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 Lyα emission line. If this identification is correct, we show that the intergalactic medium (IGM) around the galaxy must be significantly ionized; otherwise, the damping wing of Lyα 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 ##IMG## [http://ej.iop.org/icons/Entities/lesssim.gif] {lesssim} 0.4) or the stars within the galaxy must be massive ( ##IMG## [http://ej.iop.org/icons/Entities/gtrsim.gif] {gtrsim}100 M ##IMG## [http://ej.iop.org/icons/Entities/sun.gif] {sun} ) and hence capable of producing a sufficiently large H II region around it. We generalize these conclusions and derive the maximum Lyα line flux for a given UV continuum flux of galaxies prior to the epoch of reionization.