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MNRAS 479, L7–L11 (2018) doi:10.1093/mnrasl/sly087 Advance Access publication 2018 May 17 1 1‹ 1 Narendra Nath Patra, Nissim Kanekar, ,† Jayaram N. Chengalur and 2‹ Nirupam Roy National Centre for Radio Astrophysics, Tata Institute of Fundamental Research, Pune University Campus, Pune 411 007, India Indian Institute of Science, Bengaluru, India Accepted 2018 May 15. Received 2018 April 10; in original form 2018 May 12 ABSTRACT We report a deep Giant Metrewave Radio Telescope search for Galactic HI 21-cm absorption towards the quasar B0438−436, yielding the detection of wide, weak HI 21-cm absorption, with −1 a velocity-integrated HI 21-cm optical depth of 0.0188 ± 0.0036 km s . Comparing this with the H I column density measured in the Parkes Galactic All-Sky Survey gives a column density- weighted harmonic mean spin temperature of 3760 ± 365 K, one of the highest measured in the Galaxy. This is consistent with most of the H I along the sightline arising in the stable warm neutral medium. The low-peak H I 21-cm optical depth towards B0438−436 implies negligible self-absorption, allowing a multi-Gaussian joint decomposition of the HI 21-cm absorption and emission spectra. This yields a gas kinetic temperature of T ≤ (4910± <t > 1900) K, and a spin temperature of T = (1000 ± 345) K for the gas that gives rise to the H I 21-cm absorption. Our data are consistent with the HI 21-cm absorption arising from either the stable WNM, with T T ,T ≈ 5000 K, and little penetration of the background Lyman-α radiation field s k k into the neutral hydrogen, or the unstable neutral medium, with T ≈ T ≈ 1000 K. s k Key words: ISM: atoms – ISM: general – radio lines: ISM. 2003;Roy et al. 2013), finding that the CNM indeed has T ≈ 1 INTRODUCTION 20–300 K, consistent with theoretical expectations. However, the Neutral hydrogen (H I) is a key constituent of the interstellar complexity of the H I 21-cm emission profiles and the difficulty of medium (ISM) of galaxies, making up almost half the ISM in the detecting the WNM in H I 21-cm absorption have made it difficult Milky Way. The earliest H I 21-cm studies (e.g. Clark 1965) found to verify the prediction of a stable warm phase with T ≈ 5000– evidence that H I exists in two phases, the cold neutral medium 8000 K. Indeed, attempts at measuring the WNM temperature by (CNM), which gives rise to the narrow, deep components seen in fitting multi-Gaussian models to either a combination of H I 21- H I 21cm absorption spectra towards background radio sources, cm emission and absorption spectra (Heiles & Troland 2003a); and the warm neutral medium (WNM), which contributes to the Heiles & Troland 2003b) or interferometric H I 21-cm absorption relatively smooth and broad H I 21-cm emission profiles but has spectra (Kanekar et al. 2003; Roy, Kanekar & Chengalur 2013; very weak H I 21-cm absorption. Similarly, theoretical models in- Murray et al. 2015) have found significant fractions of the H I dicate that Galactic H I exists in one of the above two stable phases, to be in the thermally unstable phase, with T ≈ 1000 K. Very with low kinetic temperatures (T ≈ 40–300 K) and high number few of the above Gaussian components show temperatures within −3 densities (≈10–100 cm ) in the CNM, and high temperatures (T or larger than the stable WNM range, ≈5000–8000 K. However, −3 ≈ 5000–8000 K) and low number densities (≈0.1–1 cm )inthe most of the sightlines in the above studies have complex H I 21- WNM (e.g. Field, Goldsmith & Habing 1969; McKee & Ostriker cm absorption profiles, implying that one is typically searching for 1977;Wolfireetal. 1995, 2003). H I at intermediate temperatures WNM absorption in the presence of far stronger, multicomponent (≈500–5000 K) is expected to be unstable and to quickly move into CNM absorption. The best sightlines for a reliable detection of the one of the two stable phases. WNM in H I 21-cm absorption, and for an accurate estimate of Over the last few decades, H I 21-cm absorption studies have the WNM kinetic temperature, are those with the least complexity made much progress in characterizing conditions in the CNM in in the H I 21-cm absorption profile. Our earlier interferometric the Galaxy (e.g. Radhakrishnan et al. 1972; Dickey, Terzian & H I 21-cm absorption survey of compact radio sources achieved Salpeter 1978; Payne, Salpeter & Terzian 1983; Heiles & Troland root-mean-square (RMS) H I 21-cm optical depth noise values of −1 ≈0.001 per ≈1km s channel and yielded detections of H I 21- cm absorption in 33 of 34 sightlines. The sole sightline without a E-mail: nkanekar@ncra.tifr.res.in (NK); roy.nirupam@gmail.com (NR) detection of absorption, towards the quasar B0438−436, has one of † DST Swarnajayanti Fellow. 