UV LUMINOSITY FUNCTIONS AT REDSHIFTS z ∼ 4 TO z ∼ 10: 10,000 GALAXIES FROM HST LEGACY FIELDS

The remarkable Hubble Space Telescope (HST) data sets from the CANDELS, HUDF09, HUDF12, ERS, and BoRG/HIPPIES programs have allowed us to map the evolution of the rest-frame UV luminosity function (LF) from z similar to 10 to z similar to 4. We develop new color criteria that more optimally utilize the full wavelength coverage from the optical, near-IR, and mid-IR observations over our search fields, while simultaneously minimizing the incompleteness and eliminating redshift gaps. We have identified 5859, 3001, 857, 481, 217, and 6 galaxy candidates at z similar to 4, z similar to 5, z similar to 6, z similar to 7, z similar to 8, and z similar to 10, respectively, from the similar to 1000 arcmin(2) area covered by these data sets. This sample of >10,000 galaxy candidates at z >= 4 is by far the largest assembled to date with HST. The selection of z similar to 4-8 candidates over the five CANDELS fields allows us to assess the cosmic variance; the largest variations are at z >= 7. Our new LF determinations at z similar to 4 and z similar to 5 span a 6 mag baseline and reach to -16 AB mag. These determinations agree well with previous estimates, but the larger samples and volumes probed here result in a more reliable sampling of >L* galaxies and allow us to reassess the form of the UV LFs. Our new LF results strengthen our earlier findings to 3.4 sigma significance for a steeper faint-end slope of the UV LF at z > 4, with a evolving from alpha = -1.64 +/- 0.04 at z similar to 4 to alpha = -2.06 +/- 0.13 at z similar to 7 (and alpha = -2.02 +/- 0.23 at z similar to 8), consistent with that expected from the evolution of the halo mass function. We find less evolution in the characteristic magnitude M* from z similar to 7 to z similar to 4; the observed evolution in the LF is now largely represented by changes in phi*. No evidence for a non-Schechter-like form to the z similar to 4-8 LFs is found. A simple conditional LF model based on halo growth and evolution in the M/L ratio (alpha(1 + z)(-1.5)) of halos provides a good representation of the observed evolution.