Keck Spectroscopy of Redshift z ∼ 3 Galaxies in the Hubble Deep Field

We have obtained spectra with the 10-m Keck telescope of a sample of 24 galaxies having colors consistent with star-forming galaxies at redshifts 2 ∼z ∼ 4.5 in the Hubble Deep Field (HDF). Eleven of these galaxies are confirmed to be at high redshift (zmed = 3.0), one is at z = 0.5, and the other 12 have uncertain redshifts but have spectra consistent with their being at z > 2. The spectra of the confirmed high-redshift galaxies show a diversity of features, including weak Lyα emission, strong Lyα breaks or damped Lyα absorption profiles, and the stellar and interstellar rest-UV absorption lines common to local starburst galaxies and high-redshift star-forming galaxies reported recently by others. The narrow profiles and low equivalent widths of C IV, Si IV, and N V absorption lines may imply low stellar metallicities. Combined with the 5 high-redshift galaxies in the HDF previously confirmed with Keck spectra by Steidel et al. (1996b), the 16 confirmed sources yield a comoving volume density of n ≥ 2.5 × 10 h50 Mpc −3 for q0 = 0.05, or n ≥ 1.2 × 10 −3 h50 Mpc −3 for q0 = 0.5. These densities are 3 − 4 times higher than the recent estimates of