/Hnα ratios for the three GC LOSs.     LOS−0.11  LOS−0.02  LOS+0.693  Ratio  Modela  $$v_\mathrm{r}^{\mathrm{{\it b}}}$$  $$\frac{I_{\mathrm{H}m\beta }}{I_{\mathrm{H}n\alpha }}$$  $$v_\mathrm{r}^\mathrm{(b)}$$  $$\frac{I_{\mathrm{H}m\beta }}{I_{\mathrm{H}n\alpha }}{}^\mathrm{{\it c}}$$  $$v_\mathrm{r}^{\mathrm{{\it b}}}$$  $$\frac{I_{\mathrm{H}m\beta }}{I_{\mathrm{H}n\alpha }}{}^\mathrm{{\it c}}$$    (per cent)  (km s−1)  (per cent)  (km s−1)  (per cent)  (km s−1)  (per cent)  H99β/H79α  27.3  +20  25.8 ± 9.8  +50  22.1 ± 3.7  +70  25.2 ± 1.3      −30  37 ± 13  −40  20.9 ± 5.2      H98β/H78α  27.0  +20  33.0 ± 7.6  +50  31.1 ± 2.7  +70  23.4 ± 0.5      −30  31 ± 11  −40  25.0 ± 3.2      H96β/H77α  27.7  +20  35 ± 13  +50  26.8 ± 2.3  +70  26.7 ± 1.0      −30  23 ± 11  −40  28.8 ± 3.9      H96β/H76α  26.6  +20  39 ± 13  +50  28.6 ± 2.5  +70  26.8 ± 1.1      −30  27 ± 11  −40  25.7 ± 3.5      H94β/H75α  26.6  +20  33.3 ± 7.7  +50  28.2 ± 4.2  +70  23.9 ± 0.6      −30  38 ± 14  −40  25.2 ± 4.6          LOS−0.11  LOS−0.02  LOS+0.693  Ratio  Modela  $$v_\mathrm{r}^{\mathrm{{\it b}}}$$  $$\frac{I_{\mathrm{H}m\beta }}{I_{\mathrm{H}n\alpha }}$$  $$v_\mathrm{r}^\mathrm{(b)}$$  $$\frac{I_{\mathrm{H}m\beta }}{I_{\mathrm{H}n\alpha }}{}^\mathrm{{\it c}}$$  $$v_\mathrm{r}^{\mathrm{{\it b}}}$$  $$\frac{I_{\mathrm{H}m\beta }}{I_{\mathrm{H}n\alpha }}{}^\mathrm{{\it c}}$$    (per cent)  (km s−1)  (per cent)  (km s−1)  (per cent)  (km s−1)  (per cent)  H99β/H79α  27.3  +20  25.8 ± 9.8  +50  22.1 ± 3.7  +70  25.2 ± 1.3      −30  37 ± 13  −40  20.9 ± 5.2      H98β/H78α  27.0  +20  33.0 ± 7.6  +50  31.1 ± 2.7  +70  23.4 ± 0.5      −30  31 ± 11  −40  25.0 ± 3.2      H96β/H77α  27.7  +20  35 ± 13  +50  26.8 ± 2.3  +70  26.7 ± 1.0      −30  23 ± 11  −40  28.8 ± 3.9      H96β/H76α  26.6  +20  39 ± 13  +50  28.6 ± 2.5  +70  26.8 ± 1.1      −30  27 ± 11  −40  25.7 ± 3.5      H94β/H75α  26.6  +20  33.3 ± 7.7  +50  28.2 ± 4.2  +70  23.9 ± 0.6      −30  38 ± 14  −40  25.2 ± 4.6      aEstimated values assuming LTE conditions and optically thin radio continuum emission (see the text). bThe velocity components identified in LOS−0.11 and LOS−0.02 are labelled as +20 and −30 km s−1, and +50, and −40 km s−1, respectively. Only one velocity component labelled as +70 km s−1 is identified in LOS+0.693. cValues that do not match the expected LTE ratio are in bold print. View Large Table 8. Hemβ/Henα ratios for LOS+0.693. Ratio  Modela  $$\frac{I_{\mathrm{He}m\beta }}{I_{\mathrm{He}n\alpha }}$$    ( per cent)  ( per cent)  He99β/He79α  27.3  25 ± 18  He98β/He78α  27.0  22.7 ± 7.8  He96β/He77α  27.7  22.1 ± 6.7  He96β/He76α  26.6  28.3 ± 7.0  He94β/He75α  26.6  <25.4  Ratio  Modela  $$\frac{I_{\mathrm{He}m\beta }}{I_{\mathrm{He}n\alpha }}$$    ( per cent)  ( per cent)  He99β/He79α  27.3  25 ± 18  He98β/He78α  27.0  22.7 ± 7.8  He96β/He77α  27.7  22.1 ± 6.7  He96β/He76α  26.6  28.3 ± 7.0  He94β/He75α  26.6  <25.4  aEstimated values assuming LTE conditions and optically thin radio continuum emission (see the text). View Large Table 9. Helium-to-hydrogen line intensity ratios for the three GC sources.   LOS−0.11  LOS−0.02  LOS+0.693  Ratio  v$$_\mathrm{r}^{a}$$  $$\frac{I_{\mathrm{He}n\alpha }}{I_{\mathrm{H}n\alpha }}$$  v$$_\mathrm{r}^{a}$$  $$\frac{I_{\mathrm{He}n\alpha }}{I_{\mathrm{H}n\alpha }}$$  v$$_\mathrm{r}^{a}$$  $$\frac{I_{\mathrm{He}n\alpha }}{I_{\mathrm{H}n\alpha }}$$    (km s−1)  ( per cent)  (km s−1)  ( per cent)  (km s−1)  ( per cent)  He79α/H79α  +20  <20  +50  <10  +70  7.2 ± 0.5    −30  <40  −40  <20      He78α/H78α  +20  <10  +50  <10  +70  9.0 ± 0.7    −30  <20  −40  <20      He77α/H77α  +20  <50  +50  <10  +70  7.4 ± 1.0    −30  <100  −40  <20      He76α/H76α  +20  <20  +50  <10  +70  6.1 ± 0.5    −30  <30  −40  <20      He75α/H75α  +20  <20  +50  <10  +70  6.9 ± 1.5    −30  <30  −40  <20      He99β/H99β  +20  <50  +50  <40  +70  7.6 ± 3.8    −30  <70  −40  <90      He98β/H98β  +20  <30  +50  <10  +70  10.5 ± 2.3    −30  <80  −40  <30      He97β/H97β  +20  <40  +50  <10  +70  9.0 ± 3.1    −30  <100  −40  <30      He96β/H96β  +20  <40  +50  <10  +70  9.1 ± 1.4    −30  <80  −40  <30      He95β/H95βb  +20  –  +50  –  +70  8.5 ± 1.4    −30  –  −40  –      He94β/H94β  +20  <60  +50  <30  +70  <8.3    −30  <90  −40  <50        LOS−0.11  LOS−0.02  LOS+0.693  Ratio  v$$_\mathrm{r}^{a}$$  $$\frac{I_{\mathrm{He}n\alpha }}{I_{\mathrm{H}n\alpha }}$$  v$$_\mathrm{r}^{a}$$  $$\frac{I_{\mathrm{He}n\alpha }}{I_{\mathrm{H}n\alpha }}$$  v$$_\mathrm{r}^{a}$$  $$\frac{I_{\mathrm{He}n\alpha }}{I_{\mathrm{H}n\alpha }}$$    (km s−1)  ( per cent)  (km s−1)  ( per cent)  (km s−1)  ( per cent)  He79α/H79α  +20  <20  +50  <10  +70  7.2 ± 0.5    −30  <40  −40  <20      