The velocity modulation of galaxy properties in and near clusters:
  quantifying the decrease in star formation in backsplash galaxies
  The efficiency of recent star formation (SF) in galaxies increases with
increasing projected distance from the centre of a cluster out to several times
its virial radius (R_v). Using a complete sample of galaxies in 268 clusters
from the SDSS DR4, we investigate how, at a given projected radius from the
cluster centre, M* and SF properties of a galaxy depend on its absolute
line-of-sight velocity in the cluster rest frame, |v_LOS|. We find that for
R<0.5 R_v, the fraction of high mass non-BCG galaxies increases towards the
centre for low |v_LOS|. At a given projected radius, the fraction of Galaxies
with Ongoing or Recent (<1-3 Gyr) Efficient Star Formation (GORES, with
EW(H_delta)>2 ang & D_4000>1.5) is slightly but significantly lower for low
|v_LOS| galaxies than for their high velocity counterparts. We study these
observational trends with the help of a dark matter (DM) cosmological
simulation. We find that the backsplash particles account for at least
one-third (half) of all particles at projected radii slightly greater than the
virial radius and |v_LOS|<sigma_v. The deprojection of the GORES fraction leads
to a saturated linear increase with radius. We fit simple models of the
fraction of GORES as a function of class only or class and distance to the
cluster centre (as in our deprojected fraction). In our best-fitting model
GORES account for 13% of galaxies within the virial sphere, 11% of the virial
population, 34% of the distant (for projected radii R<2 R_v) infall population
and 19% of the backsplash galaxies. Given the 1-3 Gyr lookback time of our
GORES indicators, these results suggest that SF in a galaxy is almost
completely quenched in a single passage through the cluster.
