The galaxy UV luminosity function at z ≃ 2-4; new results on faint-end slope and the evolution of luminosity density

We present a new, robust measurement of the evolving rest-frame ultraviolet (UV) galaxy luminosity function (LF) over the key redshift range from z similar or equal to 2 to z similar or equal to 4. Our results are based on the high dynamic range provided by combining the Hubble Ultra Deep Field (HUDF), CANDELS/GOODS-South, and UltraVISTA/COSMOS surveys. We utilize the unparalleled multifrequency photometry available in this survey 'wedding cake' to compile complete galaxy samples at z similar or equal to 2, 3, 4 via photometric redshifts (calibrated against the latest spectroscopy) rather than colour-colour selection, and to determine accurate rest-frame UV absolute magnitudes (M-1500) from spectral energy distribution (SED) fitting. Our new determinations of the UV LF extend from M-1500 similar or equal to -22 (AB mag) down to M-1500 = -14.5, -15.5 and -16 at z similar or equal to 2, 3 and 4, respectively (thus, reaching similar or equal to 3-4 mag fainter than previous blank-field studies at z similar or equal to 2,3). At z similar or equal to 2, 3, we find a much shallower faint-end slope (alpha = -1.32 +/- 0.03) than reported in some previous studies (alpha similar or equal to -1.7), and demonstrate that this new measurement is robust. By z similar or equal to 4, the faint-end slope has steepened slightly, to alpha = -1.43 +/- 0.04, and we show that these measurements are consistent with the overall evolutionary trend from z = 0 to 8. Finally, we find that while characteristic number density (phi*) drops from z similar or equal to 2 to z similar or equal to 4, characteristic luminosity (M*) brightens by similar or equal to 1 mag. This, combined with the new flatter faint-end slopes, has the consequence that UV luminosity density (and hence unobscured star formation density) peaks at z similar or equal to 2.5-3, when the Universe was similar or equal to 2.5 Gyr old.