He78α/H78α  +20  <10  +50  <10  +70  9.0 ± 0.7    −30  <20  −40  <20      He77α/H77α  +20  <50  +50  <10  +70  7.4 ± 1.0    −30  <100  −40  <20      He76α/H76α  +20  <20  +50  <10  +70  6.1 ± 0.5    −30  <30  −40  <20      He75α/H75α  +20  <20  +50  <10  +70  6.9 ± 1.5    −30  <30  −40  <20      He99β/H99β  +20  <50  +50  <40  +70  7.6 ± 3.8    −30  <70  −40  <90      He98β/H98β  +20  <30  +50  <10  +70  10.5 ± 2.3    −30  <80  −40  <30      He97β/H97β  +20  <40  +50  <10  +70  9.0 ± 3.1    −30  <100  −40  <30      He96β/H96β  +20  <40  +50  <10  +70  9.1 ± 1.4    −30  <80  −40  <30      He95β/H95βb  +20  –  +50  –  +70  8.5 ± 1.4    −30  –  −40  –      He94β/H94β  +20  <60  +50  <30  +70  <8.3    −30  <90  −40  <50      aThe velocity components identified in LOS−0.11 and LOS−0.02 are labelled as +20 and −30 km s−1, and +50 and −40 km s−1, respectively. Only one velocity component labelled as +70 km s−1 is identified in LOS+0.693. bThe He95β/H95β line intensity ratios for both sources of Sgr A were not measured because the He95β RRL was not observed. View Large Table 10. Physical properties derived for the three GC sources. Source  RRL  v$$_\mathrm{r}^{a}$$  Sc  ne  log (NLyc)      (km s−1)  (mJy)  (cm−3)  (ph. s−1)  LOS−0.11  H77α  +20  122 ± 23  71 ± 7  47.14 ± 0.08      −30  153 ± 40  80 ± 10  47.24 ± 0.10    H96β  +20  42 ± 14  43 ± 7  46.70 ± 0.12      −30  35 ± 13  39 ± 7  46.62 ± 0.14  LOS−0.02  H77α  +50  451 ± 18  137 ± 3  47.71 ± 0.02      −40  339 ± 25  119 ± 4  47.58 ± 0.03    H96β  +50  118 ± 9  72 ± 3  47.15 ± 0.03      −40  96 ± 11  65 ± 4  47.06 ± 0.05  LOS+0.693  H77α  +70  2311 ± 45  310 ± 3  48.42 ± 0.01    H96β  +70  603 ± 19  163 ± 3  47.86 ± 0.01  Source  RRL  v$$_\mathrm{r}^{a}$$  Sc  ne  log (NLyc)      (km s−1)  (mJy)  (cm−3)  (ph. s−1)  LOS−0.11  H77α  +20  122 ± 23  71 ± 7  47.14 ± 0.08      −30  153 ± 40  80 ± 10  47.24 ± 0.10    H96β  +20  42 ± 14  43 ± 7  46.70 ± 0.12      −30  35 ± 13  39 ± 7  46.62 ± 0.14  LOS−0.02  H77α  +50  451 ± 18  137 ± 3  47.71 ± 0.02      −40  339 ± 25  119 ± 4  47.58 ± 0.03    H96β  +50  118 ± 9  72 ± 3  47.15 ± 0.03      −40  96 ± 11  65 ± 4  47.06 ± 0.05  LOS+0.693  H77α  +70  2311 ± 45  310 ± 3  48.42 ± 0.01    H96β  +70  603 ± 19  163 ± 3  47.86 ± 0.01  aThe velocity components identified in LOS−0.11 and LOS−0.02 are labelled as +20 and −30 km s−1, and +50 and −40 km s−1, respectively. Only one velocity component labelled as +70 km s−1 is identified in LOS+0.693. View Large 3.2 LTE conditions In order to check whether LTE conditions apply in the three GC sources, we have derived the Hmβ to Hnα integrated line intensity ratios (hereafter Hmβ to Hnα ratios), using Hnα and Hmβ lines that were observed simultaneously to avoid uncertainties related to pointing and flux calibration. We show the value of these ratios in Table 7 for the different velocity components and the three GC sources. In this table, we also list the Hmβ to Hnα ratios estimated assuming LTE conditions and optically thin radio continuum emission. We also show the Hemβ to Henα integrated line intensity ratios for LOS+0.693 in Table 8, where the expected LTE values are also listed. As seen in Table 7, the three GC sources have Hmβ to Hnα ratios that are consistent, within their uncertainties, with those predicted in LTE, but there are values (in bold print) in this table that do not match the expected LTE ratio. In LOS−0.02, the measured H99β to H79α and H98β to H78α (positive velocity component) ratios are inconsistent with the LTE values likely due to uncertainty in the baseline correction of the H99β and H98β lines. The same reason may explain why the H99β, H98β, and H94β lines, in LOS+0.693, show intensities lower than those expected in LTE. On the other hand, the measured Hemβ to Henα ratios (see Table 8) are consistent within their uncertainties with the values expected in LTE. In summary, the Hmβ to Hnα ratios derived for the three GC sources and the Hemβ to Henα ratios derived for LOS+0.693 show that the ionized gas in the studied sources can be reasonably assumed to be emitted under LTE conditions. 