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

Abstract

Measuring the ionizing photon production efficiency ξ ion,0 —the ratio of the ionizing photon output rate Q H 0 to the UV continuum luminosity L UV —in galaxies at z  > 6 is crucial for constraining their contribution to cosmic reionization. We present integrated and spatially resolved measurements of ξ ion,0 for 12 exceptionally bright ( M UV  ∼ −22 mag) star-forming galaxies at z  ∼ 7 from the Reionization Era Bright Emission Line Survey (REBELS). These measurements are based on JWST NIRSpec/Integral Field Unit PRISM spectroscopy, probing the rest-frame UV and optical regime. Notably, in 8 of the 12 galaxies, the spectral coverage includes H α , 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 ( ξ ion,0 ) values ranging from 25.19 ± 0.11 to 25.61 ± 0.11, with a weighted mean of 25.44 ± 0.15, consistent with the canonical value of ∼25.3. Using a sample of 25 star-forming clumps within these galaxies, we explore local variations in the ionizing photon production efficiency, finding a broader range, from 24.52 ± 0.21 to 26.18 ± 0.61. We identify strong correlations between ξ ion,0 and the specific star formation rate, star formation surface density, H β equivalent width, and stellar mass. Clumps with the highest ξ ion,0 exhibit EW 0 (H β ) ≥ 150 Å, consistent with young stellar ages. From previous Ly α measurements in three galaxies, we estimate a typical Ly α escape fraction of f esc,Ly α  ∼ 2%, suggesting similar or lower escape fractions for LyC photons. Combining this with our H α measurements, we infer ionized bubble sizes ∼1 pMpc, aligned with expectations from Ly α -detected systems and reionization models.