The UV luminosity functions of bright z > 8 galaxies: determination from 0.41 deg² of HST observations along 300 independent sightlines

Abstract

We determine the bright end of the rest-frame UV luminosity function (UVLF) at z = 8-10 by selecting bright z ≳ 8 photometric candidates from the largest systematic compilation of HST (pure-)parallel observations to date, the Super-Brightest-of-Reionizing-Galaxies (SuperBoRG) data set. The data set includes ~300 independent sightlines from WFC3 observations, totalling 800-1300 arcmin² (depending on redshift). We identify 31 z ≳ 8 candidates via colour selection and photo-z analysis with observed magnitude (24.1 < H₁₆₀ < 26.6) and 1σ range probability of $58\!-\!95~{{\ \rm per\ cent}}$ of being true high-z galaxies. Following detailed completeness and source recovery simulations, as well as modelling of interloper contamination, we derive rest-frame UVLFs at z = 8-10 down to M_UV ≃ -23. We find that the bright end of the galaxy luminosity function can be described both by a Schechter and by a double power-law function, with our space-based large area determination showing some tentative discrepancies with the luminosity functions derived from ground-based observations at the same redshifts. We identify two possible explanations. The first is that the UVLF includes a substantial contribution from AGNs at magnitudes M_UV < -22. The UVLF we derive at z = 8 is consistent with no evolution of the bright end from z = 6-7 UVLFs that include AGNs. An alternative (and non-exclusive) possibility is that the dust content of the observed galaxies decreases with redshift, thereby countering the naturally expected decrease in the UV luminosity functions because of a decreased star-formation rate with increasing redshift. Both scenarios raise interesting prospects to further understand galaxy formation in extreme objects during the epoch of reionization.