3.3 Helium to hydrogen ratio As mentioned above, helium RRLs have only been detected towards LOS+0.693. We have derived the He-to-H line intensity ratios (see Table 9) for those RRL transitions where the same principal quantum number has been detected for the two elements. Otherwise, we provide upper limits assuming that the line intensity of the non-detected RRLs is lower than 3σ. We note that the Henα RRLs are located at ≈122 km s−1 with respect to Hnα RRLs, as expected by the difference of their rest frequencies (Towle et al. 1996). Thus, the derived ratios are not affected by possible effects of calibration since both spectral lines are observed simultaneously at close frequencies. We find an average He-to-H intensity ratio of 7.3±0.2 per cent or 4He mass fraction Y = 0.29±0.01 for LOS+0.693. This ratio is consistent with those found in interferometry studies (Roelfsema et al. 1987; Mehringer et al. 1993) that trace more compact regions (≲0.7 pc) than our diffuse regions (∼1.7 pc). The most stringent upper limits on the He−to−H ratio derived for LOS−0.02 and LOS−0.11 are consistent with the He-to-H number ratio of <10 per cent found in GC H ii regions (Roelfsema et al. 1987). The estimation of 0.29±0.01 helium abundance by mass differs by 14 per cent from that of 0.25 as predicted by big bang nucleosynthesis (Coc et al. 2012; Tsivilev et al. 2013). This finding suggests, as expected (Wilson & Rood 1994; Gordon & Sorochenko 2009), that high-mass stars in the GC have enriched the ISM with helium-4, in a past intense burst of star formation in this region, thus increasing its abundance compared to the primordial value. 3.4 Electron densities and number of Lyman ionizing photons In this section, we derive the average electron density ne of the ionized gas following the equation as in Mezger & Henderson (1967), where ne is given by   \begin{eqnarray} \begin{array}{rl}\left(\frac{n_\mathrm{e}}{\mathrm{cm^{-3}}}\right) =&6.351 \cdot 10^2 u_1 a^{0.5} \left(\frac{T_\mathrm{e}}{10^4 \mathrm{\ K}}\right)^{0.175} \left(\frac{\nu }{\mathrm{GHz}}\right)^{0.05} \\ &\left(\frac{S_\mathrm{c}}{\mathrm{Jy}}\right)^{0.5} \left(\frac{D}{\mathrm{kpc}}\right)^{-0.5} \left(\frac{\Theta }{\mathrm{arcmin}}\right)^{-1.5} \mathrm{,} \end{array} \end{eqnarray} (1)where Te is the electron temperature, ν is the frequency, Sc is the continuum flux, D is the distance to the GC [7.86 kpc, Boehle et al. (2016)], and Θ is the source size (which is assumed to be equal to the telescope beam size corresponding to ≈1.7 pc at the GC distance). The parameter a accounts for the deviation between the exact equation for the optical depth for free–free emission and its approximation (Mezger & Henderson 1967). For our study, we have used an average value of a equal to 0.98. We have assumed that our three GC sources have spherical geometry, and in this case the model conversion factor u1 is equal to 0.775 (Mezger & Henderson 1967). The Sc is derived from the H77α and H96β RRL emission assuming an optically thin regime and the average Te found for compact H ii regions of the GC, i.e. Te ≈ 6300 K (Goss et al. 1985). Sc values derived from the H77α and H96β RRLs are similar, within their uncertainties, to those obtained from the other detected Hnα and Hmβ lines, respectively. For this reason Table 10 lists only the Sc values derived from the H77α and H96β lines. For the three GC sources the estimated values of ne are given in Table 10. Using the formula given in Rohlfs & Wilson (1999) (see equation 13.2) we have also calculated the number of Lyman ionizing photons, NLyc, as follows:   \begin{eqnarray} N_\mathrm{Lyc}=\frac{4}{3}\pi \left(\frac{\Theta }{2}\right)^3 n_\mathrm{e} n_\mathrm{p} \alpha ^{(2)} \mathrm{,} \end{eqnarray} (2)where np is the proton density (which is equal to ne under LTE conditions, see Section 3.2), and α(2) is the recombination coefficient (Spitzer 2004). The derived values of NLyc for the three GC sources are shown in Table 10. 4 DISCUSSION 4.1 Extended and diffuse RRL emission towards the three GC LOS As previously mentioned, the only compact H ii region which falls inside the GBT beam of our observations is towards LOS−0.02. This suggests that our GBT observations trace extended RRL emission towards LOS−0.11 and LOS+0.693. In the case of LOS+0.693, this idea is also supported by the extended H69α emission map of Sgr B2 shown in Fig. 6. This figure is obtained using the HOPS data (Purcell et al. 2012). The HOPS data has a spatial resolution of 2.4 arcmin at the frequency (19.59 GHz) of the H69α line, which is a factor ∼3 worse than the average spatial resolution of our observations. Unfortunately, the HOPS data has a rms noise of ∼40 mK, which is not enough to obtain H69α line emission maps for regions where both Sgr A sources were observed. Figure 6. View largeDownload slide H69α integrated line emission of Sgr B2 obtained using HOPS data (Purcell et al. 2012). The range of velocity integration is from 20 to 80 km s−1. LOS+0.693 is indicated with a red circle with the size of the Half-Power Beam Width of the GBT observations (48 arcsec at 13.09 GHz). Figure 6. View largeDownload slide H69α integrated line emission of Sgr B2 obtained using HOPS data (Purcell et al. 2012). The range of velocity integration is from 20 to 80 km s−1. LOS+0.693 is