than the dark matter, however, and so the stellar mass of a galaxy which is accreted by a larger system probably changes only slightly until most of the dark matter has been stripped off. Therefore the subhalo mass at the time of observation is probably not a good tracer for the potential well that shaped the galaxy properties. A better tracer is the subhalo mass at the time that it was accreted by the host halo, or its maxmimum mass over its history[^2]. This was first proposed by @conroy06.

The population of dark matter halos used in this work is drawn from an $N$-body simulation run with the simulation code [GADGET]{}-2 [@springel05a] on a SGI AltixII at the University Observatory Munich. The cosmological parameters of the simulation are chosen to match results from [WMAP]{}-3 [@spergel06] for a flat $\Lambda$CDM cosmological model: $\Omega_m=0.26$, $\Omega_{\Lambda}=0.74$, $h=H_0/(100$ km s$^{-1}$ Mpc$^{-1})=0.72$, $\sigma_8=0.77$ and $n=0.95$. The initial conditions were generated using the GRAFIC software package [@bertschinger2001]. The simulation was done in a periodic box with side length $100$ Mpc, and contains $512^3$ particles with a particle mass of $2.8\times 10^8{\!\,{\rm M}_\odot}$ and a force softening of $3.5$ kpc.

Dark matter halos are identified in the 