Annual Meeting of the Deep Sky Section, 2011 March 12

Galaxy Clusters for the Amateur

Mr Brazell opened by explaining that he would be talking about objects that were variously termed as either galaxy groups or galaxy clusters, but that the distinction between these terms was not well defined. Galaxy clusters generally had more known members than groups – perhaps more than thirty to fifty – but in some cases the nomenclature was determined by historical convention. For example, the Local Group of galaxies, of which the Milky Way was a member, was termed a group despite having more than forty known members.

Several catalogues provided amateurs with lists of targets that might be attempted. Perhaps the most approachable of these was the Hickson Compact Groups of Galaxies (HCG) catalogue⁹, compiled by Paul Hickson in 1982. This presented the amateur with a manageable number of groups – exactly one hundred – typically comprising 8–10 members and comfortably within range of large-aperture amateur telescopes given a dark site. Among the catalogue's members were a number of well-known objects, including Stephan's Quintet (HCG 92) in Pegasus.

The more ambitious amateur might turn to the Shakhbazian Compact Groups of Galaxies (SHK) catalogue, compiled by Armenian astronomer Romelia Shakhbazian et al. over the years 1973–79 by systematically searching the plates of the Palomar Observatory Sky Survey (POSS). It listed 377 groups, typically comprising 5–15 members – so often richer than the members of the Hickson catalogue – but often at seriously challenging magnitudes in the range 14–19.

The Abell Catalogue of Rich Clusters of Galaxies presented a larger sample of 2,712 northern-hemisphere clusters, also found by searching the plates of the Palomar Observatory Sky Survey (POSS), and published by George Abell in 1958. The selection criterion used by Abell was that clusters had to contain around thirty or more bright members to be included. In 1989, this catalogue had been extended to cover the southern sky, enlarging it to 4,076 members.

Other catalogues of historical interest included Fwitz Zwicky's Catalogue of Galaxies and Clusters of Galaxies (CGCG), compiled over the years 1961–68, which offered a more complete set of 9,134 objects, but this was now rarely used. Halton Arp's Atlas of Peculiar Galaxies, published in 1966, listed 377 members, many of which were closely interacting pairs of galaxies.

To observe any galaxy group or cluster, it was essential to have a dark sky – few locations in the UK were suitable – and a good finder chart; even those using telescopes with digital setting circles would need the latter in order to identify and navigate around the members of richer clusters such as the Virgo cluster. Several computer packages and online resources were able to provide such charts. Mr Brazell added that, though he had a personal preference for undriven telescopes, a drive was hugely beneficial when seeking out very faint objects. Access to high quality eyepieces with a range of powers was also helpful – low powers being used for navigation and higher powers for trying to resolve fine detail within individual galaxies.

Turning to show images of a few of the best known clusters, Mr Brazell began with the Virgo cluster as it was the most accessible, containing a number of Messier objects as well as many much fainter members; it offered challenges with a wide range of difficulty levels. Wide-field imagers might like to try stitching together an image of the whole cluster, though the large area that it covered on the sky made it particularly necessary to have a good finder chart; it was easy to get lost. The Coma cluster, a little to the north and at a greater distance away from us, offered a tighter and slightly fainter condensation. One of the challenges here was the magnitude 7.2 field star HIP 63405 in the midst of the cluster, which could be rather dazzling in a large-aperture telescope.

Other targets which were accessible from the UK at this time of year included the Leo Triplet, comprising M65, M66 and the more challenging NGC 3628. Nearby, Hickson 44 offered a more challenging quartet of NGCs 3185, 3187, 3190 and 3193. The faintest member, NGC 3187, was especially challenging.

To close, Mr Brazell listed various books which were available to amateurs interested in observing these objects. The relevant volume of the Webb Society's Deep Sky Observer's Handbook¹⁰ was now out of print, but amateur astronomer Alvin Huey maintained a website¹¹ which offered a wealth of information together with several self-published observing guides available for purchase.

Following the applause, Dr Nick Hewitt commented that collecting observations of all of the Hickson groups might be a nicely sized observing project for the Section to take on, given the catalogue's modest number of members. The meeting then broke for lunch, during which some audience members observed a solar flare through a solar telescope fitted with an Hα filter. After the break, the Director invited Grant Privett to speak.