  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.
