2007ApJ...660...47D

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 i₇₇₅-z₈₅₀>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 i₇₇₅-z₈₅₀ color selection technique. Six of the confirmed galaxies show Lyα emission. Assuming that the 14 objects without redshifts are z∼6 LBGs that lack detectable Lyα emission lines, we infer that the fraction of Lyα-emitting LBGs with Lyα equivalent widths greater than 20 Å among z∼6 LBGs is ~30%, similar to that found at z∼3. Every Lyα-emitting object in our sample is compact, with half-light radii r_hl≤0.14". Furthermore, all the Lyα-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 Lyα-emitting population is a subset of the rest of the z∼6 LBG population at >97% confidence. We speculate that the small sizes of the Lyα-emitting LBGs are due to these objects being less massive than other LBGs at z∼6.