CONNECTING GALAXIES, HALOS, AND STAR FORMATION RATES ACROSS COSMIC TIME

Abstract

A simple, observationally-motivated model is presented for understanding how\nhalo masses, galaxy stellar masses, and star formation rates are related, and\nhow these relations evolve with time. The relation between halo mass and galaxy\nstellar mass is determined by matching the observed spatial abundance of\ngalaxies to the expected spatial abundance of halos at multiple epochs -- i.e.\nmore massive galaxies are assigned to more massive halos at each epoch. Halos\nat different epochs are connected by halo mass accretion histories estimated\nfrom N-body simulations. The halo--galaxy connection at fixed epochs in\nconjunction with the connection between halos across time provides a connection\nbetween observed galaxies across time. With approximations for the impact of\nmerging and accretion on the growth of galaxies, one can then directly infer\nthe star formation histories of galaxies as a function of stellar and halo\nmass. This model is tuned to match both the observed evolution of the stellar\nmass function and the normalization of the observed star formation rate --\nstellar mass relation to z~1. The data demands, for example, that the star\nformation rate density is dominated by galaxies with Mstar~10^{10.0-10.5} Msun\nfrom 0<z<1, and that such galaxies over these epochs reside in halos with\nM~10^{11.5-12.5} Msun. The star formation rate -- halo mass relation is\napproximately Gaussian over the range 0<z<1 with a mildly evolving mean and\nnormalization. This model is then used to shed light on a number of issues,\nincluding 1) a clarification of ``downsizing'', 2) the lack of a sharp\ncharacteristic halo mass at which star formation is truncated, and 3) the\ndominance of star formation over merging to the stellar build-up of galaxies\nwith Mstar<10^11 Msun at z<1.\n