Hickson Compact Groups

Contributing Editor Ted Forte offers a look at Hickson Compact Groups in the November 2017 issue of Sky & Telescope.

Hickson Compact Group 92

Five galaxies (NGC 7317, NGC 7318a, NGC 7318b, NGC 7319, and NGC 7320C [outside the frame]) form Hickson Compact Group 92. Spiral galaxy NGC 7320 isn't a proper group member; rather, it's a foreground object. The group is popularly referred to as Stephan's Quintet — visual observers include NGC 7320 in the quintet, but typically overlook the small, distant NGC 7320C. The group is shown here at optical and X-ray wavelengths.
NASA / CXC / CfA / E. O'Sullivan (X-ray); Canada-France-Hawaii-Telescope / Coelum (Optical)

In 1982, Canadian astronomer Paul Hickson published a list of 100 compact galaxy groups based on his examination of the Palomar Observatory Sky Survey (POSS) red plates. A compact group, as defined by Hickson, is "a small, relatively isolated, system of typically four or five galaxies in close proximity to one another." This sounds like a broad definition, but Hickson used a few more specific criteria to select his groups.

Hickson wanted to compile a uniform sample of groups, so as he made his list, he noted the magnitudes and morphological type of each galaxy in the group. To be included in the final catalog, a group had to consist of four or more galaxies with magnitudes differing by less than 3.0. Each group also had to have a mean surface brightness brighter than 26.0 magnitudes per arcsecond of size. He rejected any grouping where a non-member galaxy not more than 3 magnitudes fainter than the brightest member fell within three radii of the center of the smallest circle possible "drawn" around the entire group. In other words, the groups had to be sufficiently isolated from neighboring galaxies to be included in the catalog.

In the November 2017 issue of Sky & Telescope, Contributing Editor Ted Forte introduced us to seven objects from Hickson's catalog. His article includes detailed descriptions from his observing notes. If you're ready to start your own hunt for Hickson compact groups, you can find object tables, including positions, magnitudes, surface brightness, and position angles, here. Just click on the links to download the table in Excel format or as a .pdf.

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