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Revisiting Stephan's Quintet with deep optical images

Revisiting Stephan's Quintet with deep optical images MNRAS 475, L40–L44 (2018) doi:10.1093/mnrasl/sly004 Advance Access publication 2018 January 9 1‹ 2 3 Pierre-Alain Duc, Jean-Charles Cuillandre and Florent Renaud Universite´ de Strasbourg, CNRS, Observatoire astronomique de Strasbourg, UMR 7550, F-67000 Strasbourg, France IRFU, CEA, Universite´ Paris-Saclay, Universite´ Paris Diderot, AIM, Sorbonne Paris Cite, ´ CEA, CNRS, Observatoire de Paris, PSL Research University, F-91191 Gif-sur-Yvette Cedex, France Department of Astronomy and Theoretical Physics, Lund Observatory, Box 43, SE-221 00 Lund, Sweden Accepted 2018 January 5. Received 2018 January 5; in original form 2017 November 6 ABSTRACT Stephan’s Quintet, a compact group of galaxies, is often used as a laboratory to study a number of phenomena, including physical processes in the interstellar medium, star formation, galaxy evolution, and the formation of fossil groups. As such, it has been subject to intensive multiwavelength observation campaigns. Yet, models lack constrains to pin down the role of each galaxy in the assembly of the group. We revisit here this system with multiband deep optical images obtained with MegaCam on the Canada–France–Hawaii Telescope (CFHT), focusing on the detection of low surface brightness (LSB) structures. They reveal a number of extended LSB features, some new, and some already visible in published images but not discussed before. An extended diffuse, reddish, lopsided, halo is detected towards the early- type galaxy NGC 7317, the role of which had so far been ignored in models. The presence of this halo made of old stars may indicate that the group formed earlier than previously thought. Finally, a number of additional diffuse filaments are visible, some close to the foreground galaxy NGC 7331 located in the same field. Their structure and association with mid-infrared emission suggest contamination by emission from Galactic cirrus. Key words: techniques: photometric – galaxies: interactions – galaxies: photometry – galaxies: stellar content. and to explore the underlying physical processes. Current models in- 1 INTRODUCTION volve a number of events in the last half Gyr, including the collision Stephan’s Quintet (SQ), a compact group of five galaxies located between the spiral galaxy NGC 7319 and NGC 7320c, creating the at a distance of 85 Mpc, is arguably the poster child for this class so-called outer tail, between NGC 7319 and the early-type galaxy of objects. This system exhibits a remarkable variety of physical NGC 7318a creating the inner tail, and finally the late high speed processes at all scales, pertaining to cooling and heating of the collision with the spiral NGC 7318b, producing shocks in the gas interstellar medium (e.g. Appleton et al. 2017), star formation in stripped by the previous interactions (see Fig. 1). So far, the lack of shock regions (e.g. Guillard et al. 2012), star cluster formation observed tidal features between a fifth galaxy, NGC 7317, and the (e.g. Trancho et al. 2012), tidal dwarf galaxy (TDG) formation other group members suggested this galaxy had not yet interacted (e.g. Lisenfeld et al. 2004), not to mention the repeated galaxy with the group. collisions and group formation. Not surprisingly, this remarkable Past studies of SQ have mostly focused on tracers of rela- system has been observed at all wavelengths, from the X-rays (e.g. tively young events, such as the cold gas (HI, molecular), the ion- O’Sullivan et al. 2009) and ultraviolet (UV; e.g. Xu et al. 2005)to ized/shocked gas or the dust emission. However, few analyses have the far-infrared (far-IR; e.g. Appleton et al. 2013) and radio HI (e.g. yet been carried out on the old stellar populations, although they Williams, Yun & Verdes-Montenegro 2002). can be used to reconstruct the mass assembly of the group to older Several attempts have been carried out to reproduce numerically times and help constraining the scenarios. Mapping the faint diffuse the full variety of morphological features of the group, including stellar haloes and fine structures around the galaxies is particularly multiple tidal tails, and complex velocity fields (e.g. Renaud, Apple- helpful for this purpose. Such a study requires images with rela- ton & Xu 2010; Hwang et al. 2012), to propose formation scenarios tively large fields of view, which was actually the case when, until the 1980s, photography with plates and Schmidt telescopes was available. A rather deep image of the field of SQ was presented by Arp & Kormendy (1972). At that time when the mutual distances E-mail: pierre-alain.duc@astro.unistra.fr 1 −1 −1 between all group member candidates were still discussed, the mo- We adopt here H = 75 km s Mpc . At the assumed distance, 10 arcsec corresponds to 4.1 kpc tivation for such observations was the detection of stellar bridges 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/475/1/L40/4794952 by Ed 'DeepDyve' Gillespie user on 16 March 2018 Deep imaging of Stephan’s Quintet L41 Figure 1. Top left-hand panel: composite u + g + r true colour image of the SQ. Top right-hand panel: g-band surface brightness map, with the principle −2 structures labelled. The faintest emission (with surface brightness