A STEEP FAINT-END SLOPE OF THE UV LUMINOSITY FUNCTION AT z ∼ 2-3: IMPLICATIONS FOR THE GLOBAL STELLAR MASS DENSITY AND STAR FORMATION IN LOW-MASS HALOS

We use the deep ground-based optical photometry of the Lyman Break Galaxy (LBG) Survey to derive robust measurements of the faint-end slope (α) of the UV luminosity function (LF) at redshifts 1.9 ⩽ z ⩽ 3.4. Our sample includes >2000 spectroscopic redshifts and ≈31000 LBGs in 31 spatially independent fields over a total area of 3261 arcmin 2 . These data allow us to select galaxies to 0.07 L * and 0.10 L * at z ∼ 2 and z ∼ 3, respectively. A maximum-likelihood analysis indicates steep values of α( z = 2) = −1.73 ± 0.07 and α( z = 3) = −1.73 ± 0.13. This result is robust to luminosity-dependent systematics in the Lyα equivalent width and reddening distributions, and is similar to the steep values advocated at z ≳ 4, and implies that ≈93% of the unobscured UV luminosity density at z ∼ 2–3 arises from sub- L * galaxies. With a realistic luminosity-dependent reddening distribution, faint to moderately luminous galaxies account for ≳70% and ≳25% of the bolometric luminosity density and present-day stellar mass density, respectively, when integrated over 1.9 ⩽ z < 3.4. We find a factor of 8–9 increase in the star-formation rate density between z ∼ 6 and z ∼ 2, due to both a brightening of L * and an increasing dust correction proceeding to lower redshifts. Combining the UV LF with stellar mass estimates suggests a relatively steep low-mass slope of the stellar mass function at high redshift. The previously observed discrepancy between the integral of the star-formation history and stellar mass density measurements at z ∼ 2 may be reconciled by invoking a luminosity-dependent reddening correction to the star-formation history combined with an accounting for the stellar mass contributed by UV-faint galaxies. The steep and relatively constant faint-end slope of the UV LF at z ≳ 2 contrasts with the shallower slope inferred locally, suggesting that the evolution in the faint-end slope may be dictated simply by the availability of low-mass halos capable of supporting star formation at z ≲ 2.