Was the Universe Reionized at Redshift 10?

Abstract

Recently, Pello and coworkers claimed to have discovered a galaxy at a redshift z = 10 and identified a feature in its spectrum with a hydrogen Lyalpha emission line. If this identification is correct, we show that the intergalactic medium (IGM) around the galaxymust be significantly ionized; otherwise, the dampingwing of Lyalpha absorption by the neutral IGM would have greatly suppressed the emission line. We find that either the large-scale region surrounding this galaxy must have been largely reionized by z = 10 ( with a neutral fraction less than or similar to0.4) or the stars within the galaxymust be massive (greater than or similar to100 M-.) and hence capable of producing a sufficiently large H II region around it. We generalize these conclusions and derive themaximum Lyalpha line flux for a given UV continuum flux of galaxies prior to the epoch of reionization.