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 ( IGM ) by redshift z \approx 6 , ( 2 ) its enrichment by metals by z \approx 3 without disturbing the Ly \alpha forest , and ( 3 ) the presence of supermassive black holes powering the recently discovered bright quasars at z \sim 6 , strongly suggest that a population of low–mass galaxies exists beyond redshifts { { { { z { \mathrel { \mathchoice { \lower 2.9 pt \vbox { \halign { \cr } $ \displaystyle \hfil > $ \cr$% \displaystyle \hfil \sim$ } } } { \lower 2.9 pt \vbox { \halign { \cr } $ \textstyle \hfil > $ \cr% $ \textstyle \hfil \sim$ } } } { \lower 2.9 pt \vbox { \halign { \cr } $ \scriptstyle \hfil > $ \cr% $ \scriptstyle \hfil \sim$ } } } { \lower 2.9 pt \vbox { \halign { \cr } $ \scriptscriptstyle% \hfil > $ \cr$ \scriptscriptstyle \hfil \sim$ } } } } } 6 .
Although the first stars could have been born in dark matter halos with virial temperatures as low as T _ { vir } \approx 200 K , collapsing as early as z \sim 25 , the first galaxies likely appeared in significant numbers only in halos with T _ { vir } > 10 ^ { 4 } K that collapsed later ( z \sim 15 ) .
The gas in these more massive halos initially contracts isothermally to high densities by atomic Ly \alpha cooling . { H _ { 2 } } molecules can then form efficiently via non–equilibrium gas–phase chemistry , allowing the gas to cool further to T \sim 100 K , 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 Ly \alpha cooling radiation , can be detected in the future by NGST and other instruments .