indicated with a red circle with the size of the Half-Power Beam Width of the GBT observations (48 arcsec at 13.09 GHz). In order to figure out whether the emission detected by the GBT towards LOS−0.02 is arising exclusively from the compact A region or not, we have compared the RRL emission measured using the VLA with that of our GBT observations. For this, we have first determined the spectral index α of the region A. This region shows a α of 0.06±0.04 derived considering the Sc of 590±30 mJy that we have measured at 24.5 GHz (using the VLA map shown in Fig. 1) and the value of 570±20 mJy derived at 14.7 GHz by Goss et al. (1985). Thus, T$$^*_\mathrm{A}$$ ∝ ν1.16 assuming that the free−free and RRL emission is optically thin. We have measured the H64α peak line intensity of 67±22 mJy by integrating the channel map at the peak intensity of 47 km s−1 (obtained using the data cube described in Section 2.1) over the Half-Power Beam Width of the GBT. By using the previous relation, we have extrapolated the H64α peak line intensity to that expected for the H76α line, finding a T$$^*_\mathrm{A}$$ of 39±12 mJy at 14.7 GHz, which is similar to that of 40.2±1.2 mJy as measured by the GBT. This suggests that part of the region A inside the GBT beam may contribute significantly to the RRL emission detected in LOS−0.02. The T$$^*_\mathrm{A}$$ of 39±12 mJy found for the H76α line is a factor ∼2 lower than that of 114 mJy as measured by Goss et al. (1985) for the entire region A, which agrees with the fact that only half of the region A falls inside the GBT beam size towards LOS−0.02. Despite this finding, we believe that it is unlikely that the GBT data traces extended RRL emission towards LOS+0.693 and LOS−0.11 and that it does not trace extended RRL emission towards LOS−0.02. In fact, this is supported in Fig. 7 (upper panel) where we show the C i, C iiI and H79α spectra of LOS−0.11 and LOS−0.02. It can be seen that both the positive and negative velocity components of the extended ionized gas traced by the C ii emission (García 2015) are also well traced by the H79α line emission. Therefore, in this paper, we consider that the GBT traces extended ionized gas in the three LOSs. Figure 7. View largeDownload slide Upper panels: C i (black), C ii (red), and H79α (green) spectra observed towards LOS−0.11 and LOS−0.02. Bottom panels: C i (black) and C ii (red) spectra observed towards the H1 source and Sgr A*. The line intensity of several spectra is multiplied by a factor for comparison purposes. C ii spectra is affected by absorption features (indicated by dashed lines) associated with the 3 kpc, 4.5 kpc, and local spiral arms (Oka et al. 1998). The C i and C ii spectra are obtained with a spatial resolution of 46 arcsec similar to that of the H79α spectra. Figure 7. View largeDownload slide Upper panels: C i (black), C ii (red), and H79α (green) spectra observed towards LOS−0.11 and LOS−0.02. Bottom panels: C i (black) and C ii (red) spectra observed towards the H1 source and Sgr A*. The line intensity of several spectra is multiplied by a factor for comparison purposes. C ii spectra is affected by absorption features (indicated by dashed lines) associated with the 3 kpc, 4.5 kpc, and local spiral arms (Oka et al. 1998). The C i and C ii spectra are obtained with a spatial resolution of 46 arcsec similar to that of the H79α spectra. The studied ionized gas is also diffuse because we have found ne of ∼40–310 cm−3, which are much lower than those of 3600–5100 cm−3 found in compact H ii regions of the GC (Mills et al. 2011). The ne of ∼40–120 cm−3 found for the negative velocity gas of LOS−0.11 and LOS−0.02 are consistent with those of ∼100–130 cm−3 found for diffuse ionized gas of the Arched Filaments H ii complex (Langer et al. 2017). C ii emission traces a negative velocity component not only in both Sgr A sources but also in the H1 source and Sgr A* (see the bottom panel of Fig. 7). We also note in Fig. 7 that the C i emission does not trace the negative velocity component in LOS−0.11 and LOS−0.02 but it does partially in the H1 source and Sgr A*. 