The assembly of the first galaxies

The first galaxies formed at high redshifts, and were likely substantially less massive than typical galaxies in the local universe. We argue that (1) the reionization of a clumpy intergalactic medium by redshift z=6, (2) its enrichment by metals by z=3 without disturbing the Lyman alpha forest, and (3) the presence of supermassive black holes powering the recently discovered bright quasars at z=6, strongly suggest that a population of low-mass galaxies exists beyond redshifts z>6. Although the first stars could have been born in dark matter halos with virial temperatures as low as Tvir=200K, collapsing as early as z=25, the first galaxies likely appeared in significant numbers only in halos with Tvir>10,000K that collapsed later (z=15). The gas in these more massive halos initially contracts isothermally to high densities by atomic Lyman alpha cooling. H2 molecules can then form efficiently via non-equilibrium gas-phase chemistry, allowing the gas to cool further to T=100K, and fragment on stellar mass scales. These halos can harbor the first generation of ``mini-galaxies'' that reionized the universe. The continuum and line emission from these sources, as well as their Lyman alpha cooling radiation, can be detected in the future by NGST and other instruments.