above 26 mag.arcsec ) revealed by the deep CFHT MegaCam image is shown in red. Bottom −1 19 −2 left-hand panel: u − r colour map with HI/VLA map in the velocity range 6475–6755 km s superimposed. The lowest contour is 6× 10 cm (adapted from Williams et al. 2002). Selected intergalactic star-forming regions are labelled. Bottom right-hand panel: g − r colour map with archival X-ray/XMM–Newton −2 contours superimposed. For the surface brightness and colour maps, the scale in mag, respectively, mag arcsec is indicated to the right. The field of view is 10 arcmin × 10 arcmin (250 kpc × 250 kpc). North is up and east left. Palma. With a 2000-s exposure time, they reached a local surface between the group and the spiral galaxy NGC 7331. Indeed, Arp −2 brightness limit of 27.3 mag arcsec , allowing them to disclose a & Kormendy (1972) detected some streams, but could not firmly diffuse halo around SQ, and to trace back the known tidal tails in conclude on their origin. Gutierrez ´ et al. (2002) obtained so far the the system to much larger distances. The field of view was however deepest optical image of SQ, using R band observations with the too limited to investigate the structures previously found by Arp & Wide Field Camera on the 2.5-m Isaac Newton Telescope at La Kormendy (1972) between SQ and NGC 7331. Meanwhile, with the advent of new generation of wide field of view cameras coupled with new techniques optimised for the de- Which is now known to be in the foreground and unrelated to SQ, but tection of low surface brightness (LSB) structures, deep optical perhaps not to the other foreground galaxy NGC 7320. MNRASL 475, L40–L44 (2018) Downloaded from https://academic.oup.com/mnrasl/article-abstract/475/1/L40/4794952 by Ed 'DeepDyve' Gillespie user on 16 March 2018 L42 P.-A. Duc, J.-C. Cuillandre and F. Renaud imaging of nearby galaxies has been rejuvenated (e.g. Mart´ınez- nucleus should instead be concentric as illustrated by the light pro- Delgado et al. 2010; Ferrarese et al. 2012; Mihos et al. 2013; files of the bright foreground stars surrounding the galaxy. Duc et al. 2015; Trujillo & Fliri 2016). We revisit here SQ and its Since the stellar halo traces a dynamically hot morphology (as surroundings using multiband deep optical images obtained at the opposed to sharp tidal tails), it is likely that its stars have been Canada–France–Hawaii Telescope (CFHT. We gain new insights, stripped from the elliptical galaxy NGC 7317 during an interaction in particular, on the role of NGC 7317, and gather clues to prepare with one (or more) of the group members. The lack of tidal arms for the next generation of models for this group, in particular, and pointing towards a specific disc galaxy forbids us to designate a of studies of repeated interactions, in general. culprit though an implication of NGC 7318b should be excluded because of it high radial velocity. Such arm(s) either never formed (e.g. because of a retrograde encounter), or have been erased by 2 OBSERVATIONS AND DATA REDUCTION subsequent interactions, which would suggest that NGC 7317 has been involved early in the construction of the group. Deep multiband images of the field encompassing SQ and the spiral galaxy NGC 7331 have been obtained with the MegaCam camera installed on the CFHT, through Director Discretionary Time (DDT, 3.2 Young and old tails PI: J.-C. Cuillandre) from 2015 July to December. Images have been SQ is known for hosting multiple intergalactic blue regions, with star acquired with an observing strategy optimized for the detection of formation triggered by shocks or compressive tides in the stripped LSB objects. It relies on (a) acquiring consecutively seven individ- gas (e.g. Iglesias-Paramo ´ et al. 2012). The MegaCam u-band image ual images with large offsets between them, such that the distance reveals new ones and provides some details on their structure. We between each image centre is at least 10 arcmin, a scale larger than draw attention on the region referred as SQ-HII in Fig. 1 (bottom the LSB features around SQ studied here; (b) building a master left-hand panel) that coincides with the northern tip of an HI tail, and sky median-stacking the individual images; and (c) subtracting a forms a jet-like stream with regularly-spaced giant star complexes. smoothed and rescaled version of the master sky from each indi- Several other filamentary structures are observed in the eastern vidual image before stacking them. The procedure, fully described region of the SQ. The principle ones are labeled on the top right- in Duc et al. (2015), is identical to the one used for the MATLAS hand panel of Fig. 1. The tail apparently coming from NGC 7319, project. The final stacked images consist of 4 × 7 frames with total known as the inner tail, hosts massive star clusters, which are 150– exposure time of 56 min for the u band, and 28 min for the g and r 200 Myr old (Fedotov et al. 2011), and prominent star-forming bands. Following the DDT policy, observations were obtained with condensations, including the TDG candidate, SQ-B (Lisenfeld et al. poor seeing conditions with respect to the CFHT standards (aver- 2004). At the location of a CO-detected dust lane towards SQ-B, the age seeing 1.8 arcsec), but under photometric conditions. Images tail seems to split. One branch, labelled IT1, extending eastwards, were