Spectroscopic confirmation of faint Lyman break galaxies near redshift five in the Hubble Ultra Deep field

We present the faintest spectroscopically confirmed sample of z ~ 5 Lyman break galaxies (LBGs) to date. The sample is based on slitless grism spectra of the Hubble Ultra Deep Field region from the Grism ACS Program for Extragalactic Science (GRAPES) and Probing Evolution and Reionization Spectroscopically (PEARS) projects, using the G800L grism on the Hubble Space Telescope Advanced Camera for Surveys. We report here confirmations of 39 galaxies, preselected as candidate LBGs using photometric selection criteria. We compare a "traditional" V-dropout selection, based on the work of Giavalisco et al., to a more liberal one (with V -- i > 0.9), and find that the traditional criteria are about 64% complete and 81% reliable. We also study the Lyalpha emission properties of our sample. We find that Lyalpha emission is detected in ~1/4 of the sample, and that the liberal V-dropout color selection includes ~55% of previously published line-selected Lyalpha sources. Finally, we examine our stacked two-dimensional spectra. We demonstrate that strong, spatially extended (~1'') Lyalpha emission is not a generic property of these LBGs, but that a modest extension of the Lyalpha photosphere (compared to the starlight) may be present in those galaxies with prominent Lyalpha emission