Can Blue UV-Bright Galaxies Drive Reionization at Cosmic Dawn? An $M_{\mathrm{UV}}=-19.6$ Galaxy with $f_{\mathrm{esc}}\approx 0.85$ at $z=9.244$

Abstract

We revisit EBG-1, a gravitationally lensed galaxy at $z=9.244$, using newly obtained deep (14 hr) JWST/NIRSpec G235M ($R\sim 1000$) spectroscopy. EBG-1 is one of the bluest galaxies observed at $z>9$. Along with its remarkably blue continuum of $\beta =-2.99\pm 0.15$, we find very weak nebular emission from EBG-1. We obtain a $3\sigma$ upper limit of EW $<8.4$ Angstrom for C III], a marginal H$\beta$ measurement (EW $=256.2\pm 122.2$ Angstrom), and a $5\sigma$ detection of [O III] $\lambda 5007$ (EW $=882.8\pm 180.6$ Angstrom). These lines suggest an oxygen abundance of $12+\log (O/H)=7.16\pm 0.25$ ($\simeq 3\%Z_{\odot }$) and $\log (C/O)<-0.88$ ($3\sigma$), placing EBG-1 on the C/O-metallicity sequence traced by lower-redshift galaxies. To constrain the escape fraction ($f_{\mathrm{esc}}$), we compare the observed UV slope and emission-line equivalent widths with synthetic spectra generated using BEAGLE. The models favor $f_{\mathrm{esc}}\gtrsim 0.75$, with $f_{\mathrm{esc}}\approx 0.85$ providing the best overall agreement with the observations. These results suggest that relatively UV-bright galaxies like EBG-1 can achieve the exceptionally high ionizing efficiencies supported by late reionization scenarios.

arXiv comment15 pages, 6 figures, 2 tables. Submitted to ApJL. Comments welcome!