GEMS: Which Galaxies Dominate thez ∼ 0.7 Ultraviolet Luminosity Density?

Abstract

We combine high-resolution images from GEMS with redshifts and spectral energy distributions from COMBO-17 to explore the morphological types of galaxies that dominate the z ~ 0.7 UV luminosity density. We analyzed rest-frame 280 nm and V-band luminosities of 1483 galaxies with 0.65 < z < 0.75, combining these with visual morphological classifications from F850LP images (approximately rest-frame V band) taken with HST ACS on the Extended Chandra Deep Field-South. We derive UV luminosity functions and j280 luminosity densities for spheroid-dominated galaxies, spiral galaxies, Magellanic irregular galaxies, and clearly interacting galaxies with morphologies suggestive of ongoing major mergers. We check the reliability of GEMS morphologies against the deeper GOODS images and quantify an incompleteness of the GEMS merger identification at the faint end. We derive the fractions of the global UV luminosity density j280 originating from the galaxy types and find that spiral and Magellanic irregular galaxies dominate with about 40% each. Interacting and merging galaxies account for roughly 20% of j280, while the contribution of early types is negligible. These results imply that the strong decline in the UV luminosity density of the universe observed from z ~ 1 until today is dominated by the decreasing UV luminosities of normal spiral galaxies, accompanied by the migration of UV-luminous star formation in irregular galaxies to systems of progressively lower mass and luminosity. These conclusions suggest that major-merger-driven star formation cannot dominate the declining cosmic star formation rate, unless major mergers are substantially more obscured than intensely star-forming spiral galaxies and the decline in observed cosmic star formation rate is substantially stronger than the already precipitous decline in uncorrected UV luminosity density.