2018 The Author(s) Published by Oxford University Press on behalf of the Royal Astronomical Society Downloaded from https://academic.oup.com/mnrasl/article-abstract/479/1/L7/4998859 by Ed 'DeepDyve' Gillespie user on 03 July 2018 L8 N. N. Patra et al. the lowest H I column densities of the sample (N = 1.29 × 10 HI −2 cm ; Kalberla & Haud 2015) and is at a high Galactic latitude, implying low CNM contamination. The absence of CNM absorption on this sightline, despite the high sensitivity of our survey, suggests that B0438−436 is a good target for a clean search for the WNM in absorption. We hence used the Giant Metrewave Radio Telescope (GMRT) to carry out a deep search for H I 21-cm absorption towards B0438−436, the results of which are described in this Letter. 2 OBSERVATIONS, DATA ANALYSIS, AND RESULTS Our GMRT search for Galactic H I 21-cm absorption towards B0438−436 was carried out over 2011 May 6–15, using the L-band receivers, with a total observing time of ≈30 h over six observing sessions. The GMRT Software Backend (GSB) was used as the correlator, with a bandwidth of ≈1.067 MHz subdivided into 512 −1 channels, yielding a total velocity coverage of ≈225 km s and a −1 velocity resolution of ≈0.43 km s . We used frequency-switching at the first GMRT local oscillator, on B0438−436 itself, to calibrate the system passband; the switching was carried out every 5 min Figure 1. The final GMRT H I 21-cm absorption spectrum towards with a throw of 5 MHz. Observations of 3C 48 and 3C 147 at the B0438−436, at (A) the Hanning-smoothed and re-sampled velocity res- start and end of each run were used to calibrate the flux density −1 olution of ≈0.86 km s (top panel), and (B) smoothed to, and re-sampled scale. Since B0438−436 is a phase calibrator for the GMRT, no −1 at, a resolution of ≈6.0 km s (bottom panel). additional phase calibration was necessary. The use of the GSB for these observations implied a far superior data quality and a larger is only ≈2 K, far lower than the GMRT system temperature at total velocity coverage than those of our earlier GMRT observations the observing frequency. The final optical depth RMS noise on the of B0438−436 (Roy et al. 2013). Allied with our higher sensitivity, −3 −1 GMRT spectrum is ≈1.0 × 10 per 0.43 km s channel. this significantly improved our ability to detect wide H I 21-cm Fig. 1(A) shows our final Galactic H I 21-cm absorption spec- absorption lines. trum towards B0438−436, with H I 21-cm optical depth plotted All data were analysed in the Astronomical Image Processing versus LSR velocity, after Hanning-smoothing and re-sampling the System (Greisen 2003), following standard data editing, calibra- −1 spectrum, at a velocity resolution of ≈0.86 km s (with an optical tion, self-calibration, imaging, and continuum subtraction proce- −3 −1 depth RMS noise of ≈0.72 × 10 per 0.86 km s channel). Weak dures (e.g. Roy et al. 2013). The data from the observing run on absorption can be seen close to zero LSR velocity. Fig. 1(B) shows May 6 were found to be severely affected by radio frequency in- the spectrum after additional smoothing and re-sampling at a ve- terference (RFI) and were hence excluded from the later analysis. −1 locity resolution of ≈6.0 km s ; the RMS optical depth noise on The task CVEL was finally used to shift the residual visibilities to the −4 −1 this spectrum is ≈2.4 × 10 per ≈6.0 km s .The H I 21-cm ab- local standard of rest (LSR) velocity frame. These visibilities were sorption is now clearly visible, detected at ≈5.3σ significance: the then imaged to produce a spectral cube, using natural weighting velocity-integrated H I 21-cm optical depth (over line channels) is and excluding baselines shorter than 1 kλ to reduce contamination −1 τ dV = (0.0188 ± 0.0036) km s . The rest of the spectrum, away from H I 21-cm emission