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

TL;DR

This study delivers the first spatially resolved measurements of the ionizing photon production efficiency, $ξ_{ion,0}$, in 12 UV-bright galaxies during the EoR using JWST/NIRSpec-IFU data. Globally, $ξ_{ion,0}$ is largely consistent with the canonical value, but its values within individual clumps span nearly two orders of magnitude, revealing strong local variation tied to young, highly star-forming regions and lower stellar masses. The analysis highlights robust galaxy- and clump-scale correlations between $ξ_{ion,0}$ and EW(Hβ), sSFR, and ΣSFR, while dust-correction systematics introduce significant uncertainties. The results imply that massive, UV-bright galaxies can drive ionized bubbles of order ~1 pMpc, even with modest LyC escape fractions, underscoring the importance of internal $ξ_{ion,0}$ distributions for modeling reionization and its topology.

Abstract

Measuring the ionizing photon production efficiency $ξ_{\mathrm{ion,0}}$ -- 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 $ξ_{\mathrm{ion,0}}$ 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$α$, 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 $\logξ_{\mathrm{ion,0}}$ 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 $ξ_{\mathrm{ion,0}}$ and specific star formation rate, star formation surface density, H$β$ equivalent width, and stellar mass. Clumps with the highest $ξ_{\mathrm{ion,0}}$ exhibit $\mathrm{EW}_0(\mathrm{H}β) \ge 150$ Angstrom, consistent with young stellar ages. From previous Ly$α$ measurements in three galaxies, we estimate a typical Ly$α$ escape fraction of $f_{\rm esc, Lyα} \sim 2\%$, suggesting similar or lower escape fractions for LyC photons. Combining this with our H$α$ measurements, we infer ionized bubble sizes $\sim 1$ pMpc, aligned with expectations from Ly$α$-detected systems and reionization models.

Figures (8)

• Figure 1: Illustration of the procedure used to determine the mask for extracting individual clumps, shown here for REBELS-15. From left to right, the panels show: continuum-subtracted $\mathrm{H}\alpha$ flux, used for clump identification, within multi-wavelength $7\sigma$ isophotes (Section \ref{['sec:obs']}; Stefanon et al. in prep), with an inset UV continuum image ($1250-2600$ Å)
• Figure 2: Ionizing photon production efficiencies $\xi_{\mathrm{ion,0}}$ in the REBELS-IFU sample vs. redshift. Each galaxy is plotted with a distinct marker symbol, as indicated in the legend. Global, galaxy-integrated values are shown in red, and the spatially resolved measurements in individual clumps are shown in blue. The average $\xi_{\mathrm{ion,0}}$ on each scale is shown as a horizontal band of the corresponding color, with its width representing the 1$\sigma$ dispersion of the measurements. The canonical value of $\xi_{\mathrm{ion,0}}$Robertson2013Wilkins2016 is marked as a black line. Measurements of $\xi_{\mathrm{ion,0}}$ present a large ($\geq 1$ dex) dispersion, but are on average consistent with the canonical value.
• Figure 3: Comparison of $\log(\xi_{\rm ion, 0})$ in the REBELS-IFU sample to physical properties on the galaxy (red markers) and individual-clump (blue markers) scale: a) observed rest-frame $1500$ Å luminosity density; b) observed $\mathrm{H}\alpha$ luminosity; c) $M_{*,\rm tot}$; d) $\rm EW_0(H\beta)$; e) UV slope $\beta$; f) $\rm sSFR_{H\beta}$; g) $\Sigma_{\mathrm{SFR, H\beta}}$; h) $\log(U)$; and i) $12+\log(\rm O/H)$. Spearman's rank correlation coefficients $\rho$ and p-values are shown for each subset of measurements in the corresponding colors in the bottom-right corner. For the significant correlations, we show the linear regression results with dashed lines and linear equations of the corresponding color. The same marker convention is used as in Figure \ref{['fig:global_resolved']}.
• Figure 4: Comparison of $\log(\xi_{\rm ion, 0})$ in the REBELS-IFU sample to measurements in the literature across redshift, with various studies represented by different grey symbols, as indicated in the legend. Galaxy-scale REBELS measurements are shown as red squares, with the green square representing the weighted mean of the sample. Resolved, clump-scale measurements are shown as blue stars. The yellow curve is the parameterization of the redshift evolution in $\xi_{\mathrm{ion,0}}$ by Matthee2017. The canonical value of $\xi_{\mathrm{ion,0}}$ is shown as a grey band.
• Figure 5: Estimates of the radii of ionized bubbles driven by the galaxies in our sample as a function of redshift, assuming $f_{\rm esc,LyC} =1\%$. The Strömgren radii are computed from dust-corrected $\mathrm{H}\alpha$ or $\mathrm{H}\beta$ ($z\geq7.0$) luminosities, assuming $n_H = 1.9 \times10^{-7}~(1+z)^3~\rm cm^{-3}$ and recombination coefficient $\alpha_B = 2.6 \times 10^{-13} \rm~cm^3~s^{-1}$ for $T=10^4 ~\rm K.$ Left: $R_{\rm S}$ vs. redshift. Right: $R_{\rm S}$ vs. stellar mass.