Lyman-Break Galaxies at z ≳ 4 and the Evolution of the Ultraviolet Luminosity Density at High Redshift*

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Abstract
We present initial results of a survey for star-forming galaxies in the redshift range 3.8 z 4.5. This sample consists of a photometric catalog of 244 galaxies culled from a total solid angle of 0.23 deg2 to an apparent magnitude of IAB = 25.0. Spectroscopic redshifts in the range 3.61 ≤ z ≤ 4.81 have been obtained for 48 of these galaxies; their median redshift is z = 4.13. Selecting these galaxies in a manner entirely analogous to our large survey for Lyman-break galaxies at smaller redshift (2.7 z 3.4) allows a relatively clean differential comparison between the populations and integrated luminosity density at these two cosmic epochs. Over the same range of UV luminosity, the spectroscopic properties of the galaxy samples at z ~ 4 and z ~ 3 are indistinguishable, as are the luminosity function shapes and the total integrated UV luminosity densities [ρUV(z = 3)/ρUV(z = 4) = 1.1 ± 0.3]. We see no evidence at these bright magnitudes for the steep decline in the star formation density inferred from fainter photometric Lyman-break galaxies in the Hubble deep field (HDF). The HDF provides the only existing data on Lyman-break galaxy number densities at fainter magnitudes. We have reanalyzed the z ~ 3 and z ~ 4 Lyman-break galaxies in the HDF using our improved knowledge of the spectral energy distributions of these galaxies, and we find, like previous authors, that faint Lyman-break galaxies appear to be rarer at z ~ 4 than z ~ 3. This might signal a large change in the faint-end slope of the Lyman-break galaxy luminosity function between redshifts z ~ 3 and z ~ 4, or, more likely, be due to significant variance in the number counts within the small volumes probed by the HDF at high redshifts (~160 times smaller than the ground-based surveys discussed here). If the true luminosity density at z ~ 4 is somewhat higher than implied by the HDF, as our ground-based sample suggests, then the emissivity of star formation as a function of redshift would appear essentially constant for all z > 1 once internally consistent corrections for dust are made. This suggests that there may be no obvious peak in star formation activity and that the onset of substantial star formation in galaxies might occur at z 4.5.

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