The UV luminosity functions of Bright z>8 Galaxies: Determination from ~0.41 deg2 of HST Observations along ~sim 300 independent sightlines

TL;DR

This study measures the bright end of the UV luminosity function at redshifts 8-10 using extensive HST data, revealing potential contributions from AGNs and dust evolution affecting early galaxy formation understanding.

Contribution

It provides the largest systematic measurement of the bright UV luminosity function at z=8-10 from HST observations, including detailed analysis of potential AGN and dust effects.

Findings

Bright end of UVLF can be modeled by Schechter or double power-law functions.

Possible significant AGN contribution at magnitudes M_UV<-22.

No clear evolution of the bright end from z=6-7 to z=8, considering AGN presence.

Abstract

We determine the bright end of the rest-frame UV luminosity function (UVLF) at $z=8-10$ by selecting bright $z\gtrsim 8$ 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 $\sim300$ independent sightlines from WFC3 observations, totalling $800-1300$ arcmin$^2$ (depending on redshift). We identify 31 $z\gtrsim8$ candidates via colour selection and photo-$z$ analysis with observed magnitude ($24.1< H_{160} <26.6$) and $1\sigma$ range probability of $58-95\%$ 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 $M_{UV}\simeq-23$. 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 reionisation.