processed by the ELIXIR-LSB pipeline. The estimated local lim- was not known before. It is diffuse, and lacks star-forming regions. −2 iting surface brightnesses of 29.0, 28.6, and 27.6 mag arcsec for, The other branch, labeled IT2, bifurcates to the north, crossing the respectively, the u, g,and r bands assess the ability of detecting other TDG candidate, SQ-tip, and ends in the scattered star-forming structures of typical size 5 arcsec × 5 arcsec. knots mentioned above. The difference in gas content (and thus star formation activity) in the two branches remains to be explained. The so-called outer tail, observed to the south, disappears on 3 RESULTS its western side towards the foreground galaxy NGC 7320. The The CFHT images provide a panoramic view of the stellar popu- deep CFHT image undoubtedly confirms that it points to the east lations that are present all across the group, up to large distances: towards NGC 7320c, the barred galaxy believed to have crossed the intergalactic young stars and star clusters, distributed along blue, group about 0.5 Gyr ago. Interestingly, the rather blue colour of this narrow, and clumpy filaments, old stars present in reddish diffuse tail (u − r = 1.4, see Fig. 1, bottom left-hand panel), testifies the tails or extended haloes, with a possible contamination by filamen- presence of intermediate age stars, but star formation seems to have tary structures associated with cirrus emission. stopped there: The eastern tip of the tail is not detected in UV in GALEX images we collected from the archives. As shown in Fig. 1 (bottom left-hand panel), there is no HI cloud with a column density 3.1 The diffuse halo 19 −2 larger than 6× 10 cm at this location. SQ appears to be embedded in a diffuse reddish halo, which is especially prominent towards the south-west, near NGC 7317. This 3.3 Isolated filamentary structures: cirrus emission early-type galaxy is off-centred with respect to the outer isophotes of the halo. The diffuse emission extends up to 60 kpc from the galaxy. Fig. 2 shows a composite image of the whole MegaCam field cover- As seen in the true colour image (Fig. 1, top left-hand panel), but ing about 1 square degree. Several extended filamentary structures even more clearly on the calibrated u − r and g − r colour maps with colours varying between green and yellow (indicating that they (Fig. 1, bottom), the diffuse light is much redder to the south-west are most prominent in the g and r bands) can be seen between SQ than towards the north of the compact group. The average colours in the south, and the spiral galaxy NGC 7331 in the north, in addi- (u − r = 2.6 mag, g − r = 0.9 mag) correspond to typical Gyr old tion to many faint foreground stars, extended ghost haloes of bright stellar populations. Only a mild negative g − r colour gradient of nearby stars, and background galaxies. 0.05 dex per 10 kpc is observed. Most of the several isolated narrow filamentary features scattered We note that PSF effects that may affect the light and colour in the whole field (but more numerous to the north, and globally distributions of galaxies (see the study of Karabal et al. 2017, based sharing an east–west orientation) show a clear counterpart in the on MegaCam images) cannot explain the specific properties of the mid-infrared (mid-IR) images (12 µm, Fig. 3), which we queried diffuse, off-centred, light around NGC 7317. At its location on from the WISE archives using the CDS Aladin tool. These struc- the MegaCam image, instrumental ghosts generated by the galaxy tures are thus undoubtedly Galactic cirrus, and not extragalactic MNRASL 475, L40–L44 (2018) Downloaded from https://academic.oup.com/mnrasl/article-abstract/475/1/L40/4794952 by Ed 'DeepDyve' Gillespie user on 16 March 2018 Deep imaging of Stephan’s Quintet L43 Figure 2. Composite u + g + r band image of the whole CFHT/MegaCam field, encompassing SQ (bottom right-hand panel) and NGC 7331 (top left-hand panel). The field of view is 70 arcmin × 60 arcmin. North is up and east left. An arcsinh scaling has been used to enhance the LSB features. tidal structures as initially suggested, among other hypotheses, by 7320c is clearly confirmed, proving the active role of this galaxy in Arp & Kormendy (1972). Scattered optical light by dust clouds in the dynamical history of the group. the Milky Way hampers extragalactic studies based on deep images, The reddish halo surrounding SQ, which is most prominent to- as they mimic stellar streams. They however provide valuable infor- wards NGC 7317, provides indications on the implication of this mation on the structure of the ISM at small scales, as emphasized galaxy in the group’s history. The size of the halo, reminiscent of by Miville-Deschenes ˆ et al. (2016). the intracluster light in clusters and fossil groups, could be con- Getting closer to SQ, the plume apparently dragging on from the sistent with a group formation several Gyr ago. A remarkable cor- diffuse halo of NGC 7317 towards the south-west, and best seen in respondence between the diffuse component of the X-ray emis- the r band, might be a Galactic cirrus as well. Indeed it has a similar sion (as traced by XMM–Newton) and the diffuse optical light had structure and orientation as the other confirmed mid-IR detected been noted by Trinchieri et al. (2005).As showninFig. 