Isolated galaxies in hierarchical galaxy formation models - present-day
  properties and environmental histories
  In this study, we have carried out a detailed, statistical analysis of
isolated model galaxies, taking advantage of publicly available hierarchical
galaxy formation models. To select isolated galaxies, we employ 2D methods
widely used in the observational literature, as well as a more stringent 3D
isolation criterion that uses the full 3D-real space information. In
qualitative agreement with observational results, isolated model galaxies have
larger fractions of late-type, star forming galaxies with respect to randomly
selected samples of galaxies with the same mass distribution. We also find that
the samples of isolated model galaxies typically contain a fraction of less
than 15 per cent of satellite galaxies, that reside at the outskirts of their
parent haloes where the galaxy number density is low. Projection effects cause
a contamination of 2D samples of about 18 per cent, while we estimate a typical
completeness of 65 per cent. Our model isolated samples also include a very
small (few per cent) fraction of bulge dominated galaxies (B/T > 0.8) whose
bulges have been built mainly by minor mergers. Our study demonstrates that
about 65-70 per cent of 2D isolated galaxies that are classified as isolated at
z = 0 have indeed been completely isolated since z = 1 and only 7 per cent have
had more than 3 neighbours within a comoving radius of 1 Mpc. Irrespectively of
the isolation criteria, roughly 45 per cent of isolated galaxies have
experienced at least one merger event in the past (most of the mergers are
minor, with mass ratios between 1:4 and 1:10). The latter point validates the
approximation that isolated galaxies have been mainly influenced by internal
processes.
