First infall time of satellite progenitors onto their host-group progenitor (see text for details). Left: probability distribution function of t_{first infall}. The Gaussians illustrate the presence of two populations of satellites: those whose progenitors first entered before 3 Gyr ago (old population, in blue) and those whose progenitors first entered in the last 3 Gyr (young population, in green). Negative values represent satellites whose first infall time could not be computed (empty merger tree for low-mass subhalos, near the resolution limit, for instance). Right: first infall time as a function of host halo mass M_halo at z ∼ 0. Lines represent the median values for the old (blue line), total (grey line), and young (green line) populations of satellites.