within the primary beam. The H I 21-cm from the absorption feature, shows no evidence for any structure spectrum was obtained by taking a cut through the spectral cube at in the baseline. The Kolmogorov–Smirnov rank-1 and Anderson– the location of the quasar; a second-order baseline was then fitted to Darling tests find that the offline channels are consistent with being line-free regions, and subtracted out, to obtain the final H I 21-cm drawn from a Gaussian distribution. absorption spectrum. We measured a flux density of 4.2 Jy for B0438−436 from the final GMRT continuum image; the error on this value is dominated 3 DISCUSSION by uncertainties in the GMRT flux density scale, which we estimate to be ≈15 per cent. The flux density is in reasonable agreement Neutral hydrogen is usually characterized by two ‘temperatures’, with the value of 5.0 Jy listed in the Very Large Array Calibrator the kinetic temperature, T ,and theH I 21-cm line excitation tem- Manual. perature, the spin temperature, T . In the case of the high-density For Galactic H I 21-cm absorption studies, the spectral RMS CNM, the H I hyperfine levels are expected to be thermalized by noise depends on the observing frequency due to the contribution a combination of collisions and Lyman-α scattering, with the spin from the brightness temperature of the H I emission in the beam temperature approximately equal to the kinetic temperature (e.g. (Roy et al. 2013). We followed the procedure of Roy et al. (2013)to Wouthuysen 1952; Field 1958;Liszt 2001). Observationally, low combine the brightness temperature measured in the Parkes Galactic spin temperatures (≈100 K) have indeed been found to be associ- All-Sky Sky Survey (GASS; McClure-Griffiths et al. 2009;Kalberla ated with the narrow H I 21-cm absorption features that are expected & Haud 2015)H I 21-cm emission spectrum with the GMRT system to arise in the CNM (e.g. Dickey et al. 1978; Heiles & Troland 2003; temperature (≈73 K at 1420 MHz) to determine the RMS noise Roy et al. 2013). However, in the case of the WNM, the low number spectrum. Note that this correction is very small for B0438−436 as density implies that collisions are not very effective in thermalizing the peak H I 21-cm brightness temperature in the GASS spectrum the H I 21-cm transition. Resonant scattering of Lyman-α photons MNRASL 479, L7–L11 (2018) Downloaded from https://academic.oup.com/mnrasl/article-abstract/479/1/L7/4998859 by Ed 'DeepDyve' Gillespie user on 03 July 2018 Detection of the Galactic warm neutral medium in HI 21-cm absorption L9 is hence expected to be the main process in driving T towards T s k (e.g. Field 1965; Deguchi & Watson 1985;Liszt 2001). It has long been unclear whether the fraction of the Galactic Lyman-α back- ground radiation threading into the WNM is sufficient to drive the WNM spin temperature to the kinetic temperature (e.g. Deguchi & Watson 1985;Liszt 2001). Theoretical studies suggest that T < T s k in the WNM due to both the low WNM number density and the expected low penetration of background Lyman-α photons into the WNM (e.g. Liszt 2001). A few early observational studies did obtain high spin temperature estimates, 5000 K (e.g. Kalberla, Mebold & Reich 1980; Payne, Salpeter & Terzian 1983; Kulkarni, Heiles & Dickey 1985), albeit usually on sightlines with complex H I 21-cm absorption and with concerns about stray radiation affecting the H I 21-cm emission spectra. Further, while T values 1000 K have recently been found at specific velocity ranges along mul- tiple Galactic sightlines (e.g. Carilli, Dwarakanath & Goss 1998; Dwarakanath, Carilli & Goss 2002;Roy et al. 2013), it is not usually straightforward to estimate T at the same velocity ranges, making it difficult to test whether the H I 21-cm hyperfine excitation is indeed sub-thermal in the WNM. Next, the H I column density along a sightline is related to the velocity-integrated H I 21-cm optical depth by the equation Figure 2. The three panels of the figure show (A) the HI 21-cm emission spectrum from the GASS survey (Kalberla & Haud 2015), (B) the GMRT N = 1.823 × 10 T τ dV ,, (1) HI s 21cm HI 21-cm optical depth spectrum, and (C) the spin temperature T ,for the −1 sightline towards B0438−436, plotted versus LSR velocity, in km s .The spectra have been smoothed