THE GROWTH OF GALAXY STELLAR MASS WITHIN DARK MATTER HALOS

We study the evolution of stellar mass in galaxies as a function of host halo mass, using the MPA and Durham semi-analytic models, implemented on the Millennium Run simulation. For both models, the stellar mass of the central galaxies increases rapidly with halo mass at the low-mass end and more slowly in halos of larger masses at the three redshifts probed (z similar to 0, 1, 2). About 45% of the stellar mass in central galaxies in present-day halos less massive than similar to 10(12) h(-1) M-circle dot is already in place at z similar to 1, and this fraction increases to similar to 65% for more massive halos. The baryon conversion efficiency into stars has a peaked distribution with halo mass, and the peak location shifts toward lower mass from z similar to 1 to z similar to 0. The stellar mass in low-mass halos grows mostly by star formation since z similar to 1, while in high-mass halos most of the stellar mass is assembled by mergers, reminiscent of downsizing. We compare our findings to empirical results from the Sloan Digital Sky Survey and DEEP2 surveys utilizing galaxy clustering measurements to study galaxy evolution. The theoretical predictions are in qualitative agreement with these phenomenological results, but there are large discrepancies. The most significant one concerns the number of stars already in place in the progenitor galaxies at z similar to 1, which is about a factor of two larger in both semi-analytic models. We demonstrate that methods studying galaxy evolution from the galaxy-halo connection are powerful in constraining theoretical models and can guide future efforts of modeling galaxy evolution. Conversely, semi-analytic models serve an important role in improving such methods.