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Understanding low and high velocity dispersion compact groups

Published online by Cambridge University Press:  12 April 2016

G. A. Mamon
G. A. Mamon*
Affiliation:
IAP, F-75014 Paris, FRANCE

Abstract

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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 1013 M⊙ 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.

Type
Part 2. Properties of Small Groups
Information
International Astronomical Union Colloquium , Volume 174: Small Galaxy Groups , 2000 , pp. 217 - 225
Copyright
Copyright © Astronomical Society of the Pacific 2000

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