Abundance and group coalescence timescales of compact groups of galaxies
  in the EAGLE simulation
  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 $z<0.2$.