4.2 Kinematics of the ionized gas Our RRLs show diffuse ionized gas with negative and positive velocities in LOS−0.11 and LOS−0.02. In Fig. 8, we show the velocities of both Sgr A sources on a position-velocity diagram for the C ii emission. Four gas streams from the model of Kruijssen et al. (2015) are also shown in this figure. The $$v$$r of both velocity components of LOS−0.11 are consistent with the velocities of streams 1 and 4, while the $$v$$r of both velocity components of LOS−0.02 agree with the velocities of streams 1 and 2. This suggests that along the two LOSs the GBT traces diffuse and extended ionized gas that are part of the gas streams orbiting the GC. The positions of LOS−0.11 and LOS−0.02 (see Fig. 8, upper panel) also support that the positive velocity gas in both sources is part of stream 1. The velocities and positions of the regions H1–H5 (see Fig. 8) show that these sources are likely associated with stream 2. If this hypothesis is correct, then the negative velocity gas of LOS−0.02 could coexist with the H1–H5 sources. Our hypothesis is in agreement with the finding of Langer et al. (2017) that the kinematics of the ionized gas in the Sgr A and Sgr B2 complexes, as traced by the C ii emission, is well explained by the gas streams proposed by Kruijssen et al. (2015). Figure 8. View largeDownload slide Upper panel: four streams used to model the GC gas kinematics (Kruijssen et al. 2015). The NH3(1,1) emission map obtained using HOPS data (Purcell et al. 2012) is shown in grey-scale. The map is integrated over the velocity range −100–+100 km s−1. Filled triangles indicate positions of LOS−0.11 and LOS−0.02, while filled squares show the positions of H1–H5 sources. The black cross shows the position of Sgr A*. Bottom panel: The four streams are drawn on the position-velocity C ii map obtained using HIFI data (García et al. 2016). This map covers 0.1° in latitude centred at 0°. Central line velocities of LOS−0.11 and LOS−0.02 are shown with filled triangles, while the central line velocities of H1–H5 sources (Zhao et al. 1993) are indicated with filled squares. Velocity error bars overlap with filled triangles. Figure 8. View largeDownload slide Upper panel: four streams used to model the GC gas kinematics (Kruijssen et al. 2015). The NH3(1,1) emission map obtained using HOPS data (Purcell et al. 2012) is shown in grey-scale. The map is integrated over the velocity range −100–+100 km s−1. Filled triangles indicate positions of LOS−0.11 and LOS−0.02, while filled squares show the positions of H1–H5 sources. The black cross shows the position of Sgr A*. Bottom panel: The four streams are drawn on the position-velocity C ii map obtained using HIFI data (García et al. 2016). This map covers 0.1° in latitude centred at 0°. Central line velocities of LOS−0.11 and LOS−0.02 are shown with filled triangles, while the central line velocities of H1–H5 sources (Zhao et al. 1993) are indicated with filled squares. Velocity error bars overlap with filled triangles. 4.3 Sources of ionization 4.3.1 Positive velocity gas in the three GC LOSs The ionized gas studied in LOS−0.11 is at a projected distance of ∼3.8 pc from the region G (see Fig. 1), whose massive stars are thought to be the closest compact source of ionization to the positive velocity LOS−0.11 gas (Ho et al. 1985). On the other hand, since the region L lies close to LOS+0.693 (see Fig. 1, bottom panel), it is expected that the main ionization source of the LOS+0.693 gas could be the massive stars responsible for the ionization of the region L. As can be seen in the upper panel of Fig. 1, part of the emission arising in the region A falls within the GBT beam towards LOS−0.02. Thus, we expect that the gas with positive velocities in this GC source is mainly ionized by the massive stars in the H ii region A. To test whether the H ii region A could be the main source of ionization of the positive velocity gas in LOS−0.02, we estimate the number of photons inside the GBT beam, NΩ, and that ∼50 per cent of the region A falls inside the GBT beam (see Fig. 1). Considering the location of the GBT beam centre, then the compact H ii region A would actually be displaced from the telescope beam centre. For this geometry, we can estimate an upper limit to NΩ following the expression given by Rodríguez-Fernández & Martín-Pintado (2005):   \begin{eqnarray} N_{\Omega }=\frac{N_\mathrm{Lyc}}{4\pi r^2} \Omega D^2 \mathrm{,} \end{eqnarray} (3)where r is the radius of the H ii region. We derive the upper limit of 1050.95 photons s−1 for the value of NΩ by using the NLyc value provided by Mills et al. (2011) for the H ii region A, Ω = 45 arcsec and r = 1.7 pc in equation (3). It seems that the positive velocity gas of LOS−0.02 is mainly ionized by the massive stars in the H ii region A because the NLyc values of LOS−0.02, given in Table 10, are consistent with the upper limit of 1050.95 photons s−1. 