be tightly coupled to the depth of the halo potential and thus to the halo mass.

There are various different approaches to link the properties of galaxies to those of their halos. A first method attempts to derive the halo properties from the properties of its galaxy population using e.g. galaxy kinematics [@erickson1987; @zaritsky1993; @carlberg1996; @more2009a; @more2009b], gravitational lensing [@mandelbaum05; @mandelbaum06; @cacciato2008], or X-ray studies [@lin2003; @linmohr2004].

A second approach is to attempt to model the physics that shapes galaxy formation *ab initio* using either large numerical simulations including both gas and dark matter [@katz1996; @springel2003] or semi-analytic models (SAMs) of galaxy formation [e.g. @kauffmann1993; @cole1994; @somerville1999]. In “hybrid” SAMs [e.g. @croton06; @bower2006], dark matter “merger trees” are extracted from a dark matter only N-body simulation, and gas processes are treated with semi-analytic recipes. An advantage of this method is that high-resolution N-body simulations can track the evolution of individual subhalos [@klypin1999; @springel01] and thus provide the precise positions and velocities of galaxies within a halo. However, many of the physical processes involved in galaxy formation (such as star formation and various kinds of feedback) are still not well understood, and in many cases simulations are not able to reproduce 