to, and re-sampled at, a velocity resolution of where T is the column density-weighted harmonic mean spin s −1 ≈6.0 km s . See main text for discussion. temperature along the sightline. Note that T is biased towards low CNM temperatures: for example, if the H I along the sightline −τ is equally divided between phases with T ≈ 100 K and T ≈ 8000 Fig. 2(C) shows the spin temperature, T = T /(1 − e ), plotted s s s B K, one would obtain T ≈ 200 K. Even if 90 per cent of the H I versus LSR velocity, at velocities where the H I 21-cm emission is along the sightline has T ≈ 8000 K, and 10 per cent has T ≈ 100 detected at ≥5σ significance. Velocity channels with detections of s s K, one would measure T ≈ 900 K. Exacerbating this issue, spin H I 21-cm absorption at ≥3σ significance are shown as filled circles temperatures in the WNM are expected to be lower than the kinetic with error bars, while channels with non-detections of H I 21-cm temperature and are hence likely to be significantly lower than the absorption (i.e. with <3σ significance) are shown as 3σ lower limits assumed 8000 K (e.g. Liszt 2001). As such, high values of T , to the spin temperature. While the two velocity channels with ≥3σ 1000 K, can only be obtained for sightlines with almost all the detections of absorption have T ≈ 1750 K, the two higher-velocity gas in the WNM. channels have T (3σ ) 2500 K. These T values lie in the expected s s In the case of the sightline towards B0438−436, the GASS H I range for WNM spin temperatures (e.g. Liszt 2001). emission spectrum yields an H I column density of 1.29 × 10 The above results demonstrate that most or all of the gas towards −2 cm . Combining this with our measured integrated H I 21- B0438−436 is warm, with high spin temperatures, 1750 K, con- −1 cm optical depth of τ dV = (0.018 ± 0.036) km s then sistent with an origin in the stable WNM, at all channels with 21 cm yields T = 3760 ± 765 K. This is one of the highest column significant H I 21-cm emission. We also carried out a Gaussian density-weighted harmonic mean spin temperatures ever measured decomposition of the H I 21-cm absorption profile of Fig. 1(A) at −1 along a sightline in the Milky Way, comparable to values seen in a resolution of 0.86 km s ; a single Gaussian, shown as the solid low-metallicity damped Lyman-α absorbers at high redshifts (e.g. curves in Figs 1(A) and (B), provides an excellent fit to the spectrum; Kanekar & Chengalur 2003; Kanekar et al. 2014). The sightline to- this has a full width at half maximum (FWHM) of 12.0 ± 3.1 km −1 wards B0438−436 is clearly dominated by the WNM, with almost s , corresponding to a temperature of 3150 ± 1635 K. We empha- no cold gas present towards the quasar. The lack of CNM along this size that this estimate corresponds to the maximum allowed value low-N sightline is consistent with the H I column density thresh- of the kinetic temperature, sometimes referred to as the ‘Doppler HI 20 −2 old of ≈2 × 10 cm that has been suggested for the formation of temperature’ (T ;e.g. Royetal. 2013), as non-thermal motions significant amounts of cold atomic gas in the Milky Way (Kanekar, may contribute to the line broadening. This estimate of the Doppler Braun&Roy 2011). temperature is very similar to the column density-weighted har- Figs 2(A) and (B) compare the GMRT H I 21-cm absorption monic mean spin temperature estimate of T = 3760 ± 765 K spectrum towards B0438−436 with the GASS H I 21-cm emission and may suggest that the gas towards B0438−436 is in the unstable spectrum at a neighbouring location, with both spectra plotted ver- phase, with T ≈ T . However, we emphasize that the above ki- s k sus LSR velocity, after smoothing to, and re-sampling at, a velocity netic temperature estimate is consistent (within ≈1σ significance) −1 resolution of 6.0 km s . It is clear that the peak H I 21-cm absorp- with the standard stable WNM kinetic temperature range (≈5000– −1 tion occurs at a slightly lower velocity (≈0km s ) than the peak 8000 K). This conclusion is rendered even more unlikely because −1 of the H I 21-cm emission (≈ +8 km s ). Indeed, the bulk of gas T ≈ 1750 K at the velocity channels with the strongest H I 21- detected in H I 21-cm emission appears to not be detected in H I cm absorption. Deeper H I 21-cm absorption studies are