2018ApJ...867..150M

We present a full data analysis of the pure-parallel Hubble Space Telescope (HST) imaging observations in the Brightest of Reionizing Galaxies Survey (BoRG[z9]) in Cycle 22. The medium-deep exposures with five HST/WFC3IR+UVIS filter bands from 79 independent sightlines (∼370 arcmin²) provide the least biased determination of number density for z >= 9 bright galaxies against cosmic variance. After a strict two-step selection for candidate galaxies, including dropout color and photometric redshift analyses, and revision of previous BoRG candidates, we identify one source at z ∼ 10 and two sources at z ∼ 9. The z ∼ 10 candidate shows evidence of line-of-sight lens magnification (µ ∼ 1.5), yet it appears surprisingly luminous (M_UV∼-22.6±0.3 mag), making it one of the brightest candidates at z> 8 known (∼0.3 mag brighter than the z = 8.68 galaxy EGSY8p7, spectroscopically confirmed by Zitrin and collaborators). For z ∼ 9 candidates, we include previous data points at fainter magnitudes and find that the data are well fitted by a Schechter luminosity function with α=-2.1_–0.3^+0.3, M_UV^* =-21.0_–1.4^+0.7 mag, and logφ^* =-4.2_–0.9^+0.6 Mpc^–3 mag^–1, for the first time without fixing any parameters. The inferred cosmic star formation rate density is consistent with unaccelerated evolution from lower redshift.