Luminous Lyman Break Galaxies at z > 5 and the Source of Reionization^*

We have discovered six galaxies with spectroscopically confirmed redshifts of 4.8 < z < 5.8 in a single 44 arcmin² field deeply imaged in R, I, and z bands. All the spectra show an emission line in the region around 7000-8400 Å with a spectroscopically detected faint continuum break across the line. These six were drawn from 13 sources with I_AB < 26.2 and R_AB - I_AB > 1.5 in the field; this photometric cut is designed to select galaxies at z > 4.8. The line fluxes range between 0.2 and 2.5 × 10^-17 ergs cm^-2 s^-1, indicating luminosities of around 10⁴²-10⁴³ ergs s^-1 for Lyα, and their high emission line equivalent widths suggest very young ages (≲10⁸ yr). A further line-emitting object with no detectable continuum was serendipitously detected by spectroscopy. If this line is Lyα, then it is from a source at z = 6.6, making this the most distant galaxy known. However, the redshift cannot be considered secure as it is based on a single line. No broad emission line objects (quasars) were detected. The 13 sources at I_AB < 26.2 are less than that expected if the luminosity function of dropout galaxies remained unchanged between z = 3 and 6, although the deficit is not highly significant given possible cosmic variance. The UV luminosity density from galaxies brighter than our flux limit is considerably less than that necessary to keep the volume probed by our field at ~ 5.3 ionized. These galaxies are observed within several hundred megayears of the end of the epoch of reionization (z = 6-7), with little time for the luminosity function to evolve. This and the lack of detected quasars imply that the bulk of the UV flux that reionized the universe came from faint galaxies with M_AB(1700 Å) > -21.