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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 (median z=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. Combined with 5 previously confirmed high-redshift galaxies in the HDF, the 16 confirmed sources yield a comoving volume density of n>2.5^-4 h50^3 Mpc^-3 for q0=0.05, or n>1.2^-3 h50^3 Mpc^-3 for q0=0.5. These densities are comparable to estimates of the local volume density of galaxies brighter than L*, and could be almost three times higher still if all 29 of the unconfirmed candidates in our original sample are indeed also at high redshift. The galaxies are small but luminous, with half-light radii 1.8 < r(1/2) < 6.5 h50^-1 kpc and absolute magnitudes -21.5 > M_B > -23. The HST images show a wide range of morphologies, including several with very close, small knots of emission embedded in wispy extended structures. Using rest-frame UV continuum fluxes with no dust correction, we calculate star formation rates in the range 7 - 24 or 3 - 9 h50^-2 Msun/yr for q0=0.05 and q0=0.5, respectively. The variety of morphologies and the high number density of z=3 galaxies in the HDF suggest that they represent a range of physical processes and stages of galaxy formation and evolution, rather than any one class of object, such as massive ellipticals. A key issue remains the measurement of masses. These high-redshift objects are likely to be the low-mass, starbursting building blocks of more massive galaxies seen today.