1 (bot- cirrus, though the former is likely too faint to be detected by WISE tom right-hand panel), the spatial matching is best observed for the (see Fig. 3). halo around NGC 7317. If the X-ray emission traces its potential well, this might be another indication that the group is older (and more relaxed) than generally believed. Although the peak of the 3.4 The spiral galaxy NGC 7331 X-ray/optical halo emission matches the position of NGC 7317, the galaxy appears off-centred with respect to the outer isophotes. This NGC 7331 is a highly inclined galaxy with prominent spiral arms. disturbed shape could thus be due to a still on-going interaction Given its distance (13.5 Mpc, taken from the NED data base), it with NGC 7319. The presence of a diffuse X-ray emitting hot gas has been argued to form an interacting pair with NGC 7320 (itself halo suggests that ram pressure could perhaps explain why the tips foreground of SQ). However, even at the depth reached here, we of the outer tail and eastern branch of the inner tail are gas poor. do not detect any morphological sign of perturbation in the galactic This study shows how deep imaging can, with the proper ob- disc. As mentioned above, the filamentary structures to the west and serving strategy requiring limited telescope time, bring additional south of the spiral are most likely Galactic cirrus and not tidal tails. constraints on the modelling of interacting systems by providing information on the oldest collisional events traced by the old stellar population. More specifically, all these observed new features call 4 DISCUSSION AND CONCLUSIONS for revised formation scenarios and detailed models of the SQ. To date, models of the SQ attribute most of its tidal features to the interactions between NGC 7319, NGC 7320c, and NGC 7318 a/b, while the role of the early-type galaxy NGC 7317 is generally ACKNOWLEDGEMENTS ignored due to a lack of observational clues. We have revisited this system with deep optical multiband CFHT Based on observations obtained with MegaPrime MegaCam, a images, which exhibit a number of extended LSB structures, some joint project of CFHT and CEA/DAPNIA, at the Canada–France– new, some that were visible in previously published images, but Hawaii Telescope (CFHT), which is operated by the National Re- largely ignored. The connection between the outer tail and NGC search Council (NRC) of Canada, the Institut National des Science MNRASL 475, L40–L44 (2018) Downloaded from https://academic.oup.com/mnrasl/article-abstract/475/1/L40/4794952 by Ed 'DeepDyve' Gillespie user on 16 March 2018 L44 P.-A. Duc, J.-C. Cuillandre and F. Renaud of the CFHT calendar produced in collaboration with Edizioni Sci- entifiche Coelum (R. Zabotti, G. Anselmi). A crop of it is shown in Fig. 1. We thank the referee for his prompt feedback and useful suggestions. FR acknowledges funding from the Knut and Alice Wallemberg Foundation. REFERENCES Appleton P. N. et al., 2013, ApJ, 777, 66 Appleton P. N. et al., 2017, ApJ, 836, 76 Arp H., Kormendy J., 1972, ApJ, 178, L101 Duc P. -A. et al., 2015, MNRAS, 446, 120 Fedotov K., Gallagher S. C., Konstantopoulos I. S., Chandar R., Bastian N., Charlton J. C., Whitmore B., Trancho G., 2011, AJ, 142, 42 Ferrarese L. et al., 2012, ApJS, 200, 4 Guillard P. et al., 2012, ApJ, 749, 158 Gutierrez ´ C. M., Lopez-Corredoira ´ M., Prada F., Eliche M. C., 2002, ApJ, 579, 592 Hwang J.-S., Struck C., Renaud F., Appleton P. N., 2012, MNRAS, 419, Iglesias-Paramo ´ J., Lopez-Mart ´ ´ın L., V´ılchez J. M., Petropoulou V., Sulentic J. W., 2012, A&A, 539, A127 Karabal E., Duc P.-A., Kuntschner H., Chanial P., Cuillandre J.-C., Gwyn S., 2017, A&A, 601, A86 Lisenfeld U., Braine J., Duc P.-A., Brinks E., Charmandaris V., Leon S., 2004, A&A, 426, 471 Mart´ınez-Delgado D. et al., 2010, AJ, 140, 962 Mihos J. C., Harding P., Rudick C. S., Feldmeier J. J., 2013, ApJ, 764, L20 Miville-Deschenes ˆ M.-A., Duc P.-A., Marleau F., Cuillandre J.-C., Didelon P., Gwyn S., Karabal E., 2016, A&A, 593, A4 O’Sullivan E., Giacintucci S., Vrtilek J. M., Raychaudhury S., David L. P., 2009, ApJ, 701, 1560 Renaud F., Appleton P. N., Xu C. K., 2010, ApJ, 724, 80 Trancho G., Konstantopoulos I. S., Bastian N., Fedotov K., Gallagher S., Mullan B., Charlton J. C., 2012, ApJ, 748, 102 Trinchieri G., Sulentic J., Pietsch W., Breitschwerdt D., 2005, A&A, 444, Figure 3. WISE 12µm (bottom) and optical CFHT g-band (top) images Trujillo I., Fliri J., 2016, ApJ, 823, 123 of the same field, showing contamination by emission from Galactic cirrus. Williams B. A., Yun M. S., Verdes-Montenegro L., 2002, AJ, 123, 2417 The RA and Dec. coordinates in J2000 are indicated. Xu C. K. et al., 2005, ApJ, 619, L95 de l’Univers of the Centre National de la Recherche Scientifique (CNRS) of France, and the University of Hawaii. We warmly thank the CFHT QSO team. The data used in this Letter were initially made to produce a true colour image shown in the 2018 version This paper has been typeset from a T X/LT X file prepared by the author. E E MNRASL 475, L40–L44 (2018) Downloaded from https://academic.oup.com/mnrasl/article-abstract/475/1/L40/4794952 by Ed 'DeepDyve' Gillespie user on 16 March 2018 http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png Monthly Notices of the Royal Astronomical Society Letters Oxford University Press

Revisiting Stephan's Quintet with deep optical images

