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physically detached 'compact groups'A small fraction of galaxies appear to reside in dense compact groups, whose inferred crossing times are much shorter than a Hubble time. These short crossing times have led to considerable disagreement among researchers attempting to deduce the dynamical state of these systems. In this paper, we suggest that many of the observed groups are not physically bound but are chance projections of galaxies well separated along the line of sight. Unlike earlier similar proposals, ours does not require that the galaxies in the compact group be members of a more diffuse, but physically bound entity. The probability of physically separated galaxies projecting into an apparent compact group is nonnegligible if most galaxies are distributed in thin filaments. We illustrate this general point with a specific example: a simulation of a cold dark matter universe, in which hydrodynamic effects are included to identify galaxies. The simulated galaxy distribution is filamentary and end-on views of these filaments produce apparent galaxy associations that have sizes and velocity dispersions similar to those of observed compact groups. The frequency of such projections is sufficient, in principle, to explain the observed space density of groups in the Hickson catalog. We discuss the implications of our proposal for the formation and evolution of groups and elliptical galaxies. The proposal can be tested by using redshift-independent distance estimators to measure the line-of-sight spatial extent of nearby compact groups.
Document ID
19950054903
Document Type
Reprint (Version printed in journal)
External Source(s)
Authors
Hernquist, Lars
(Univ. of California at Santa Cruz, Santa Cruz, CA United States)
Katz, Neal
(Univ. of Washington, Seattle, WA United States)
Weinberg, David H.
(Institute for Advanced Study, Princeton, NJ United States)
Date Acquired
August 16, 2013
Publication Date
March 20, 1995
Publication Information
Publication: The Astrophysical Journal, Part 1
Volume: 442
Issue: 1
ISSN: 0004-637X
Subject Category
ASTROPHYSICS
Funding Number(s)
CONTRACT_GRANT: NSF AST-90-18526
CONTRACT_GRANT: NSF PHY-92-45317
CONTRACT_GRANT: NAGW-2422
CONTRACT_GRANT: NAG5-2213
CONTRACT_GRANT: NSF ASC-93-18185
CONTRACT_GRANT: NAGW-2523
Distribution Limits
Public
Copyright
Other