The Star Formation History of Field Galaxies

We develop a method for interpreting faint galaxy data which focuses on the integrated light radiated from the galaxy population as a whole. The emission history of the universe at ultraviolet, optical, and near-infrared wavelengths is 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 Hubble Deep Field (HDF) imaging survey. In a q₀ = 0.5, h₅₀ = 1 cosmology, the global spectrophotometric properties of field galaxies can be 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 (IMF) extending from 0.1 to 125 M_☉. While a Salpeter IMF with a modest amount of dust reddening or a somewhat steeper mass function, ϕ(m) ∝ m^-2.7, can both reproduce the data reasonably well, a Scalo IMF produces too much long-wavelength light and is unable to match the observed mean galaxy colors. In the best-fit models, the global SFR rises sharply, by about an order of magnitude, from a redshift of zero to a peak value at z ≈ 1.5 in the range 0.12-0.17 M_☉ yr^-1 Mpc^-3, to fall again at higher redshifts. After integrating the inferred star formation rate over cosmic time, we find a stellar mass density at the present epoch of Ω_s h²₅₀≳0.005, hence a mean stellar mass-to-light ratio ≳4 in the B-band and ≳1 in K, consistent with the values observed in nearby galaxies of various morphological types. The models are able to account for the entire background light recorded in the galaxy counts down to the very faint magnitude levels probed by the HDF. Since only ~20% of the current stellar content of galaxies is produced at z > 2, a rather low cosmic metallicity is expected at these early times, in good agreement with the observed enrichment history of the damped Lyα systems. 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. A "monolithic collapse" model, where half of the present-day stars formed at z > 2.5 and were shrouded by dust, can be made consistent with the global history of light, but overpredicts the metal mass density at high redshifts as sampled by quasi-stellar object absorbers.