The velocity modulation of galaxy properties in and near clusters: quantifying the decrease in star formation in backsplash galaxies

Ongoing or recent star formation in galaxies is known to increase with increasing projected distance from the centre of a cluster out to several times its virial radius (R-v). Using a complete sample (M-r 2 angstrom & D(n)4000 > 1.5] is slightly but significantly lower for low vertical bar v(LOS)vertical bar galaxies than for their high-velocity counterparts. We study these observational trends with the help of a dark matter (DM) cosmological simulation. We classify DM particles as virial, infall and backsplash according to their present positions in (r, v(r)) radial phase space and measure the frequencies of each class in cells of (R, vertical bar v(LOS)vertical bar) projected phase space. As expected, the virial class dominates at projected radii R <R-v, while the infall particles dominate outside, especially at high vertical bar v(LOS)vertical bar. However, the backsplash particles account for at least one-third (half) of all particles at projected radii slightly greater than the virial radius and vertical bar v(LOS)vertical bar <sigma(v) (vertical bar v(LOS)vertical bar