Studying structure formation with the sloan digital sky survey

I review some of the recent results from the SDSS related to galaxies and large scale structure, including: (1) discovery of coherent, unbound structures in the stellar halo of the Milky Way, (2) demonstration that the Pal 5 globular cluster has tidal tails and that the Draco dwarf spheroidal does not, (3) precise measurement of the galaxy luminosity function and its variation with galaxy surface brightness, color, and morphology, (4) detailed examination of the Fundamental Plane from a sample of 9000 early type galaxies, (5) measurement, via galaxy-galaxy lensing, of the extended dark matter distributions around galaxies and their variation with galaxy luminosity, morphology, and environment, (6) measurements of the galaxy angular power spectrum and of the spatial correlation function and pairwise velocity dispersion as a function of galaxy luminosity and color. I then turn to a more abstract discussion of what we can hope to learn, in the long run, from galaxy clustering in the SDSS and the 2dFGRS. The clustering of a galaxy sample depends on the mass function and clustering of the dark halo population, and on the Halo Occupation Distribution (HOD), which specifies the way that galaxies populate the halos. Hydrodynamic simulations and semi-analytic models of galaxy formation make similar predictions for the probability P(N\M) that a halo of virial mass M contains N galaxies of a specified type: a non-linear form of the mean occupation N-avg(M), sub-Poisson fluctuations about the mean in low mass halos, and a strong dependence of N-avg(M) on the age of a galaxy's stellar population. Different galaxy clustering statistics respond to different features of the HOD, making it possible to determine the HOD empirically given an assumed cosmological model. Furthermore, changes to Omega(m) and/or the linear power spectrum produce changes in the halo population that would be difficult to mask by changing the HOD. Ultimately, we can hope to have our cake and eat it too, obtaining strong guidance to the physics of galaxy formation by deriving the HOD of different classes of galaxies, while simultaneously carrying out precision tests of cosmological models.