4.3.2 Negative velocity gas in LOS−0.11 and LOS−0.02 The ionized gas components with negative velocities found towards both sources of Sgr A raises the question of the source of ionization. The top–down view shown in fig. 6 of Kruijssen et al. (2015) gives us information about the distances between Sgr A* and the four streams considered in their kinematical model. In this scenario Sgr A* is located between both the 20 and 50 km s−1 clouds and their background gas streams 3 and 4, at a projected distance of ∼60 pc from these features. If the negative velocity LOS−0.02 gas is part of the gas stream 2, as discussed in Section 4.2, then it may be ionized by the photons arising in massive O6–O7 stars, which also ionize the presumably closest UC–H ii regions, i.e. H1–H5 (Zhao et al. 1993), located at least ∼12 pc away from the negative velocity gas observed towards LOS−0.02 (see Fig. 8, upper panel). Of course, other ionizing sources apart from those proposed may exist in the environment of the negative velocity LOS−0.11 gas. On the other hand, the negative velocity LOS−0.02 gas is likely part of the stream 4, as discussed in Section 4.2. So far there are no compact H ii regions or massive stars whose velocities and positions are consistent with those of the gas stream 4 around LOS−0.11, hence the identification of ionizing sources of the negative velocity LOS−0.11 gas remains unclear. A possibility is that the negative velocity LOS−0.11 gas is actually part of stream 2, despite the difference in their velocites (see Fig. 8, bottom panel), thus being also ionized by the massive stars inside H1–H5 sources as for LOS−0.02. Considering the gas stream model proposed by Kruijssen et al. (2015), the massive young stars orbiting Sgr A* can be ruled out as ionizing sources of the negative velocity LOS−0.11 and LOS−0.02 gas since in this scenario Sgr A* is ∼60 pc away from gas streams 2 and 4 along the two LOSs. 5 CONCLUSIONS Using the GBT telescope, we have detected extended and diffuse ionized emission towards three GC LOSs. The main conclusions of the present work are as follows: We found that the ionized gas observed towards the three GC sources is emitted under LTE conditions based on the Hmβ-to-Hnα integrated line intensity ratios. We found a 4He mass fraction Y of 0.29±0.01 that supports the hypothesis that high-mass stars in the GC have enriched the helium-4 abundance in the ISM as compared to the primordial value. For LOS−0.11, LOS−0.02, and LOS+0.693, we have derived ne and NLyc values. The studied gas is characterized by ne of ∼40–310 cm−3. The ionized gas detected towards regions of the 20 and 50 km s−1 clouds is likely associated, following the Kruijssen et al. (2015) model, with gas stream 1 orbiting the GC, while the ionized gas moving with negative velocities in LOS−0.02 and LOS−0.11 is likely associated with the gas streams 2 and 4, respectively, located in projection ∼12 pc above stream 1. The LOS−0.02 gas at positive velocities is mainly ionized by ultraviolet (UV) photons produced in the massive stars also ionizing the H ii region A. The massive stars inside the H ii regions L and G are considered the closest sources of gas ionization of LOS+0.693 and LOS−0.11 (positive velocity component), respectively. We propose that the gas with negative velocities observed towards LOS−0.02 may be ionized by UV photons originating in the massive stars of the presumably closest H ii regions H1–H5. The negative velocity gas observed towards LOS−0.11 is likely associated with gas stream 4. We were not able to propose any possible ionizing sources of the negative velocity LOS−0.11 gas because, so far, there are no compact H ii regions or massive stars having both velocities and positions similar to those expected for gas stream 4 around LOS−0.11. However, if the negative velocity components of both Sgr A sources are part of the stream 2, then the massive stars in the H1–H5 regions could be the main sources of UV photons ionizing the gas with negative velocities of both Sgr A sources. We compared C i spectra with our H79α spectra, finding that C i emission does not trace the negative velocity component of either of the Sgr A sources. This indicates that this diffuse gas component is fully ionized. ACKNOWLEDGEMENTS We thank the anonymous referee for comments, which helped to improve this paper. AB-R acknowledges support from a DGAPA postdoctoral grant (year 2015) to UNAM. JM-P acknowledges partial support by the MINECO under grants ESP2015−65597 −C4−1 and ESP2017− and Comunidad de Madrid grant number S2013/ICE−2822 SpaceTec−CM. Footnotes 1 The name is given by its local standard of rest radial velocities. 2 The National Radio Astronomy Observatory is a facility of the National Science Foundation, operated under a cooperative agreement by Associated Universities, Inc. 3 gbtidl is an NRAO data reduction package, written in the IDL language for the reduction of GBT data. 4 This package have been developed at the Centro de Astrobiología. More information about this package in http://cab.inta-csic.es/madcuba/Portada.html. 5 https://archive.nrao.edu/ 6 http://casa.nrao.edu/ REFERENCES Armstrong D. A., Jackson J. M., Ho P. T. P., 1989, in Morris M., ed. IAU Symp. 136, The Center of the Galaxy . Kluwer, Dordrecht, p. 389 Google Scholar CrossRef Search ADS   Bally J., Stark A. A., Wilson R. W., 1987, ApJS , 65, 13 https://doi.org/10.1086/191217 CrossRef Search ADS   Boehle A. et al.  , 2016, ApJ , 830, 17 https://doi.org/10.3847/0004-637X/830/1/17 CrossRef Search ADS   Coc A., Goriely S., Xu Y., Saimpert M., Vangioni E., 2012, ApJ , 744, 158 https://doi.org/10.1088/0004-637X/744/2/158 CrossRef Search ADS   De Pree C. G., Wilder D. J., Deblasio J., Mercer A. J., Davis L. E., 2005, ApJ , 624, L101 https://doi.org/10.1086/430738 CrossRef Search ADS   Ekers R. D., van Gorkom J. H., Schwarz U. J., Goss W. M., 1983, A&A , 122, 143 García P., 2015, PhD thesis , Univ, Cologne García P., Simon R., Stutzki J., Güsten R., Requena-Torres M. A., Higgins R., 2016, A&A , 588, A131 CrossRef Search ADS   Gaume R. A., Claussen M. J., De Pree C. G., Goss W. M., Mehringer D. M., 1995, ApJ , 449, 663 https://doi.org/10.1086/176087 CrossRef Search ADS   Gordon M. A, Sorochenko R. L., 2009, Radio Recombination Lines: Their Physics and Astronomical Applications . Springer, New York Google Scholar CrossRef Search ADS   Gordon M. A., Berkermann U., Mezger P. G., Zylka R., Haslam C. G. T., Kreysa E., Sievers A., Lemke R., 1993, A&A , 280, 208 Goss W. M., Schwarz U. J., van Gorkom J. H., Ekers R. D., 1985, MNRAS , 215, 69 https://doi.org/10.1093/mnras/215.1.69P CrossRef Search ADS   Henshaw J. D. et al.  , 2016, MNRAS , 457, 2675 https://doi.org/10.1093/mnras/stw121 CrossRef Search ADS   Ho P. T. P., Jackson J. M., Barrett A. H., Armstrong J. T., 1985, ApJ , 288, 17 https://doi.org/10.1086/162823 CrossRef Search ADS   Kruijssen J. M. D., Dale J. E., Longmore S. N., 2015, MNRAS , 447, 1059 https://doi.org/10.1093/mnras/stu2526 CrossRef Search ADS   Lang C. C., Goss W. M., Morris M., 2001, AJ , 121, 2681 https://doi.org/10.1086/320373 CrossRef Search ADS   Langer W. D., Velusamy T., Morris M. R., Goldsmith P. F., Pineda J. L., 2017, A&A , 599, A136 CrossRef Search ADS   Lau R. M., Herter T. L., Morris M. R., Adams J. D., 2014, ApJ , 794, 108 https://doi.org/10.1088/0004-637X/794/2/108 CrossRef Search ADS   Lu F. J., Wang Q. D., Lang C. C., 2003, AJ , 126, 319 https://doi.org/10.1086/375754 CrossRef Search ADS   Martín S., Requena-Torres M. A., Martín-Pintado J., Mauersberger R., 2008, ApJ , 678, 245 https://doi.org/10.1086/533409 CrossRef Search ADS   Mehringer D. M., Palmer P., Goss W. M., Yuzef-Zadeh F., 1993, ApJ , 412, 684 https://doi.org/10.1086/172954 CrossRef Search ADS   Mezger P. G., Henderson A. P., 1967, ApJ , 147, 471 https://doi.org/10.1086/149030 CrossRef Search ADS   Mills E., Morris M. R., Lang C. C., Dong H., Wang Q. D., Cotera A., Stolovy S. R., 2011, ApJ , 735, 84 https://doi.org/10.1088/0004-637X/735/2/84 CrossRef Search ADS   Molinari A. et al.  , 2011, ApJL , 735, L33 https://doi.org/10.1088/2041-8205/735/2/L33 CrossRef Search ADS   Oka T., Hasegawa T., Sato F., Tsuboi M., Miyazaki A., 1998, ApJS , 118, 455 https://doi.org/10.1086/313138 CrossRef Search ADS   Purcell C. R. et al.  , 2012, MNRAS , 426, 3 https://doi.org/10.1111/j.1365-2966.2012.21800.x CrossRef Search ADS   Rodríguez-Fernández N. J., Martín-Pintado J., 2005, A&A , 429, 923 CrossRef Search ADS   Roelfsema P. R., Goss W. M., Whiteoak J. B., Gardner F. F., Pankonin V., 1987, A&A , 175, 219 Rohlfs K., Wilson T. L., 1999, Tools of Radio Astronomy . Springer-Verlag, Heidelberg Royster M. J., Yusef-Zadeh F., 2014, in Sjouwerman L., Ott J., Lang C., eds, Proc. IAU Symp. 303, The Galactic Center: Feeding and Feedback in a Normal Galactic Nucleus . Kluwer, Dordrecht, p. 92 Serabyn E., Lacy J. H., Achtermann J. M., 1992, ApJ , 395, 166 https://doi.org/10.1086/171640 CrossRef Search ADS   Spitzer L., 2004, Physical processes in the Intertellar Medium , Wiley-VCH, p. 107 Towle J. P., Feldman P. A., Watson J. K. G., 1996, ApJS , 107, 747 https://doi.org/10.1086/192380 CrossRef Search ADS   Tsivilev A. P., Parfenov S. Yu., Sobolev A. M., Krasnov V. V., 2013, Astron. Lett., Springer , 39, 737 https://doi.org/10.1134/S106377371310006X CrossRef Search ADS   Wilson T. L., Rood R. T., 1994, ARA&A , 32, 191 CrossRef Search ADS   Yusef-Zadeh F., Wardle M., Muno M., Law C., Pound M., 2005, Adv. Space Res. , 35, 1074 https://doi.org/10.1016/j.asr.2005.02.057 CrossRef Search ADS   Zhao J.-H., Desai K., Goss W. M., Yusef-Zadeh F., 1993, ApJ , 418, 235 https://doi.org/10.1086/173385 CrossRef Search ADS   © 2018 The Author(s) Published by Oxford University Press on behalf of the Royal Astronomical Society

