Numerous similar groups exist nearby

Numerous similar groups exist nearby - when surveyed to...

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Numerous similar groups exist nearby - Sculptor, M81, the M101 or CVn cloud, etc. In Virgo we find a much richer aggregation, at the cluster core. Here are giant ellipticals (with M87 at the center being a D galaxy), spirals of all types, dwarfs, etc., making this a crucial environment for calibrating distace indicators. A few times farther away, we find the Hydra I and Centaurus clusters (not to be confused with the Cen A group), beyond which things get populous as more volume is sampled. Coma (about 5 times the Virgo distance) is the nearest rich, regular cluster and a useful standard of comparison. Even some ``small groups" contain of order 100 members
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Unformatted text preview: when surveyed to faint levels, as seen in the map of the Fornax group from Ferguson and Sandage (AJ 100, 1, courtesy of the AAS): Groups may be picked out (with redshift data) in several ways. Local density contrast and "friends-of-friends" techniques are popular, remembering that velocities don't tell much about distance within a group since group velocity dispersions are comparable to the internal velocities in luminous galaxies (~ 300 km/s). Numerous papers have catalogued groups in various ways; the main point may be that most galaxies can be plausibly assigned to groups at the scale of ours or more numerous....
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