Abundance and group coalescence timescales 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 timescales for formation and destruction of CGs is challenging if the typical timescale 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 $1000\,\mathrm{km\,s}^{-1}$ within the group members, the majority of CGs ($\approx 60$ per cent) are elongated along the line-of-sight by at least a factor of two. These CGs are mostly transient as they are only compact in projection. In more spherical systems $\approx 80$ 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 timescale for this process is 2-3 Gyr. We conclude that the combination of large fractions of interlopers and the longer median group coalescence timescale of CGs alleviates the need for a fast formation process to explain the observed abundance of CGs for $z<0.2$.