THE EVOLUTION OF THE GLOBAL STAR-FORMATION HISTORY AS MEASURED FROM THE HUBBLE DEEP FIELD

The Hubble Deep Field (HDF) is the deepest set of multicolor optical p hotometric observations ever undertaken, and it offers a valuable data set with which to study galaxy evolution. Combining the optical WFPC2 data with ground-based near-infrared photometry, we derive photometri cally estimated redshifts for HDF galaxies with J < 23.5. We demonstra te that incorporating the near-infrared data reduces the uncertainty i n the estimated redshifts by approximately 40% and is required to remo ve systematic uncertainties within the redshift range 1 < z < 2. Utili zing these photometric redshifts, we determine the evolution of the co moving ultraviolet (2800 Angstrom) luminosity density (presumed to be proportional to the global star formation rate) from a redshift of z = 0.5 to z = 2. We find that the global star formation rate increases r apidly with redshift, rising by a factor of 12 from a redshift of zero to a peak at z approximate to 1.5. For redshifts beyond 1.5, it decre ases monotonically. Our measures of the star formation rate are consis tent with those found by Lilly et al. from the Canada-France Redshift Survey at z < 1 and by Madau et al. from Lyman break galaxies at z > 2 , and they bridge the redshift gap between those two samples. The over all star formation or metal enrichment rate history is consistent with the theoretical models of White and Frenk and the predictions of Pei and Fall based on the evolving H I content of Ly alpha QSO absorption line systems.