The Physical Properties of Luminous $z\gtrsim8$ Galaxies and Implications for the Cosmic Star Formation Rate Density From ~0.35 deg$^{2}$ of (Pure-)Parallel HST Observations

Guido Roberts-Borsani, Takahiro Morishita, Tommaso Treu, Nicha Leethochawalit, Michele Trenti

Published 2021-06-11Version 1

We present the largest systematic search to date for luminous $z\gtrsim8$ galaxy candidates using ~1267 arcmin$^{2}$ of (pure-)parallel HST observations from the SuperBoRG data set, a compilation of 316 random sightlines with ACS and WFC3 observations, which together represent a factor ~1.4x larger than existing data sets. Using NIR color cuts and careful photo-$z$ analyses, we find 49 $z\sim8-12$ galaxy candidates over 44 unique sightlines, and derive global galaxy properties such as UV magnitudes and continuum slopes, sizes, and rest-frame optical properties (e.g., SFRs, stellar masses, $A_{\rm v}$). Taking advantage of the (pure-)parallel nature of our data set - making it one of the most representative thus far - and derived SFRs, we evaluate the cosmic star formation rate density for the bright end of the luminosity at $z\sim8-10$ and test the validity of luminosity function-derived results using a conversion factor. We find our method yields comparable results to those derived with luminosity functions. Furthermore, we present follow up observations of 4 (Super)BoRG targets with Keck/MOSFIRE, finding no evidence of Ly$\alpha$ in >3 hrs of $Y-$band observations in either, consistent with a largely neutral medium at $z\sim8$. Our results offer a definitive HST legacy on the bright end of the luminosity function and provide a valuable benchmark as well as targets for follow up with JWST.