Spectroscopic Confirmation of a Population of Normal Star-forming Galaxies at Redshiftsz> 3

We report the discovery of a substantial population of star--forming galaxiesat $3.0 \simlt z \simlt 3.5$. These galaxies have been selected using colorcriteria sensitive to the presence of a Lyman continuum break superposed on anotherwise very blue far-UV continuum, and then confirmed with deep spectroscopyon the W. M. Keck telescope. The surface density of galaxies brighter than R=25with 3 < z < 3.5 is $0.4\pm0.07$ galaxies arcmin$^{-2}$, approximately 1.3\% ofthe deep counts at these magnitudes; this value applies both to ``random''fields and to fields centered on known QSOs. The corresponding co-moving spacedensity is approximately half that of luminous ($L \simgt L^{\ast}$)present--day galaxies. Our sample of $z > 3$ galaxies is large enough that wecan begin to detail the spectroscopic characteristics of the population as awhole. The spectra of the $z>3$ galaxies are remarkably similar to those ofnearby star-forming galaxies, the dominant features being stronglow--ionization interstellar absorption lines and high--ionization stellarlines, often with P-Cygni profiles characteristic of Wolf-Rayet and O--starwinds. Lyman $\alpha$ emission is generally weak ($< 20$ \AA\ rest equivalentwidth) 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$\sun$ yr$^{-1}$ for $q_0=0.5$. Together with the morphological properties ofthe $z>3$ galaxy population, which we discuss in a companion paper (Giavalisco\et 1996), all of these findings strongly suggest that we have identified thehigh-redshift counterparts of the spheroid component of present--day luminousgalaxies. In any case, it is clear that massive galaxy formation was alreadywell underway by $z \sim 3.5$. (shortened abstract). arch-ive/yymmnnnComment: To be published in ApJ Letters, 12 pages, AAS Latex, including 2 figures. Also available at ftp://astro.caltech.edu/users/ccs/zg3spec.tar.g