EVOLUTION OF THE GALAXY-DARK MATTER CONNECTION AND THE ASSEMBLY OF GALAXIES IN DARK MATTER HALOS

We present a new model to describe the galaxy-dark matter connection across cosmic time, which unlike the popular subhalo abundance-matching technique is self-consistent in that it takes account of the facts that (1) subhalos are accreted at different times and (2) the properties of satellite galaxies may evolve after accretion. Using observations of galaxy stellar mass functions (SMFs) out to z similar to 4, the conditional SMF at z similar to 0.1 obtained from Sloan Digital Sky Survey galaxy group catalogs, and the two-point correlation function (2PCF) of galaxies at z similar to 0.1 as a function of stellar mass, we constrain the relation between galaxies and dark matter halos over the entire cosmic history from z similar to 4 to the present. This relation is then used to predict the median assembly histories of different stellar mass components within dark matter halos (central galaxies, satellite galaxies, and halo stars). We also make predictions for the 2PCFs of high-z galaxies as function of stellar mass. Our main findings are the following: (1) Our model reasonably fits all data within the observational uncertainties, indicating that the Lambda CDM concordance cosmology is consistent with a wide variety of data regarding the galaxy population across cosmic time. (2) At low-z, the stellar mass of central galaxies increases with halo mass as M-0.3 and M greater than or similar to(4.0) at the massive and low-mass ends, respectively. The ratio M-*,M-c/M reveals a maximum of similar to 0.03 at a halo mass M similar to 10(11.8) h (1) M-circle dot, much lower than the universal baryon fraction (similar to 0.17). At higher redshifts the maximum in M-*,M-c/M remains close to similar to 0.03, but shifts to higher halo mass. (3) The inferred timescale for the disruption of satellite galaxies is about the same as the dynamical friction timescale of their subhalos. (4) The stellar mass assembly history of central galaxies is completely decoupled from the assembly history of its host halo; the ratio M-*,M- c/M initially increases rapidly with time until the halo mass reaches similar to 10(12) h(-1) M-circle dot, at which point M-*,M-c/M similar to 0.03. Once M greater than or similar to 10(12) h(-1) M-circle dot, there is little growth in M-*,M-c, causing the ratio M-*,M-c/M to decline. In Milky Way (MW)-sized halos more than half of the central stellar mass is assembled at z less than or similar to 0.5. (5) In low-mass halos, the accretion of satellite galaxies contributes little to the formation of their central galaxies, indicating that most of their stars must have formed in situ. In massive halos more than half of the stellar mass of the central galaxy has to be formed in situ, and the accretion of satellites can only become significant at z less than or similar to 2. (6) The total mass in halo stars is more than twice that of the central galaxy in massive halos, but less than 10% of M-*,M-c in MW-sized halos. (7) The 2PCFs of galaxies on small scales hold important information regarding the evolution of satellite galaxies, which at high-z is predicted to be much steeper than at low-z, especially for more massive galaxies. We discuss various implications of our findings regarding the formation and evolution of galaxies in a Lambda CDM cosmology.