A New Method to Quantify Environment and Model Ram-Pressure Stripping in
  N-Body Simulations
  We introduce a Local Background Environment (LBE) estimator that can be
measured in and around every galaxy or its dark matter subhalo in
high-resolution cosmological simulations. The LBE is designed to capture the
influence of environmental effects such as ram-pressure stripping on the
formation and evolution of galaxies in semi-analytical models. We define the
LBE directly from the particle data within an adaptive spherical shell, and
devise a Gaussian mixture estimator to separate background particles from
previously unidentified subhalo particles. Analyzing the LBE properties, we
find that the LBE of satellite galaxies is not at rest with respect to their
host halo, in contrast to typical assumptions. The orientations of the
velocities of a subhalo and its LBE are well aligned in the outer infall
regions of haloes, but decorrelated near halo center. Significantly, there is
no abrupt change in LBE velocity or density at the halo virial radius. This
suggests that stripping should also happen beyond this radius. Therefore, we
use the time-evolving LBE of galaxies to develop a method to better account for
ram-pressure stripping within the Munich semi-analytical model, L-Galaxies.
Overall, our new approach results in a significant increase in gas stripping
across cosmic time. Central galaxies, as well as satellites beyond the virial
radius, can lose a significant fraction of their hot halo gas. As a result, the
gas fractions and star formation rates of satellite galaxies are suppressed
relative to the fiducial model, although the stellar masses and global stellar
mass functions are largely unchanged.
