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

We report the discovery of a substantial population of star--forming galaxies at 3. 0 z 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 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 emission is generally weak (50% of the galaxies. The star formation rates, measured directly from the far-UV continua, lie in the range 4-25 h₅₀^-2 M yr^-1 for q₀=0. 5. Together with the morphological properties of the z>3 galaxy population, which we discuss in a companion paper (Giavalisco 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 3. 5. (shortened abstract). arch-ive/yymmnnn