Spectroscopic Confirmation of a Population of Normal Star-Forming Galaxies at Redshifts z>3

We report the discovery of a substantial population of star–forming galaxies at $3.0\text{simlt}z\text{simlt}3.5$. These galaxies have been selected using color criteria sensitive to the presence of a Lyman continuum break superposed on an otherwise very blue far-UV continuum, and then confirmed with deep spectroscopy on the W. M. Keck telescope. The surface density of galaxies brighter than R=25 with 3 < z < 3.5 is $0.4\pm 0.07$ galaxies arcmin${}^{-2}$, approximately 1.3% of the deep counts at these magnitudes; this value applies both to ``random’’ fields and to fields centered on known QSOs. The corresponding co-moving space density is approximately half that of luminous ($L\text{simgt}{L}^{\ast }$) present–day galaxies. Our sample of $z>3$ galaxies is large enough that we can begin to detail the spectroscopic characteristics of the population as a whole. The spectra of the $z>3$ galaxies are remarkably similar to those of nearby star-forming galaxies, the dominant features being strong low–ionization interstellar absorption lines and high–ionization stellar lines, often with P-Cygni profiles characteristic of Wolf-Rayet and O–star winds. Lyman $\alpha$ emission is generally weak ($<20$ \AA\ rest equivalent width) and is absent for >50% of the galaxies. The star formation rates, measured directly from the far-UV continua, lie in the range 4-25 $h_{50}^{-2}$ M$\text{sun}$ yr${}^{-1}$ for $q_0=0.5$. Together with the morphological properties of the $z>3$ galaxy population, which we discuss in a companion paper (Giavalisco \et 1996), all of these findings strongly suggest that we have identified the high-redshift counterparts of the spheroid component of present–day luminous galaxies. In any case, it is clear that massive galaxy formation was already well underway by $z\sim 3.5$. (shortened abstract). arch-ive/yymmnnn