Spectroscopy of z ~ 6 i-Dropout Galaxies: Frequency of Lyα Emission and the Sizes of Lyα-emitting Galaxies1,
Rosati, P.; van der Wel, A.; Bouwens, R. J.; Illingworth, G. D.; Franx, M.; van Dokkum, P. G.; Holden, B. P.; Ford, H.; Magee, D.; Zirm, A.; Dow-Hygelund, C. C.
United States, Netherlands, Germany
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 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 rhl<=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.
Based on observations made with the NASA/ESA Hubble Space Telescope, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS 5-26555. These observations are associated with programs 7817, 9270, 9301, 9583, and 9803. Based on observations collected at the European Southern Observatory, Paranal, Chile (LP166.A-0701 and 169.A-045).