The Ultraviolet Luminosity Density of the Universe from Photometric Redshifts of Galaxies in the Hubble Deep Field

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Studies of the Hubble Deep Field (HDF) and other deep surveys have revealed an apparent peak in the ultraviolet (UV) luminosity density, and therefore the star-formation rate density, of the Universe at redshifts 1<z<2. We use photometric redshifts of galaxies in the HDF to determine the comoving UV luminosity density and find that, when errors (in particular, sampling error) are properly accounted for, a flat distribution is statistically indistinguishable from a distribution peaked at z 1.5. Furthermore, we examine the effects of cosmological surface brightness (SB) dimming on these measurements by applying a uniform SB cut to all galaxy fluxes after correcting them to redshift z=5. We find that, comparing all galaxies at the same intrinsic surface brightness sensitivity, the UV luminosity density contributed by high intrinsic SB regions increases by almost two orders of magnitude from z 0 to z 5. This suggests that there exists a population of objects with very high star formation rates at high redshifts that apparently do not exist at low redshifts. The peak of star formation, then, likely occurs somewhere beyond z>2.