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MNRAS 475, L40–L44 (2018) doi:10.1093/mnrasl/sly004 Advance Access publication 2018 January 9 1‹ 2 3 Pierre-Alain Duc, Jean-Charles Cuillandre and Florent Renaud Universite´ de Strasbourg, CNRS, Observatoire astronomique de Strasbourg, UMR 7550, F-67000 Strasbourg, France IRFU, CEA, Universite´ Paris-Saclay, Universite´ Paris Diderot, AIM, Sorbonne Paris Cite, ´ CEA, CNRS, Observatoire de Paris, PSL Research University, F-91191 Gif-sur-Yvette Cedex, France Department of Astronomy and Theoretical Physics, Lund Observatory, Box 43, SE-221 00 Lund, Sweden Accepted 2018 January 5. Received 2018 January 5; in original form 2017 November 6 ABSTRACT Stephan’s Quintet, a compact group of galaxies, is often used as a laboratory to study a number of phenomena, including physical processes in the interstellar medium, star formation, galaxy evolution, and the formation of fossil groups. As such, it has been subject to intensive multiwavelength observation campaigns. Yet, models lack constrains to pin down the role of each galaxy in the assembly of the group. We revisit here this system with multiband deep optical images obtained with MegaCam on the Canada–France–Hawaii Telescope (CFHT), focusing on the detection of low surface brightness (LSB) structures. They reveal a number of extended LSB features, some new, and some already visible in published images but not discussed before. An extended diffuse, reddish, lopsided, halo is detected towards the early- type galaxy NGC 7317, the role of which had so far been ignored in models. The presence of this halo made of old stars may indicate that the group formed earlier than previously thought. Finally, a number of additional diffuse filaments are visible, some close to the foreground galaxy NGC 7331 located in the same field. Their structure and association with mid-infrared emission suggest contamination by emission from Galactic cirrus. Key words: techniques: photometric – galaxies: interactions – galaxies: photometry – galaxies: stellar content. and to explore the underlying physical processes. Current models in- 1 INTRODUCTION volve a number of events in the last half Gyr, including the collision Stephan’s Quintet (SQ), a compact group of five galaxies located between the spiral galaxy NGC 7319 and NGC 7320c, creating the at a distance of 85 Mpc, is arguably the poster child for this class so-called outer tail, between NGC 7319 and the early-type galaxy of objects. This system exhibits a remarkable variety of physical NGC 7318a creating the inner tail, and finally the late high speed processes at all scales, pertaining to cooling and heating of the collision with the spiral NGC 7318b, producing shocks in the gas interstellar medium (e.g. Appleton et al. 2017), star formation in stripped by the previous interactions (see Fig. 1). So far, the lack of shock regions (e.g. Guillard et al. 2012), star cluster formation observed tidal features between a fifth galaxy, NGC 7317, and the (e.g. Trancho et al. 2012), tidal dwarf galaxy (TDG) formation other group members suggested this galaxy had not yet interacted (e.g. Lisenfeld et al. 2004), not to mention the repeated galaxy with the group. collisions and group formation. Not surprisingly, this remarkable Past studies of SQ have mostly focused on tracers of rela- system has been observed at all wavelengths, from the X-rays (e.g. tively young events, such as the cold gas (HI, molecular), the ion- O’Sullivan et al. 2009) and ultraviolet (UV; e.g. Xu et al. 2005)to ized/shocked gas or the dust emission. However, few analyses have the far-infrared (far-IR; e.g. Appleton et al. 2013) and radio HI (e.g. yet been carried out on the old stellar populations, although they Williams, Yun & Verdes-Montenegro 2002). can be used to reconstruct the mass assembly of the group to older Several attempts have been carried out to reproduce numerically times and help constraining the scenarios. Mapping the faint diffuse the full variety of morphological features of the group, including stellar haloes and fine structures around the galaxies is particularly multiple tidal tails, and complex velocity fields (e.g. Renaud, Apple- helpful for this purpose. Such a study requires images with rela- ton & Xu 2010; Hwang et al. 2012), to propose formation scenarios tively large fields of view, which was actually the case when, until the 1980s, photography with plates and Schmidt telescopes was available. A rather deep image of the field of SQ was presented by Arp & Kormendy (1972). At that time when the mutual distances E-mail: pierre-alain.duc@astro.unistra.fr 1 −1 −1 between all group member candidates were still discussed, the mo- We adopt here H = 75 km s Mpc . At the assumed distance, 10 arcsec corresponds to 4.1 kpc tivation for such observations was the detection of stellar bridges 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/475/1/L40/4794952 by Ed 'DeepDyve' Gillespie user on 16 March 2018 Deep imaging of Stephan’s Quintet L41 Figure 1. Top left-hand panel: composite u + g + r true colour image of the SQ. Top right-hand panel: g-band surface brightness map, with the principle −2 structures labelled. The faintest emission (with surface brightness above 26 mag.arcsec ) revealed by the deep CFHT MegaCam image is shown in red. Bottom −1 19 −2 left-hand panel: u − r colour map with HI/VLA map in the velocity range 6475–6755 km s superimposed. The lowest contour is 6× 10 cm (adapted from Williams et al. 2002). Selected intergalactic star-forming regions are labelled. Bottom right-hand panel: g − r colour map with archival X-ray/XMM–Newton −2 contours superimposed. For the surface brightness and colour maps, the scale in mag, respectively, mag arcsec is indicated to the right. The field of view is 10 arcmin × 10 arcmin (250 kpc × 250 kpc). North is up and east left. Palma. With a 2000-s exposure time, they reached a local surface between the group and the spiral galaxy NGC 7331. Indeed, Arp −2 brightness limit of 27.3 mag arcsec , allowing them to disclose a & Kormendy (1972) detected some streams, but could not firmly diffuse halo around SQ, and to trace back the known tidal tails in conclude on their origin. Gutierrez ´ et al. (2002) obtained so far the the system to much larger distances. The field of view was however deepest optical image of SQ, using R band observations with the too limited to investigate the structures previously found by Arp & Wide Field Camera on the 2.5-m Isaac Newton Telescope at La Kormendy (1972) between SQ and NGC 7331. Meanwhile, with the advent of new generation of wide field of view cameras coupled with new techniques optimised for the de- Which is now known to be in the foreground and unrelated to SQ, but tection of low surface brightness (LSB) structures, deep optical perhaps not to the other foreground galaxy NGC 7320. MNRASL 475, L40–L44 (2018) Downloaded from https://academic.oup.com/mnrasl/article-abstract/475/1/L40/4794952 by Ed 'DeepDyve' Gillespie user on 16 March 2018 L42 P.-A. Duc, J.-C. Cuillandre and F. Renaud imaging of nearby galaxies has been rejuvenated (e.g. Mart´ınez- nucleus should instead be concentric as illustrated by the light pro- Delgado et al. 2010; Ferrarese et al. 2012; Mihos et al. 2013; files of the bright foreground stars surrounding the galaxy. Duc et al. 2015; Trujillo & Fliri 2016). We revisit here SQ and its Since the stellar halo traces a dynamically hot morphology (as surroundings using multiband deep optical images obtained at the opposed to sharp tidal tails), it is likely that its stars have been Canada–France–Hawaii Telescope (CFHT. We gain new insights, stripped from the elliptical galaxy NGC 7317 during an interaction in particular, on the role of NGC 7317, and gather clues to prepare with one (or more) of the group members. The lack of tidal arms for the next generation of models for this group, in particular, and pointing towards a specific disc galaxy forbids us to designate a of studies of repeated interactions, in general. culprit though an implication of NGC 7318b should be excluded because of it high radial velocity. Such arm(s) either never formed (e.g. because of a retrograde encounter), or have been erased by 2 OBSERVATIONS AND DATA REDUCTION subsequent interactions, which would suggest that NGC 7317 has been involved early in the construction of the group. Deep multiband images of the field encompassing SQ and the spiral galaxy NGC 7331 have been obtained with the MegaCam camera installed on the CFHT, through Director Discretionary Time (DDT, 3.2 Young and old tails PI: J.-C. Cuillandre) from 2015 July to December. Images have been SQ is known for hosting multiple intergalactic blue regions, with star acquired with an observing strategy optimized for the detection of formation triggered by shocks or compressive tides in the stripped LSB objects. It relies on (a) acquiring consecutively seven individ- gas (e.g. Iglesias-Paramo ´ et al. 2012). The MegaCam u-band image ual images with large offsets between them, such that the distance reveals new ones and provides some details on their structure. We between each image centre is at least 10 arcmin, a scale larger than draw attention on the region referred as SQ-HII in Fig. 1 (bottom the LSB features around SQ studied here; (b) building a master left-hand panel) that coincides with the northern tip of an HI tail, and sky median-stacking the individual images; and (c) subtracting a forms a jet-like stream with regularly-spaced giant star complexes. smoothed and rescaled version of the master sky from each indi- Several other filamentary structures are observed in the eastern vidual image before stacking them. The procedure, fully described region of the SQ. The principle ones are labeled on the top right- in Duc et al. (2015), is identical to the one used for the MATLAS hand panel of Fig. 1. The tail apparently coming from NGC 7319, project. The final stacked images consist of 4 × 7 frames with total known as the inner tail, hosts massive star clusters, which are 150– exposure time of 56 min for the u band, and 28 min for the g and r 200 Myr old (Fedotov et al. 2011), and prominent star-forming bands. Following the DDT policy, observations were obtained with condensations, including the TDG candidate, SQ-B (Lisenfeld et al. poor seeing conditions with respect to the CFHT standards (aver- 2004). At the location of a CO-detected dust lane towards SQ-B, the age seeing 1.8 arcsec), but under photometric conditions. Images tail seems to split. One branch, labelled IT1, extending eastwards, were processed by the ELIXIR-LSB pipeline. The estimated local lim- was not known before. It is