The BoRG-$JWST$ Survey: Abundance and Mass-to-light Ratio of Luminous $z=7-9$ Galaxies from Independent Sight Lines with NIRSpec

TL;DR

This study uses JWST spectroscopy across multiple sight lines to accurately measure the abundance and properties of bright galaxies at redshifts 7-9, revealing a higher UV luminosity function than previous estimates and insights into their stellar mass-to-light ratios.

Contribution

It provides the first spectroscopically-confirmed measurement of the bright end of the UVLF at z=7-9 from multiple independent sight lines, reducing cosmic variance effects.

Findings

Bright end of UVLF is higher than previous imaging-based estimates.

Stellar mass scales nearly linearly with UV luminosity.

Large intrinsic scatter in M/L ratios suggests stochastic star formation.

Abstract

We present new results on the rest-frame UV luminosity function (UVLF) and stellar mass-to-light (M/L) ratio of bright (M$_{\rm UV}\lesssim-20$ mag) spectroscopically-confirmed galaxies at $z=7-9$ derived from the BoRG-$JWST$ survey, a unique data set of NIRSpec prism follow up of $HST$-selected sources from random-pointing imaging. By selecting galaxies from over 200 independent sight lines, the survey minimizes cosmic variance ensuring a statistically robust sample of the bright-galaxy population during the epoch of reionization. The data is used to constrain, for the first time, the bright end of the UVLF at $z=7-9$ from spectroscopically-confirmed galaxies over eight independent fields. We find that the bright end of the UVLF is higher than found using imaging over $JWST$ legacy fields, suggesting the latter may be significantly affected by cosmic variance, and thus reducing the tension with recent findings from $JWST$ at $z>10$ and comparable to models invoking little dust attenuation and bursty star formation. Additionally, we use the galaxies' $JWST$ spectra to infer their stellar masses and M/L ratios relative to other $HST$ and $JWST$ studies. We show that the stellar mass scales almost linearly with UV luminosity (M$_* \propto L_{\rm UV}^{0.85\pm0.12}$), albeit with large ($\sim0.5$ dex) intrinsic scatter, consistent with stochastic bursts of star formation in early galaxy formation.