In popular cold dark matter cosmological scenarios , stars may have first appeared in significant numbers around a redshift of 10 or so , as the gas within protogalactic halos with virial temperatures T _ { vir } \mathrel { \hbox to 0.0 pt { \lower 3.0 pt \hbox { $ \mathchar 536 $ } \hss } % \raise 2.0 pt \hbox { $ \mathchar 318 $ } } 10 ^ { 4.3 } K ( corresponding to masses comparable to those of present–day dwarf ellipticals ) cooled rapidly due to atomic processes and fragmented .
It is this ‘ second generation ’ of subgalactic stellar systems , aided perhaps by an early population of accreting black holes in their nuclei , which may have generated the ultraviolet radiation and mechanical energy that ended the cosmic “ dark ages ” and reheated and reionized most of the hydrogen in the universe by a redshift of 6 .
The detailed history of the universe during and soon after these crucial formative stages depends on the power–spectrum of density fluctuations on small scales and on a complex network of poorly understood ‘ feedback ’ mechanisms , and is one of the missing link in galaxy formation and evolution studies .
The astrophysics of the epoch of first light is recorded in the thermal state , ionization degree , and chemical composition of the intergalactic medium , the main repository of baryons at high redshifts .