Detecting Reionization in the Star Formation Histories of High-Redshift Galaxies

The reionization of cosmic hydrogen, left over from the big bang, increased its temperature to >~ 1.e4 K. This photo-heating resulted in an increase of the minimum mass of galaxies and hence a suppression of the cosmic star formation rate. The affected population of dwarf galaxies included the progenitors of massive galaxies that formed later. We show that a massive galaxy at a redshift z >~ 6 should show a double-peaked star formation history marked by a clear break. This break reflects the suppression signature from reionization of the region in which the galaxy was assembled. Since massive galaxies originate in overdense regions where cosmic evolution is accelerated, their environment reionizes earlier than the rest of the universe. For a galaxy of ~ 1.e12 M_solar in stars at a redshift of z ~ 6.5, the star formation rate should typically be suppressed at a redshift z >~ 10 since the rest of the universe is known to have reionized by z >~ 6.5. Indeed, this is inferred to be the case for HUDF-JD2, a massive galaxy which is potentially at z ~ 6.5 but is inferred to have formed the bulk of its 3.e11 M_solar in stars at z >~ 9.