Measuring the Distribution of Galaxies Between Halos

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 halos 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 halos. Furthermore, P(N;M) provides strong constraints on models of galaxy formation, since it depends upon the merger history of dark-matter halos and the galaxy-galaxy merger rate within halos. We show that measurements from a combination of the 2MASS and SDSS or 2dFGRS datasets will allow P(N;M) averaged over halos occupied by bright galaxies to be accurately measured for N=0-2.