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Abstract
We present the faintest spectroscopically confirmed sample of redshift four and five
Lyman break galaxies to date. The sample is based on slitless grism spectra of the
Hubble Ultra Deep Field region from the GRAPES (Grism ACS Program for Extragalactic
Science) and PEARS (Probing Evolution and Reionization Spectroscopically) projects,
using the G800L grism on the HST Advanced Camera for Surveys. We report here confirmations
of 39 galaxies, pre-selected as candidate Lyman break galaxies using photometric selection
criteria. We compare a "traditional" V-dropout selection 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 Lyman alpha emission properties of our sample. We find
that Lyman alpha emission is detected in about 1/4 of the sample, and that our broad-band
color selected sample includes 55% of previously published line-selected Lyman alpha
sources. Finally, we examine our stacked 2D spectra. We demonstrate that strong, spatially
extended (arcsecond scale) Lyman alpha emission is not a generic property of these
Lyman break galaxies, but that a modest extension of the Lyman alpha photosphere (compared
to the starlight) may be present in those galaxies with prominent Lyman alpha emission.
Comments Submitted to The Astrophysical Journal. Reduced spectra from both
GRAPES and PEARS are available from STScI, at
http://www.stsci.edu/science/grapes/ and at
http://archive.stsci.edu/prepds/pears/