Understanding low and high velocity dispersion compact groups
Abstract
Description
A galaxy system must have a minimum velocity dispersion for its mass to be greater than the sum of the masses of its galaxies. Nearly half of the nearby Hickson compact groups (HCGs) have too low a velocity dispersion in comparison with the rotational velocities of their spiral galaxies and internal velocity dispersions of their early types.
A detailed study of the low velocity dispersion group, HCG 16 -- the only known group of late-type galaxies with diffuse intergalactic X-ray emitting hot gas -- reveals that half of the diffuse X rays are associated with foreground/background sources and the remaining gas is clumpy and mostly associated with the bright galaxies of the group. The large-scale environment of the group suggests that HCG 16 lies where a cosmological filament falls perpendicularly onto a large-scale sheet.
The observed frequency of compact groups is lower than predicted from the extended Press-Schechter formalism, which also predicts that most 10^13 Msun objects in the Universe must be fairly old and hence have already coalesced into single objects, reminiscent of elliptical galaxies over-luminous in X-rays that are now being discovered.
Thus, the low survival time of dense groups against the merging instability is no longer a worry for compact groups, as they form in large enough numbers. I show why other arguments against the reality of HCGs no longer hold, partly because of the biases of Hickson's sample.
To appear in IAU Colloq. 174, Small Galaxy Groups (meeting held in Turku, Finland, June 1999), ed. M. Valtonen & Chris Flynn (San Francisco: ASP). 8 pages LaTeX2e, uses graphics.sty and journals.sty (included), 5 encapsulated PS figures. Full resolution paper available at ftp://ftp.iap.fr/pub/from_users/gam/PAPERS/iau174.ps.gz (1261967 bytes)
To appear in IAU Colloq. 174, Small Galaxy Groups (meeting held in Turku, Finland, June 1999), ed. M. Valtonen & Chris Flynn (San Francisco: ASP). 8 pages LaTeX2e, uses graphics.sty and journals.sty (included), 5 encapsulated PS figures. Full resolution paper available at ftp://ftp.iap.fr/pub/from_users/gam/PAPERS/iau174.ps.gz (1261967 bytes)