REBELS-IFU: Spatially Resolved Ionizing Photon Production Efficiencies of 12 Bright Galaxies in the Epoch of Reionization

TL;DR

This study measures the ionizing photon production efficiency in 12 bright galaxies at z~7 using JWST data, revealing global and local variations, correlations with galaxy properties, and implications for cosmic reionization.

Contribution

First spatially resolved measurements of ionizing photon efficiency in bright z~7 galaxies using JWST, linking local variations to galaxy properties and reionization models.

Findings

Global $\xi_{ m ion,0}$ values consistent with canonical estimates.

Significant local variation in $\xi_{ m ion,0}$ among star-forming clumps.

Correlations between $\xi_{ m ion,0}$ and star formation indicators.

Abstract

Measuring the ionizing photon production efficiency $\xi_{\mathrm{ion,0}}$ -- the ratio of ionizing photon output rate $Q_{\rm H^0}$ to UV continuum luminosity $L_{\rm UV}$ -- in galaxies at $z > 6$ is crucial for constraining their contribution to cosmic reionization. We present integrated and spatially resolved measurements of $\xi_{\mathrm{ion,0}}$ for 12 exceptionally bright ($M_\mathrm{UV} \sim -22$ mag) star-forming galaxies at $z \sim 7$ from the REBELS survey. These measurements are based on JWST NIRSpec/IFU PRISM spectroscopy, probing the rest-frame UV and optical regime. Notably, in 8 of the 12 galaxies, the spectral coverage includes H$\alpha$, enabling self-consistent dust attenuation estimates in both the ionized gas and stellar continuum via the Balmer decrement and rest-UV slope, respectively. We find global $\log\xi_{\mathrm{ion,0}}$ values ranging from $25.19\pm0.11$ to $25.61\pm0.11$, with a weighted mean of $25.44\pm0.15$, consistent with the canonical value of $\sim25.3$. Using a sample of 25 star-forming clumps within these galaxies, we explore local variations in LyC production efficiency, finding a broader range, from $24.52\pm0.21$ to $26.18\pm0.61$. We identify strong correlations between $\xi_{\mathrm{ion,0}}$ and specific star formation rate, star formation surface density, H$\beta$ equivalent width, and stellar mass. Clumps with the highest $\xi_{\mathrm{ion,0}}$ exhibit $\mathrm{EW}_0(\mathrm{H}\beta) \ge 150$ Angstrom, consistent with young stellar ages. From previous Ly$\alpha$ measurements in three galaxies, we estimate a typical Ly$\alpha$ escape fraction of $f_{\rm esc, Ly\alpha} \sim 2\%$, suggesting similar or lower escape fractions for LyC photons. Combining this with our H$\alpha$ measurements, we infer ionized bubble sizes $\sim 1$ pMpc, aligned with expectations from Ly$\alpha$-detected systems and reionization models.