The Hubble Deep Field and the Early Evolution of Galaxies

Abstract

I review some recent progress made in our understanding of galaxy evolution and the cosmic history of star formation. The Hubble Deep Field (HDF) imaging survey has achieved the sensitivity to capture the bulk of the extragalactic background light from discrete sources. No evidence is found in the optical number-magnitude relation down to AB=29 mag for a large amount of star formation at high redshifts. The emission history of the universe at ultraviolet, optical, and near-infrared wavelengths can be modeled from the present epoch to z~4 by tracing the evolution with cosmic time of the galaxy luminosity density, as determined from several deep spectroscopic samples and the HDF. The global spectrophotometric properties of field galaxies are well fitted by a simple stellar evolution model, defined by a time-dependent star formation rate (SFR) per unit comoving volume and a universal initial mass function which is relatively rich in massive stars. The SFR density is found to rise sharply, by about an order of magnitude, from a redshift of zero to a peak value at z~1.5, to fall again by a factor of 2 (4) out of a redshift of 3 (4). Since only 10% of the current stellar content of galaxies is produced at z>2.5, a rather low cosmic metallicity is predicted at these early times, in good agreement with the observed enrichment history of the damped Lyman-αsystems. The biggest uncertainty is represented by the poorly constrained amount of starlight that was absorbed by dust and reradiated in the IR at early epochs.