The Dawn of Galaxies

The development of primordial inhomogeneities into the non-linear regime and the formation of the first astrophysical objects within dark matter halos mark the transition from a simple, neutral, cooling universe - described by just a few parameters - to a messy ionized one - the realm of radiative, hydrodynamic, and star formation processes. The Wilkinson Microwave Anisotropy Probe (WMAP) polarization data show that this transition must have begun quite early, and that the universe was fully reionized some 350 million years after the Big Bang. It is a young generation of extremely metal-poor massive stars and/or 'seed' accreting black holes in subgalactic halos that may have generated the ultraviolet radiation and mechanical energy that reheated and reionized most of the hydrogen in the cosmos. The detailed thermal, ionization, and chemical enrichment history of the universe during the crucial formative stages around redshift 10 depends on the power-spectrum of density fluctuations on small scales, the stellar initial mass function and star formation efficiency, a complex network of poorly understood 'feedback' mechanisms, and remains one of the crucial missing links in galaxy formation and evolution studies.