Compact groups pose some special puzzles

Compact groups pose some special puzzles - and HCG 87 to...

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Compact groups pose some special puzzles. As listed, for example, by Hickson (1982 ApJ 255, 382), they consist of 4-7 galaxies within an area of only a few hundred kpc diameter. They contain more spirals than expected from the usual morphology-density relation, and have very short predicted lifetimes against merging. Furthermore, the number of position-selected members with discordant redshifts is rather high (~1/3); this makes Stephan's Quintet (below) a very good prototype of all the issues involved. Various workers have considered them to be real but young, the dregs of a once-rich population, constantly forming from more diffuse group environments, long filaments seen lengthwise, and fictitious chance alignments. Recent work on Stephan's Quintet has muddied the waters further, implying a crucial role for bound or captured high- velocity members in pumping the group's energy and keeping it from merging. See the WFPC2 images of Stephan's Quintet , Seyfert's Sextet or NGC 6027
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Unformatted text preview: , and HCG 87 to watch some of these interactions at work. Genuine clusters exhibit a wide range of galaxy population and content, and are luminous enough to be sampled across large distances. I'll stress at the outset that the visible galaxies are such a minor part of the whole assemblages, and are probably even outweighed by the hot intracluster gas, that "cluster of galaxies" is downright misleading, but we're stuck with it. To make matters worse, I've split off treatment of the gas to a separate section , and the treatment of the galaxy content of clusters and its evolution to the "environmental impact" section. Some order has been brought to cluster study by several classification schemes (see review by N. Bahcall 1977 (ARA&A 15, 505). These may be based on: concentration (compact to open) distribution of brightest members presence or absence of a cD galaxy subclustering morphology of dominant galaxies...
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