diffuse, and lacks star-forming regions. −2 iting surface brightnesses of 29.0, 28.6, and 27.6 mag arcsec for, The other branch, labeled IT2, bifurcates to the north, crossing the respectively, the u, g,and r bands assess the ability of detecting other TDG candidate, SQ-tip, and ends in the scattered star-forming structures of typical size 5 arcsec × 5 arcsec. knots mentioned above. The difference in gas content (and thus star formation activity) in the two branches remains to be explained. The so-called outer tail, observed to the south, disappears on 3 RESULTS its western side towards the foreground galaxy NGC 7320. The The CFHT images provide a panoramic view of the stellar popu- deep CFHT image undoubtedly confirms that it points to the east lations that are present all across the group, up to large distances: towards NGC 7320c, the barred galaxy believed to have crossed the intergalactic young stars and star clusters, distributed along blue, group about 0.5 Gyr ago. Interestingly, the rather blue colour of this narrow, and clumpy filaments, old stars present in reddish diffuse tail (u − r = 1.4, see Fig. 1, bottom left-hand panel), testifies the tails or extended haloes, with a possible contamination by filamen- presence of intermediate age stars, but star formation seems to have tary structures associated with cirrus emission. stopped there: The eastern tip of the tail is not detected in UV in GALEX images we collected from the archives. As shown in Fig. 1 (bottom left-hand panel), there is no HI cloud with a column density 3.1 The diffuse halo 19 −2 larger than 6× 10 cm at this location. SQ appears to be embedded in a diffuse reddish halo, which is especially prominent towards the south-west, near NGC 7317. This 3.3 Isolated filamentary structures: cirrus emission early-type galaxy is off-centred with respect to the outer isophotes of the halo. The diffuse emission extends up to 60 kpc from the galaxy. Fig. 2 shows a composite image of the whole MegaCam field cover- As seen in the true colour image (Fig. 1, top left-hand panel), but ing about 1 square degree. Several extended filamentary structures even more clearly on the calibrated u − r and g − r colour maps with colours varying between green and yellow (indicating that they (Fig. 1, bottom), the diffuse light is much redder to the south-west are most prominent in the g and r bands) can be seen between SQ than towards the north of the compact group. The average colours in the south, and the spiral galaxy NGC 7331 in the north, in addi- (u − r = 2.6 mag, g − r = 0.9 mag) correspond to typical Gyr old tion to many faint foreground stars, extended ghost haloes of bright stellar populations. Only a mild negative g − r colour gradient of nearby stars, and background galaxies. 0.05 dex per 10 kpc is observed. Most of the several isolated narrow filamentary features scattered We note that PSF effects that may affect the light and colour in the whole field (but more numerous to the north, and globally distributions of galaxies (see the study of Karabal et al. 2017, based sharing an east–west orientation) show a clear counterpart in the on MegaCam images) cannot explain the specific properties of the mid-infrared (mid-IR) images (12 µm, Fig. 3), which we queried diffuse, off-centred, light around NGC 7317. At its location on from the WISE archives using the CDS Aladin tool. These struc- the MegaCam image, instrumental ghosts generated by the galaxy tures are thus undoubtedly Galactic cirrus, and not extragalactic MNRASL 475, L40–L44 (2018) Downloaded from https://academic.oup.com/mnrasl/article-abstract/475/1/L40/4794952 by Ed 'DeepDyve' Gillespie user on 16 March 2018 Deep imaging of Stephan’s Quintet L43 Figure 2. Composite u + g + r band image of the whole CFHT/MegaCam field, encompassing SQ (bottom right-hand panel) and NGC 7331 (top left-hand panel). The field of view is 70 arcmin × 60 arcmin. North is up and east left. An arcsinh scaling has been used to enhance the LSB features. tidal structures as initially suggested, among other hypotheses, by 7320c is clearly confirmed, proving the active role of this galaxy in Arp & Kormendy (1972). Scattered optical light by dust clouds in the dynamical history of the group. the Milky Way hampers extragalactic studies based on deep images, The reddish halo surrounding SQ, which is most prominent to- as they mimic stellar streams. They however provide valuable infor- wards NGC 7317, provides indications on the implication of this mation on the structure of the ISM at small scales, as emphasized galaxy in the group’s history. The size of the halo, reminiscent of by Miville-Deschenes ˆ et al. (2016). the intracluster light in clusters and fossil groups, could be con- Getting closer to SQ, the plume apparently dragging on from the sistent with a group formation several Gyr ago. A remarkable cor- diffuse halo of NGC 7317 towards the south-west, and best seen in respondence between the diffuse component of the X-ray emis- the r band, might be a Galactic cirrus as well. Indeed it has a similar sion (as traced by XMM–Newton) and the diffuse optical light had structure and orientation as the other confirmed mid-IR detected been noted by Trinchieri et al. (2005).As showninFig. 