needed to 21-cm absorption. accurately estimate T via the Gaussian decomposition approach. MNRASL 479, L7–L11 (2018) Downloaded from https://academic.oup.com/mnrasl/article-abstract/479/1/L7/4998859 by Ed 'DeepDyve' Gillespie user on 03 July 2018 L10 N. N. Patra et al. Figure 3. Results of the multi-Gaussian joint decomposition of the (A) HI 21-cm emission and (B) HI 21-cm absorption spectra; the former spectrum is at −1 −1 a resolution of ≈0.82 km s , while the latter is at a resolution of ≈0.86 km s . The top panels show the best-fit model (solid curve) overlaid on the two spectra, while the bottom two panels show the residuals from the fit. Table 1. Gaussian joint decomposition of the H I 21-cm emission and absorption spectra. a b c d e Component T V FWHM τ T T B 0 pk k s −1 −1 −3 Kkms km s ×10 KK 11.08 ± 0.30 0.63 ± 1.57 15.0 ± 2.9 1.08 ± 0.22 4910 ± 1900 1000 ± 345 21.31 ± 0.35 10.9 ± 1.9 13.6 ± 4.8 <0.48 4045 ± 2850 >2730 3 0.358 ± 0.067 0.90 ± 1.46 56.9 ± 5.0 <0.23 70615 ± 12410 >1555 40.26 ± 0.10 −12.1 ± 1.1 8.4 ± 2.2 <0.61 1540 ± 805 >425 50.81 ± 0.52 20.8 ± 2.4 11.4 ± 2.1 <0.52 2835 ± 1045 >1560 a b c Notes. For each component, the columns are: The peak brightness temperature; The central LSR velocity; The peak H I 21-cm optical depth, or, for non-detections, the 3σ upper limit on the peak HI 21-cm optical depth at a velocity resolution equal to the line FWHM; The kinetic temperature inferred from the FWHM; The spin temperature or, for components 2–5, the 3σ lower limit to the spin temperature. Attempts have been made in the literature to carry out a joint one absorption component. The model is plotted in Figs 3(A) and multi-Gaussian decomposition of the H I 21-cm emission and the (B), with the residuals from the fit displayed in the lower two panels; H I 21-cm absorption spectra (e.g. Heiles & Troland 2003; Murray the residuals are seen to be consistent with noise. The properties et al. 2014). These have usually encountered difficulties in handling of the different Gaussian components are summarized in Table 1, H I 21-cm ‘self-absorption’ along the sightline by foreground CNM whose columns contain (1) the component number, (2) the peak clouds of their own, and of background CNM and WNM, emission. brightness temperature of the component, in K, (3) the central LSR −1 −1 The present sightline towards B0438−436 is very interesting in this velocity, in km s , (4) the component FWHM, in km s ,(5) context because the low peak H I 21-cm optical depth (≈0.001) the peak H I 21-cm optical depth, or the 3σ upper limit to the means that self-absorption is not an issue. The sightline thus allows peak optical depth (estimated at a velocity resolution equal to the the possibility of a joint Gaussian decomposition of the H I 21- component FWHM), (6) the kinetic temperature of the component, cm absorption and emission spectra without any effects of self- T , and (7) the component spin temperature T ,orthe 3σ lower limit k s absorption. to the spin temperature. We carried out a joint multi-Gaussian fit to the H I 21-cm emission Initially, we note that the third component in Table 1 is extremely −1 and H I 21-cm absorption spectra, the former at the original velocity wide, with an FWHM of ≈57 km s . Given the relatively high −1 ◦ resolution of ≈0.82 km s and the latter after Hanning-smoothing Galactic latitude of B0438−436 (b ≈−41 ), it appears unlikely −1 and resampling, at a resolution of ≈0.86 km s . We allowed for H I that this large broadening arises from either Galactic rotation or 21-cm absorption by a single Gaussian component and H I 21-cm turbulence. We suspect that it may be due to a residual spectral emission by multiple Gaussian components. In the fit, the position baseline in the GASS spectrum, although we note that a similar and FWHM of one of the emission components were constrained wide component is also visible in the H I 21-cm spectrum from the to be the same as that of the absorption component (using multiple Leiden–Argentine–Bonn survey (Kalberla et al. 2005). For now, the absorption components did not yield a better fit, in terms of an origin of the broad component remains unclear. improved reduced chi-square value). The best-fit model, with a The component with detected H I 21-cm absorption has an 2 −1 reduced chi-square χ = 1.00, contains 5 emission components and FWHM of 15.0 ± 2.9 km s , in good agreement with that ob- MNRASL 479, L7–L11 (2018) Downloaded from https://academic.oup.com/mnrasl/article-abstract/479/1/L7/4998859 by Ed 'DeepDyve' Gillespie user on 03 July 2018 Detection of the Galactic warm neutral medium in HI 21-cm absorption L11 tained from the fit to the H I 21-cm absorption alone (12.0 ± 3.1 21-cm self-absorption is negligible along this sightline. We carry −1 km s ). The inferred upper limit on the kinetic temperature is then out a joint multi-Gaussian decomposition of the H I 21-cm emission 4910 ± 1900 K in the stable WNM temperature range. We em- and the H I 21-cm absorption profiles to obtain T ≤ (4910 ± 1900) phasize, as above, that this is formally an estimate of the Doppler K for the component with detected H I 21-cm absorption, consis- temperature T and hence yields only an upper limit to T due to tent with the stable WNM temperature range. The spin temperature D k the possibility of non-thermal broadening. of this component is (1000 ± 345) K, significantly lower than the We consider the two extreme possibilities for line broadening: kinetic temperature. This suggests that the H 21-cm line excitation (1) weak non-thermal motions, for which the kinetic tempera- is sub-thermal in the WNM along this sightline, possibly due to low ture is approximately equal to the Doppler temperature, i.e. T ≈ gas pressure or low threading of the Galactic Lyman-α background T = (4910 ± 1900) K, and (2) strong non-thermal broadening, into the WNM. However, the present data cannot rule out the pos- in which case T T . In the first case, the detected H I 21-cm sibility that the detected H I 21-cm absorption might arise from k D absorption arises from the stable WNM. The spin temperature of warm gas in the thermally unstable phase, with significant non- this component is 1000 ± 345 K, significantly lower than the kinetic thermal line broadening, and the gas spin temperature comparable temperature. Figs 2 and 5 of Liszt (2001) suggest that this would to the kinetic temperature. −3 require the gas to be at low pressures, P/k 1000 cm K, and with almost no Galactic Lyman-α background penetrating into the H I. ACKNOWLEDGEMENTS The data would then support a picture of little coupling between the We thank the GMRT staff who have made these observations pos- WNM and the background Lyman-α radiation field. sible. The GMRT is run by the National Centre for Radio As- In the second case, of strong non-thermal broadening, the mea- trophysics of the Tata Institute of Fundamental Research. NK ac- sured Doppler temperature yields only an upper limit to T . knowledges support from the Department of Science and Technol- However, the measured spin temperature gives the constraint T ogy via a Swarnajayanti Fellowship (DST/SJF/PSA-01/2012-13). ≥ 1000, since T ≥ T in the WNM. This implies the range k s NR acknowledges support from the Infosys Foundation through the 1000 K ≤ T ≤ 4910 K for the component detected in absorption. Infosys Young Investigator grant. For strong non-thermal broadening, T T , the detected H I 21- k D cm absorption would then still arise in warm gas, albeit from the thermally unstable phase, with T ≈ T , i.e. with strong coupling k s REFERENCES between the unstable neutral medium and the background Lyman-α Carilli C. L., Dwarakanath K. S., Goss W. M., 1998, ApJL, 502, L79 radiation field. We cannot rule out this possibility with the present Clark B. G., 1965, ApJ, 142, 1398 data. Deguchi S., Watson W. 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A., 1952, AJ, 57, 31 The extremely low H I 21-cm optical depth further implies that H I MNRASL 479, L7–L11 (2018) Downloaded from https://academic.oup.com/mnrasl/article-abstract/479/1/L7/4998859 by Ed 'DeepDyve' Gillespie user on 03 July 2018 L12 N. N. Patra et al. This paper has been typeset from a T X/LT X file prepared by the author. E E MNRASL 479, L7–L11 (2018) Downloaded from https://academic.oup.com/mnrasl/article-abstract/479/1/L7/4998859 by Ed 'DeepDyve' Gillespie user on 03 July 2018
Monthly Notices of the Royal Astronomical Society Letters – Oxford University Press
Published: May 17, 2018
Keywords: ISM: atoms; ISM: general; radio lines: ISM
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