A High Space Density of Luminous Lyman Alpha Emitters at z~6.5

Abstract

We present the results of a systematic search for Lyman-alpha emitters (LAEs) at 6 z 7.6 using the HST WFC3 Infrared Spectroscopic Parallel (WISP) Survey. Our total volume over this redshift range is 8 105 Mpc3, comparable to many of the narrowband surveys despite their larger area coverage. We find two LAEs at z=6.38 and 6.44 with line luminosities of LLyα 4.7 1043 erg s-1, putting them among the brightest LAEs discovered at these redshifts. Taking advantage of the broad spectral coverage of WISP, we are able to rule out almost all lower-redshift contaminants. The WISP LAEs have a high number density of 7.710-6 Mpc-3. We argue that the LAEs reside in Mpc-scale ionized bubbles that allow the Lyman-alpha photons to redshift out of resonance before encountering the neutral IGM. We discuss possible ionizing sources and conclude that the observed LAEs alone are not sufficient to ionize the bubbles.