The bright-end galaxy candidates at z ~ 9 from 79 independent HST fields

TL;DR

This study analyzes HST data from 79 fields to identify bright galaxy candidates at z~9-10, providing new insights into galaxy luminosity functions and star formation rates during early cosmic epochs.

Contribution

It presents the first unconstrained Schechter luminosity function fit for z~9 galaxies using a large, unbiased sample from multiple sightlines.

Findings

Identified one z~10 galaxy candidate with high luminosity and possible lens magnification.

Found that the luminosity function at z~9 is well described by a Schechter function with specific parameters.

Cosmic star formation rate density at z~9 is consistent with a steady, unaccelerated evolution from lower redshifts.

Abstract

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 arcmin^2) 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 (mu~1.5), yet it appears surprisingly luminous (MUV ~ -22.6\pm0.3 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 alpha ~ -2.1, MUV ~ -21.5 mag, and log phi ~ -4.5 Mpc^-3mag^-1, for the first time without fixing any parameters. The inferred cosmic star formation rate density is consistent with unaccelerated evolution from lower redshift.