SUB-L∗ GALAXIES AT REDSHIFTS z∼4, 3, AND 2: THEIR UV LUMINOSITY FUNCTION AND LUMINOSITY DENSITY

We use very deep (Rlim=27) UnGRI imaging to study the evolution of the faint end of the UV-selected galaxy luminosity function from z∼4 to z∼2. We find that the luminosity function evolves with time and that this evolution is differential with luminosity: the number of sub-L galaxies increases from z∼4 to z∼3 by at least a factor of 2.3, while the bright end of the LF remains unchanged. Potential systematic biases restrict our ability to draw strong conclusions at lower redshifts, z∼2, but we can say that the number density of sub-L galaxies at z∼2.2 is at least as high as it is at z∼3. Turning to the UV luminosity density of the Universe, we find that the luminosity density starts dropping with increasing redshift already beginning at z=3 (earlier than recently thought — Steidel et al. 1999) and that this drop is dominated by the same sub-L galaxies that dominate the evolution of the LF. This differential evolution of the luminosity function suggests that differentially comparing key diagnostics of dust, stellar populations, etc. as a function of z and L should let us isolate the key mechanisms that drive galaxy evolution at high redshift.