Spectroscopy of z~6 i-dropout Galaxies: Frequency of Ly-alpha Emission and the Sizes of Ly-alpha Emitting Galaxies

Abstract

We report on deep spectroscopy using LRIS on Keck I and FORS2 on the VLT of a sample of 22 candidate z~6 Lyman Break galaxies (LBGs) selected by the i-z> 1.3 dropout criterion. Redshifts could be measured for eight objects. These redshifts are all in the range z = 5.5 - 6.1, confirming the efficiency of the i-z color selection technique. Six of the confirmed galaxies show Ly-alpha emission. Assuming that the 14 objects without redshifts are z~6 LBGs, but lack detectable Ly-alpha emission lines, we infer that the fraction of Ly-alpha emitting LBGs with Ly-alpha equivalent widths greater than 20 Angstroms among z~6 LBGs is ~30%, similar to that found at z~3. Every Ly-alpha emitting object in our sample is compact with r <= 0.14&#34;. Furthermore, all the Ly-alpha emitting objects in our sample are more compact than average relative to the observed size-magnitude relation of a large i-dropout sample (332 candidate z~6 objects). We can reject the hypothesis that the Ly-alpha emitting population is a subset of the rest of the z~6 LBG population at >97% confidence. We speculate the small sizes of Ly-alpha emitting LBGs are due to these objects being less massive than other LBGs at z~6.