### Journal

Monthly Notices of the Royal Astronomical SocietyOxford University Press

Published: May 1, 2018

## You’re reading a free preview. Subscribe to read the entire article.

### DeepDyve is your personal research library

It’s your single place to instantly
that matters to you.

over 12 million articles from more than
10,000 peer-reviewed journals.

All for just $49/month ### Explore the DeepDyve Library ### Unlimited reading Read as many articles as you need. Full articles with original layout, charts and figures. Read online, from anywhere. ### Stay up to date Keep up with your field with Personalized Recommendations and Follow Journals to get automatic updates. ### Organize your research It’s easy to organize your research with our built-in tools. ### Your journals are on DeepDyve Read from thousands of the leading scholarly journals from SpringerNature, Elsevier, Wiley-Blackwell, Oxford University Press and more. All the latest content is available, no embargo periods. ### Monthly Plan • Read unlimited articles • Personalized recommendations • No expiration • Print 20 pages per month • 20% off on PDF purchases • Organize your research • Get updates on your journals and topic searches$49/month

14-day Free Trial

Best Deal — 39% off

### Annual Plan

• All the features of the Professional Plan, but for 39% off!
• Billed annually
• No expiration
• For the normal price of 10 articles elsewhere, you get one full year of unlimited access to articles.

$588$360/year

billed annually

14-day Free Trial