HST followup observations of two bright z ~ 8 candidate galaxies from the BoRG pure-parallel survey

TL;DR

This study uses HST followup imaging to confirm the redshift of one bright galaxy candidate at z ~ 8 and refines the galaxy luminosity function, showing it fits a Schechter function without evidence of a different formation process before reionization.

Contribution

The paper provides new HST followup data that confirms one galaxy at z ~ 8 and refines the galaxy luminosity function, challenging previous assumptions about its shape at high redshift.

Findings

Confirmed one galaxy as a probable z ~ 8 source.

Identified the brightest candidate as a lower-redshift interloper.

Refined the UV luminosity function to fit a Schechter function without evidence for deviation.

Abstract

We present followup imaging of two bright (L > L*) galaxy candidates at z > 8 from the Brightest of Reionizing Galaxies (BoRG) survey with the F098M filter on HST/WFC3. The F098M filter provides an additional constraint on the flux blueward of the spectral break, and the observations are designed to discriminate between low- and high-z photometric redshift solutions for these galaxies. Our results confirm one galaxy, BoRG 0116+1425 747, as a highly probable z ~ 8 source, but reveal that BoRG 0116+1425 630 - previously the brightest known z > 8 candidate (mAB = 24.5) - is likely to be a z ~ 2 interloper. As this source was substantially brighter than any other z > 8 candidate, removing it from the sample has a significant impact on the derived UV luminosity function in this epoch. We show that while previous BoRG results favored a shallow power-law decline in the bright end of the luminosity function prior to reionization, there is now no evidence for departure from a Schechter function form and therefore no evidence for a difference in galaxy formation processes before and after reionization.