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Observations of compact groups of galaxies (CGs) indicate that theirabundance has not significantly changed since $z=0.2$. This balance between thetimescales for formation and destruction of CGs is challenging if the typicaltimescale for CG members to merge into one massive galaxy is as short ashistorically assumed ($<$ 0.1 Hubble times). Following the evolution of CGsover time in a cosmological simulation (EAGLE), we quantify the contributionsof individual processes that in the end explain the observed abundance of CGs.We find that despite the usually applied maximum line-of-sight velocitydifference of $1000\,\mathrm{km\,s}^{-1}$ within the group members, themajority of CGs ($\approx 60$ per cent) are elongated along the line-of-sightby at least a factor of two. These CGs are mostly transient as they are onlycompact in projection. In more spherical systems $\approx 80$ per cent ofgalaxies 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. Weconclude that the combination of large fractions of interlopers and the longermedian group coalescence timescale of CGs alleviates the need for a fastformation process to explain the observed abundance of CGs for $z<0.2$.

Abundance and group coalescence timescales of compact groups of galaxies in the EAGLE simulation