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 greater than or similar to 10(4) K. This photoheating resulted in an increase of the minimum mass of galaxies and hence a suppression of the cosmic star formation rate (SFR). 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 greater than or similar to 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 similar to 10(12) M-circle dot in stars at a redshift of z similar to 6.5, the SFR should typically be suppressed at a redshift z greater than or similar to 10 since the rest of the universe is known to have reionized by z greater than or similar to 6.5. Indeed, this is inferred to be the case for HUDF-JD2, a massive galaxy which is potentially at z similar to 6.5 but is inferred to have formed the bulk of its 3 x 10(11) M-circle dot in stars at z greater than or similar to 9.