KECK DEEP FIELDS. III. LUMINOSITY-DEPENDENT EVOLUTION OF THE ULTRAVIOLET LUMINOSITY AND STAR FORMATION RATE DENSITIES AT z∼4, 3, AND 2

We use our very deep Keck Deep Fields UnGRI catalog of z∼4, 3, and 2 UV-selected star-forming galaxies to study the evolution of the rest-frame 1700Å luminosity density at high redshift — a study that is motivated by our finding of luminosity-dependent evolution of the galaxy luminosity function at high redshift. Ours is the most robust UV luminosity density measurement to date at these redshifts as it uses a well-tested object selection technique, several independent sightlines, and probes deep into the galaxy luminosity function. The ability to reliably constrain the contribution of faint galaxies is critical and our data do so as they reach deep into the galaxy population, to M LBG+2 even at z∼4 and deeper still at lower redshifts (where M LBG=−21.0 is the Schechter function’s characteristic magnitude for Lyman break galaxies (LBGs) and L∗LBG is the corresponding luminosity). We find that the luminosity density at high redshift is dominated by the hitherto poorly studied galaxies fainter than L∗LBG, and, indeed, the the bulk of the UV light in the high-z Universe comes from galaxies in the rather narrow luminosity range L=0.1–1L∗LBG. It is these faint galaxies that govern the behavior of the total UV luminosity density. Overall, there is a gradual rise in luminosity density starting at z∼4 or earlier (we find twice as much UV light at z∼3 as at z∼4), followed by a shallow peak or a plateau within z∼3–1, and then followed by the well-know plunge at lower redshifts. Within this total picture, luminosity density in sub-L∗LBG galaxies evolves more rapidly at high redshift, z&2, than that in more luminous objects. However, this is reversed at lower redshifts, z.1, a reversal that is reminiscent of galaxy downsizing, albeit here thus far seen only in galaxy luminosity and not yet in galaxy mass. Within the context of the models commonly used in the observational literature, there seemingly aren’t enough faint or bright LBGs to maintain ionization of intergalactic gas even as late as z∼4. This is particularly true at earlier epochs and even more so if the faint-end evolutionary trends we observe at z∼3 and 4 continue to higher redshifts. Apparently the Universe must be easier to reionize than some recent studies have assumed. Nevertheless, sub-L∗LBG galaxies do dominate the total UV luminosity density at z&2 and this dominance further highlights the need for follow-up studies that will teach us more about these very numerous but thus far largely unexplored systems. Subject headings: galaxies: evolution — galaxies: formation — galaxies: high-redshift — galaxies: starburst