1 (bot- cirrus, though the former is likely too faint to be detected by WISE tom right-hand panel), the spatial matching is best observed for the (see Fig. 3). halo around NGC 7317. If the X-ray emission traces its potential well, this might be another indication that the group is older (and more relaxed) than generally believed. Although the peak of the 3.4 The spiral galaxy NGC 7331 X-ray/optical halo emission matches the position of NGC 7317, the galaxy appears off-centred with respect to the outer isophotes. This NGC 7331 is a highly inclined galaxy with prominent spiral arms. disturbed shape could thus be due to a still on-going interaction Given its distance (13.5 Mpc, taken from the NED data base), it with NGC 7319. The presence of a diffuse X-ray emitting hot gas has been argued to form an interacting pair with NGC 7320 (itself halo suggests that ram pressure could perhaps explain why the tips foreground of SQ). However, even at the depth reached here, we of the outer tail and eastern branch of the inner tail are gas poor. do not detect any morphological sign of perturbation in the galactic This study shows how deep imaging can, with the proper ob- disc. As mentioned above, the filamentary structures to the west and serving strategy requiring limited telescope time, bring additional south of the spiral are most likely Galactic cirrus and not tidal tails. constraints on the modelling of interacting systems by providing information on the oldest collisional events traced by the old stellar population. More specifically, all these observed new features call 4 DISCUSSION AND CONCLUSIONS for revised formation scenarios and detailed models of the SQ. To date, models of the SQ attribute most of its tidal features to the interactions between NGC 7319, NGC 7320c, and NGC 7318 a/b, while the role of the early-type galaxy NGC 7317 is generally ACKNOWLEDGEMENTS ignored due to a lack of observational clues. We have revisited this system with deep optical multiband CFHT Based on observations obtained with MegaPrime MegaCam, a images, which exhibit a number of extended LSB structures, some joint project of CFHT and CEA/DAPNIA, at the Canada–France– new, some that were visible in previously published images, but Hawaii Telescope (CFHT), which is operated by the National Re- largely ignored. The connection between the outer tail and NGC search Council (NRC) of Canada, the Institut National des Science MNRASL 475, L40–L44 (2018) Downloaded from https://academic.oup.com/mnrasl/article-abstract/475/1/L40/4794952 by Ed 'DeepDyve' Gillespie user on 16 March 2018 L44 P.-A. Duc, J.-C. Cuillandre and F. Renaud of the CFHT calendar produced in collaboration with Edizioni Sci- entifiche Coelum (R. Zabotti, G. Anselmi). A crop of it is shown in Fig. 1. We thank the referee for his prompt feedback and useful suggestions. FR acknowledges funding from the Knut and Alice Wallemberg Foundation. REFERENCES Appleton P. N. et al., 2013, ApJ, 777, 66 Appleton P. N. et al., 2017, ApJ, 836, 76 Arp H., Kormendy J., 1972, ApJ, 178, L101 Duc P. -A. et al., 2015, MNRAS, 446, 120 Fedotov K., Gallagher S. C., Konstantopoulos I. S., Chandar R., Bastian N., Charlton J. C., Whitmore B., Trancho G., 2011, AJ, 142, 42 Ferrarese L. et al., 2012, ApJS, 200, 4 Guillard P. et al., 2012, ApJ, 749, 158 Gutierrez ´ C. M., Lopez-Corredoira ´ M., Prada F., Eliche M. C., 2002, ApJ, 579, 592 Hwang J.-S., Struck C., Renaud F., Appleton P. N., 2012, MNRAS, 419, Iglesias-Paramo ´ J., Lopez-Mart ´ ´ın L., V´ılchez J. M., Petropoulou V., Sulentic J. W., 2012, A&A, 539, A127 Karabal E., Duc P.-A., Kuntschner H., Chanial P., Cuillandre J.-C., Gwyn S., 2017, A&A, 601, A86 Lisenfeld U., Braine J., Duc P.-A., Brinks E., Charmandaris V., Leon S., 2004, A&A, 426, 471 Mart´ınez-Delgado D. et al., 2010, AJ, 140, 962 Mihos J. C., Harding P., Rudick C. S., Feldmeier J. J., 2013, ApJ, 764, L20 Miville-Deschenes ˆ M.-A., Duc P.-A., Marleau F., Cuillandre J.-C., Didelon P., Gwyn S., Karabal E., 2016, A&A, 593, A4 O’Sullivan E., Giacintucci S., Vrtilek J. M., Raychaudhury S., David L. P., 2009, ApJ, 701, 1560 Renaud F., Appleton P. N., Xu C. K., 2010, ApJ, 724, 80 Trancho G., Konstantopoulos I. S., Bastian N., Fedotov K., Gallagher S., Mullan B., Charlton J. C., 2012, ApJ, 748, 102 Trinchieri G., Sulentic J., Pietsch W., Breitschwerdt D., 2005, A&A, 444, Figure 3. WISE 12µm (bottom) and optical CFHT g-band (top) images Trujillo I., Fliri J., 2016, ApJ, 823, 123 of the same field, showing contamination by emission from Galactic cirrus. Williams B. A., Yun M. S., Verdes-Montenegro L., 2002, AJ, 123, 2417 The RA and Dec. coordinates in J2000 are indicated. Xu C. K. et al., 2005, ApJ, 619, L95 de l’Univers of the Centre National de la Recherche Scientifique (CNRS) of France, and the University of Hawaii. We warmly thank the CFHT QSO team. The data used in this Letter were initially made to produce a true colour image shown in the 2018 version This paper has been typeset from a T X/LT X file prepared by the author. E E MNRASL 475, L40–L44 (2018) Downloaded from https://academic.oup.com/mnrasl/article-abstract/475/1/L40/4794952 by Ed 'DeepDyve' Gillespie user on 16 March 2018

Journal

Monthly Notices of the Royal Astronomical Society Letters – Oxford University Press

Published: Jan 9, 2018

Keywords: techniques: photometric; galaxies: interactions; galaxies: photometry; galaxies: stellar content

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