April 2026 - Picture of the Month

Copeland's Septet (Hickson Compact Group 57) in Leo

This month I bring you a wonderful image of a group of galaxies just above the hind-quarters of Leo known as Copeland's Septet, Hickson 57, Arp 320 or a number of other designations, as is often the way.

This group of galaxies was discovered in 1874 by Ralph Copeland with the Parsons 72-inch telescope whilst an assistant to Lord Rosse at Birr Castle in Ireland. For a while this group was lost due to a positioning error by Copeland which carried through to the compilation of the New General Catalogue of Nebulae and Clusters of Stars (NGC) by Dreyer in 1888. Being a septet, Copeland obviously identified seven members in this group, albeit over different nights of observing.

Paul Hickson made it a group of eight members, with the addition his H component, when it was catalogued as Hickson Compact Group 57 (HCG 57) in 1982 ¹. In this paper, Hickson examined 100 compact galaxy groups from the Palomar Observatory Sky Survey –now available as the Digitised Sky Survey (DSS)– with respect to their overall morphology and that of their component galaxies. From this study he attempted to draw conclusions about the dynamical evolution of such groups and their extrapolation to larger galaxy clusters, in which it was believed the density of galaxies was broadly the same.

HCG 57 comprises NGC 3753 (HCG 57A), NGC 3746 (HCG 57B), NGC 3750 (HCG 57C), NGC 3754 (HCG 57D), NGC 3748 (HCG 57E), NGC 3751 (HCG 57F), NGC 3745 (HCG 57G) and 2MASX J11375047+2200450 (LEDA 36010, PGC 36010 and also HCG 57H).

Hickson estimated there to be three spiral galaxies amongst them, but this image would suggest that this was an underestimate, which is a shame because a low proportion of spiral galaxies was felt to set these groups apart from the general background.

The NASA/IPAC Extragalactic Database (NED) gives the Hubble distance to HCG 57 as 136.04±9.53 Mpc (about 443.4 Mly) which matches closely with radio frequency based estimates of 135±3 Mpc, and places the group within the Coma Supercluster. The galaxies themselves range between 434.5 Mly (HCG 57A) and 470.7 Mly (HCG 57G) distant.

A study of the distribution of neutral elemental hydrogen (HI) in 16 of the Hickson Compact Groups (HCG) using data from the Very Large Array (VLA) proposed an evolutionary sequence for these groups ²: in phase 1 HI is confined to the individual galaxies themselves; in phase 2 tidal forces are clearly having their effect and the hydrogen is becoming distributed throughout the group, such tidal features are well in evidence in this image of HCG 57; and by the end of phase 3 the galaxies lack any HI, in fact it may be missing from the group entirely.

In a later paper this mechanism was revisited with an analysis of 38 HCGs and found little correlation between the evolutionary stage of the galaxy group based on its morphology and that of its HI deficiency ³ . The author notes that the phase 3 groups are all deficient in HI, however the difference between phases 1 and 2 is negligible and likely attributable to the amount of HI present in the galaxies at the time the group formed. Additionally, there's always the possibility of replenishment from captured galaxies at a later stage that requires consideration.

Galex shows emission in the far-ultraviolet, which is characteristic of star formation in galaxies, is confined to HCG 57B and HCG 57D with perhaps a hint in HCG 57C. It's probably a coincidence that these three galaxies are also at nearly the same distance.

HCG 57B was host to two supernovae since the turn of this century: SN 2002ar and SN 2005ba, type Ia and type II respectively. It has been noted that HCG 57D seems to be the least reddened of the galaxies in the group with clear signs of active star formation, it's also a LINER-type Active Galactic Nucleus (AGN).

A recent paper examined this group from a high-energy perspective using data from Chandra and XMM-Newton X-ray ⁴. It identified point-like X-ray emissions from HCG 57A, HCG 57B and HCG 57D confirming the presence of AGNs. The authors also note that HCG 57A and HCG 57D form an interacting pair, right in the middle of this image, with HCG 57D having passed through the disk of HCG 57A about 50 million years ago, leaving a substantial gas bridge and areas of shock heating. The turbulent expansion of the resulting shock through the two galaxies are also suggested as a possible mechanism to explain the significant suppression of star formation in the larger HCG 57A. Being much smaller, the shock passed through HCG 57D long ago.

Visual observation of the group doesn't appear to be easy by any means based on Steve Gottlieb's comments, and a large telescope will be necessary, probably 350mm (14-in) or bigger. He notes that the easiest of the galaxies is NGC 3753 (HCG 57A), which is coincidentally also the closest, and that the whole group is possible with persistence and a good sky. As you can see, these compact galaxy groups do make spectacular imaging targets.

James Whinfrey - Website Administrator.

Reference

1. Hickson, P., “Systematic properties of compact groups of galaxies.”, The Astrophysical Journal, vol. 255, IOP, pp. 382–391, 1982. doi:10.1086/159838. https://ui.adsabs.harvard.edu/abs/1982ApJ...255..382H/abstract (Viewed on 25 March 2026).
2. Verdes-Montenegro, L., Yun, M. S., Williams, B. A., Huchtmeier, W. K., Del Olmo, A., and Perea, J., “Where is the neutral atomic gas in Hickson groups?”, Astronomy and Astrophysics, vol. 377, EDP, pp. 812–826, 2001. doi:10.1051/0004-6361:20011127. https://ui.adsabs.harvard.edu/abs/2001A%26A...377..812V/abstract (Viewed on 25 March 2026).
3. Jones, M. G., “Disturbed, diffuse, or just missing? A global study of the H I content of Hickson compact groups”, Astronomy and Astrophysics, vol. 670, Art. no. A21, EDP, 2023. doi:10.1051/0004-6361/202244622. https://ui.adsabs.harvard.edu/abs/2023A%26A...670A..21J/abstract (Viewed on 25 March 2026).
4. O'Sullivan, E., “HCG 57: Evidence for Shock-heated Intergalactic Gas from X-Rays and Optical Emission Line Spectroscopy”, The Astrophysical Journal, vol. 979, no. 2, Art. no. 240, IOP, 2025. doi:10.3847/1538-4357/ada14b. https://ui.adsabs.harvard.edu/abs/2025ApJ...979..240O/abstract (Viewed on 25 March 2026).