Measuring the distribution of galaxies between haloes

We develop a method to measure the probability, P(N; M), of finding N galaxies in a dark matter halo of mass M from the theoretically determined clustering properties of dark matter haloes and the observationally measured clustering properties of galaxies. Knowledge of this function and the distribution of the dark matter completely specifies all clustering properties of galaxies on scales larger than the size of dark matter haloes. Furthermore, P(N; M) provides strong constraints on models of galaxy formation, since it depends upon the merger history of dark matter haloes and the galaxy-galaxy merger rate within haloes. We show that measurements from a combination of the Two Micron All Sky Survey and Sloan Digital Sky Survey or Two-degree Field Galaxy Redshift Survey data sets will allow P(N; M) averaged over haloes occupied by bright galaxies to be accurately measured for N = 0-2.