Compact groups from the Millennium Simulations: I. Their Nature and the completeness of the Hickson sample

Abstract

We identify compact groups of galaxies (CGs) within mock galaxy catalogues from the Millennium Simulation at z=0 with three semi-analytic models (SAMs) of galaxy formation. CGs are identified using the same 2D criteria as those visually applied by Hickson (1982) to his CGs (HCGs), but with a brightest galaxy magnitude limit, and the blending of close projected pairs. Half of the mock CGs identified in projection contain at least 4 accordant velocities (mvCGs), versus 70% for HCGs. In comparison to mvCGs, the HCGs are only 8% complete at distances < 9000 km/s, missing the CGs with small angular sizes, a strongly dominant galaxy, and (for one SAM) the CGs that are fainter and those with lower surface brightness. We explore different ways to determine the fraction of physically dense groups. Binding energy criteria turn out to be inapplicable given the segregation between galaxies and dark matter particles. We rely instead on the combination of the 3D length of the CGs (maximum real space galaxy separation) and their elongation along the line-of-sight (ratio of maximum line-of-sight to maximum projected separations), restricting ourselves in both cases to smallest quartets within the CGs. We find that between 64% and 80% (depending on the SAM) of the mvCGs have 3D lengths shorter than 200 kpc/h, between 71% and 80% have line-of-sight elongations less than 2, while between 59% and 76% have either 3D lengths shorter than 100 kpc/h or both lengths shorter than 200 kpc/h and elongations smaller than 2. Therefore, chance alignments (CAs) of galaxies concern at most 40% of the mvCGs. These CAs are mostly produced from larger host groups, but a few have galaxies extending a few Mpc beyond the host group. The mvCGs built with the Hickson selection with (without) the close projected pair blending criterion have 10% higher (lower) fractions of physically dense systems.

19 pages, 10 figures. Accepted for publication in MNRAS. The new version has a different title and a different author list. The analysis of the nature of compact groups has been thoroughly modified, while the subsections on compact group properties and correlations have been omitted and should appear elsewhere

References in corpus (11)

• The hierarchical formation of the brightest cluster galaxies
• Resolving Cosmic Structure Formation with the Millennium-II Simulation
• A fitting formula for the merger timescale of galaxies in hierarchical clustering
• The recycling of gas and metals in galaxy formation: predictions of a dynamical feedback model
• The virialized mass of dark matter haloes
• Compact groups in theory and practice - III. Compact groups of galaxies in the Sixth Data Release of the Sloan Digital Sky Survey
• Compact groups in theory and practice - I. The spatial properties of compact groups
• The K luminosity-metallicity relation for dwarf galaxies and the tidal dwarf galaxies in the tails of HCG 31
• Properties of Hickson Compact Groups and of the Loose Groups within which they are Embedded
• The nature of the nearest compact group of galaxies from precise distance measurements
• The scale dependence of mass assembly in galaxies

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