Abundance and group coalescence time-scales of compact groups of galaxies in the EAGLE simulation

Abstract

Observations of compact groups of galaxies (CGs) indicate that their abundance has not significantly changed since z = 0.2. This balance between the time-scales for formation and destruction of CGs is challenging if the typical time-scale for CG members to merge into one massive galaxy is as short as historically assumed (<0.1 Hubble times). Following the evolution of CGs over time in a cosmological simulation (EAGLE), we quantify the contributions of individual processes that in the end explain the observed abundance of CGs. We find that despite the usually applied maximum line-of-sight velocity difference of 1000kms−1 within the group members, the majority of CGs (≈60 per cent) are elongated along the line of sight by at least a factor of 2. These CGs are mostly transient as they are only compact in projection. In more spherical systems ≈80 per cent of galaxies at z > 0.4 merge into one massive galaxy before the simulation end (z = 0) and we find that the typical time-scale for this process is 2–3 Gyr. We conclude that the combination of large fractions of interlopers and the longer median group coalescence time-scale of CGs alleviates the need for a fast formation process to explain the observed abundance of CGs for z < 0.2.