The Hubble Space Telescope (HST) image of Stephan’s Quintet — a group of four galaxies and NGC 7320, a foreground cuckoo in the nest — is perhaps one of the most famous HST images ever. It provides a fascinating opportunity to study the interactions that occur between neighbouring galaxies. Now, using the MegaCam camera installed on the Canada–France–Hawaii telescope, Pierre-Alain Duc and collaborators (Mon. Not. R. Astron. Soc. Lett. 475, L40–L44; 2018) have obtained a deep- and wide-field view of this region (pictured) in order to study the low surface brightness features that may be as-of-yet unrecognized details of the interaction processes.
The image is a 70 × 60 arcmin view of the region around Stephan’s Quintet (circled) created from a composite of u-, g- and r-band images. Several of the tidal features within the Quintet itself, including an outer tail resulting from a collision between NGC 7319 and NGC 7320c, and an inner tail resulting from a collision between NGC 7319 and NGC 7318, have been well studied in the past. However, the lack of wide-field observations has hindered the study of the large-scale low-brightness features, such as stellar haloes and bridges. With this stacked MegaCam image, which took 56, 28 and 28 minutes (u, g, r) to observe, features with a limiting local surface brightness of 29.0, 28.6 and 27.6 mag arcsec–2 can be seen. Green-yellow narrow filamentary features scattered in particular between Stephan’s Quintet and the bright spiral galaxy NGC 7331 in the upper part of the image are Galactic cirrus, and not tidal structures, as was previously suggested. Similarly, there are no signs of interaction between the foreground galaxies NGC 7331 and NGC 7320.
The most significant finding from this deep image is the reddish stellar halo surrounding the Quintet itself (not visible in the wide-field image), representative of older stellar populations. Its size is consistent with the formation of the galaxy group several billion years ago. The halo is most prominent towards NGC 7317, the westernmost member of the group, which corresponds to a region of diffuse X-ray emission in XMM-Newton images. The slight difference in X-ray and optical peak emission may indicate ongoing interaction with northern neighbour NGC 7319.