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

Abstract

Measuring the ionizing photon production efficiency ${\xi }_{\mathrm{ion},0}$ -- the ratio of ionizing photon output rate $Q_{{\mathrm{H}}^0}$ to UV continuum luminosity $L_{\mathrm{UV}}$ -- 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 $25.19\pm 0.11$ to $25.61\pm 0.11$, with a weighted mean of $25.44\pm 0.15$, consistent with the canonical value of $\sim 25.3$. Using a sample of 25 star-forming clumps within these galaxies, we explore local variations in LyC production efficiency, finding a broader range, from $24.52\pm 0.21$ to $26.18\pm 0.61$. 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 $f_{\mathrm{esc},\mathrm{Ly}\alpha }\sim 2\%$, 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.

20 pages, 8 figures. Submitted to ApJ

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