Studying Structure Formation with the Sloan Digital Sky Survey

Abstract

I review recent SDSS results related to galaxies and large scale structure, including: (1) discovery of coherent, unbound structures in the stellar halo, (2) demonstration that Pal 5 has tidal tails and Draco doesn't, (3) precise measurement of the galaxy luminosity function and its variation with surface brightness, color, and morphology, (4) detailed examination of the Fundamental Plane of 9000 early type galaxies, (5) measurement, via galaxy-galaxy lensing, of the extended dark matter distributions around galaxies and their variation with luminosity, morphology, and environment, (6) measurement of galaxy angular clustering, spatial clustering and velocity dispersion as a function of luminosity and color. I then discuss what we can hope to learn in the long run from galaxy clustering in the SDSS and the 2dFGRS. Galaxy clustering depends on the dark halo population and on the Halo Occupation Distribution (HOD), which specifies the way that galaxies populate the halos. SPH simulations and semi-analytic models 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 mean occupation N_avg(M), sub-Poisson fluctuations about the mean in low mass halos, and strong dependence of N_avg(M) on stellar population age. The complementary sensitivity of different clustering statistics should make it possible to determine the HOD empirically given an assumed cosmological model. Furthermore, changes to the cosmological model change the halo population in ways 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.