Why does the environmental influence on group and cluster galaxies
  extend beyond the virial radius?
  In the local Universe, galaxies in groups and clusters contain less gas and
are less likely to be forming stars than their field counterparts. This effect
is not limited to the central group/cluster regions, but is shown by recent
observations to persist out to several virial radii. To gain insight into the
extent and cause of this large-scale environmental influence, we use a suite of
high-resolution cosmological hydrodynamic simulations to analyse galaxies
around simulated groups and clusters of a wide range of mass (log M/M_sun =
[13.0, 15.2]). In qualitative agreement with the observations, we find a
systematic depletion of both hot and cold gas and a decline in the star forming
fraction of galaxies as far out as ~ 5 r200 from the host centre. While a
substantial fraction of these galaxies are on highly elliptical orbits and are
not infalling for the first time (~ 50 per cent at 2 r200, independent of host
mass) or are affected by `pre-processing' (less than 10 per cent of galaxies
around groups, increasing to ~ 50 per cent around a massive cluster), even a
combination of these indirect mechanisms does not fully account for the
environmental influence, particularly in the case of the hot gas content.
Direct ram pressure interaction with an extended gas `halo' surrounding groups
and clusters is shown to be sufficiently strong to strip the hot gas
atmospheres of infalling galaxies out to ~ 5 r200. We show that this influence
is highly anisotropic, with ram pressure along filaments enhanced by up to a
factor of 100 despite significant co-flow of gas and galaxies.
