The GLARE Survey II. Faint z=6 Ly-alpha Line Emitters in the HUDF

Abstract

The galaxy population at z~6 has been the subject of intense study in recent years, culminating in the Hubble Ultra Deep Field (HUDF) -- the deepest imaging survey yet. A large number of high redshift galaxy candidates have been identified within the HUDF, but until now analysis of their properties has been hampered by the difficulty of obtaining spectroscopic redshifts for these faint galaxies. Our ''Gemini Lyman-Alpha at Reionisation Era'' (GLARE) project has been designed to undertake spectroscopic follow up of faint (z'<28.5) i'-drop galaxies at z~6 in the HUDF. In a previous paper we presented preliminary results from the first 7.5 hours of data from GLARE. In this paper we detail the complete survey. We have now obtained 36 hours of spectroscopy on a single GMOS slitmask from Gemini-South, with a spectral resolution of lambda/Delta(lambda) ~ 1000. We identify five strong Lyman-alpha emitters at z>5.5, and a further nine possible line emitters with detections at lower significance. We also place tight constraints on the equivalent width of Lyman-alpha emission for a further ten i'-drop galaxies and examine the equivalent width distribution of this faint spectroscopic sample of z~6 galaxies. We find that the fraction of galaxies with little or no emission is similar to that at z~3, but that the z~6 population has a tail of sources with high rest frame equivalent widths. Possible explanations for this effect include a tendency towards stronger line emission in faint sources, which may arise from extreme youth or low metallicity in the Lyman-break population at high redshift, or possibly a top-heavy initial mass function.