THE STAR-FORMATION HISTORY OF FIELD GALAXIES

We develop a method for interpreting faint galaxy data which focuses o n the integrated light radiated from the galaxy population as a whole. The emission history of the universe at ultraviolet, optical, and nea r-infrared wavelengths is modeled from the present epoch to z approxim ate to 4 by tracing the evolution with cosmic time of the galaxy lumin osity density, as determined from several deep spectroscopic samples a nd the Hubble Deep Field (HDF) imaging survey. In a q(0)=0.5, h(50)=1 cosmology, the global spectrophotometric properties of field galaxies can be well fitted by a simple stellar evolution model, defined by a t ime-dependent star formation rate (SFR) per unit comoving volume and a universal initial mass function (IMF) extending from 0.1 to 125 M.. W hile a Salpeter IMF with a modest amount of dust reddening or a somewh at steeper mass function, phi(m)proportional to m(-2.7), can both repr oduce the data reasonably well, a Scale IMF produces too much long-wav elength light and is unable to match the observed mean galaxy colors. In the best-fit models, the global SFR rises sharply, by about an orde r of magnitude, from a redshift of zero to a peak value at z approxima te to 1.5 in the range 0.12-0.17 M. yr(-1) Mpc(-3), to fall again at h igher redshifts. After integrating the inferred star formation rate ov er cosmic time, we find a stellar mass density at the present epoch of Omega(s)h(50)(2) greater than or similar to 0.005, hence a mean stell ar mass-to-light ratio greater than or similar to 4 in the B-band and greater than or similar to 1 in K, consistent with the values observed in nearby galaxies of various morphological types. The models are abl e to account for the entire background light recorded in the galaxy co unts down to the very faint magnitude levels probed by the HDF. Since only similar to 20% of the current stellar content of galaxies is prod uced at z>2, a rather low cosmic metallicity is expected at these earl y times, in good agreement with the observed enrichment history of the damped Ly alpha systems. The biggest uncertainty is represented by th e poorly constrained amount of starlight that was absorbed by dust and reradiated in the IR at early epochs. A ''monolithic collapse'' model , where half of the present-day stars formed at z>2.5 and were shroude d by dust, can be made consistent with the global history of light, bu t overpredicts the metal mass density at high redshifts as sampled by quasi-stellar object absorbers.