A galaxy at z = 6.545 and constraints on the epoch of reionization

We report the discovery of a Lyα-emitting galaxy at redshift z = 6.545 serendipitously identified in the course of spectroscopic follow-up of hard X-ray sources on behalf of the Serendipitous Extragalactic X-Ray Source Identification (SEXSI) survey. The line flux of the galaxy, 2.1 × 10 ^-17 ergs cm^-2 s^-1, is similar to line fluxes probed by narrowband imaging surveys; the 5.2 arcmin² surveyed implies a surface density of z ≈ 6.5 Lyα emitters somewhat higher than that inferred from narrowband surveys. This source marks the sixth Lyα-emitting galaxy identified at z ≈ 6.5, a redshift putatively beyond the epoch of reionization when the damping wings of the neutral hydrogen of the intergalactic medium is capable of severely attenuating Lyα emission. By comparing the Lyα luminosity functions at z ≈ 5.7 and z ≈ 6.5, we infer that the intergalactic medium may remain largely reionized from the local universe out to z ≈ 6.5.