Papers › The Occurrence of Compact Groups of Galaxies Through Cosmic Time
The Occurrence of Compact Groups of Galaxies Through Cosmic Time
Christopher D. Wiens, Trey V. Wenger, Panayiotis Tzanavaris, Kelsey E. Johnson, S. C. Gallagher, Liting Xiao
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We use the outputs of a semi-analytical model of galaxy formation run on the Millennium Simulation to investigate the prevalence of 3D compact groups (CGs) of galaxies from z = 11 to 0. Our publicly available code identifies CGs using the 3D galaxy number density, the mass ratio of secondary+tertiary to the primary member, mass density in a surrounding shell, the relative velocities of candidate CG members, and a minimum CG membership of three. We adopt "default" values for the first three criteria, representing the observed population of Hickson CGs at z = 0. The percentage of non-dwarf galaxies (M > 5 ×10⁸h⁻¹ M_⊙) in CGs peaks near z ∼2 for the default set, and between z ∼1 - 3 for other parameter sets. This percentage declines rapidly at higher redshifts (z ≳4), consistent with the galaxy population as a whole being dominated by low-mass galaxies excluded from this analysis. According to the most liberal criteria, ≲3% of non-dwarf galaxies are members of CGs at the redshift where the CG population peaks. Our default criteria result in a population of CGs at z < 0.03 with number densities and sizes consistent with Hickson CGs. Tracking identified CG galaxies and merger products to z = 0, we find that ≲16% of non-dwarf galaxies have been CG members at some point in their history. Intriguingly, the great majority (96%) of z = 2 CGs have merged to a single galaxy by z= 0. There is a discrepancy in the velocity dispersions of Millennium Simulation CGs compared to those in observed CGs, which remains unresolved.
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