A GLIMPSE into the UV Continuum Slopes of the Faintest Galaxies in the Epoch of Reionization

TL;DR

The paper tackles the uncertain ionizing contribution of faint galaxies ($M_{\rm UV} \gtrsim -16$) during the Epoch of Reionization by leveraging ultra-deep JWST/NIRCam GLIMPSE imaging of Abell S1063 and strong gravitational lensing to measure rest-frame UV slopes $\beta$ for 555 galaxies at $z>6$ down to $M_{\rm UV} \sim -12.5$. UV slopes are derived from rest-frame UV photometry (and cross-checked with SED-based estimates), and indirect inferences of the LyC escape fraction $f_{\rm esc}$ are made using the local $\beta$–$f_{\rm esc}$ relation, supplemented by exploration of nonzero escape in SED models. The results reveal a diverse faint-galaxy population, with no widespread extremely blue slopes and a mean $f_{\rm esc}$ around 14%, peaking near $M_{\rm UV} \approx -16$. When integrated with GLIMPSE's UV luminosity function and ionizing photon production efficiency $\xi_{\rm ion}$, the inferred reionization history aligns with Planck and Ly$\alpha$ forest constraints, indicating that galaxies with $-18 \lesssim M_{\rm UV} \lesssim -14$ predominantly drive reionization, while the faintest galaxies contribute less due to reduced $f_{\rm esc}$ or $\xi_{\rm ion}$.

Abstract

As observations have yet to constrain the ionizing properties of the faintest (M$_{\rm UV}$ > -16) galaxies, their contribution to cosmic reionization remains unclear. The rest-frame ultraviolet (UV) continuum slope ($β$) is a powerful diagnostic of stellar populations and one of the few feasible indicators of the escape fraction of ionizing photons (f$_{\rm esc}$) for such faint galaxies at high-redshift. Leveraging ultra-deep JWST/NIRCam GLIMPSE imaging of strong lensing field Abell S1063, we estimate UV continuum slopes of 555 galaxies at z $>$ 6 with absolute magnitudes down to M$_{\rm UV}$ $\simeq -$12.5. We find a modest evolution of $β$ with redshift and a flattening in the $β$-M$_{\rm UV}$ relation such that galaxies fainter than M$_{\rm UV}$ $\sim -$16.5 no longer exhibit the bluest UV slopes. The 138 ultra-faint galaxies with M$_{\rm UV}$ $> -$16 are a diverse population encompassing dusty (30\%), old (15\%), and low-mass (50\%) galaxies. We apply the empirical $β$-f$_{\rm esc}$ relation from local Lyman continuum leakers, finding the mean f$_{\rm esc}$ peaks at $\sim 20\%$ at M$_{\rm UV}=-$16.5 and declines towards fainter galaxies, while remaining consistent with f$_{\rm esc}$ = 14\% within uncertainties, in agreement with recent radiative transfer simulations. Incorporating GLIMPSE constraints on the UV luminosity function, ionizing photon production efficiency, and escape fractions produces a reionization history consistent with independent observational constraints. Our results indicate galaxies with M$_{\rm UV}$ between $-18$ and $-14$ supplied $\sim 60\%$ of the ionizing photons to cosmic reionization, while the lower f$_{\rm esc}$ of fainter galaxies produces a natural cutoff in the ionizing photon production rate density.

Figures (12)

• Figure 1: Representative galaxies ranging in UV slope ($\beta_{\rm PL}$), absolute magnitude (M$_{\mathrm {UV}}$), and photometric redshift (z) to illustrate the calculation of $\beta_{\rm PL}$, ordered from reddest to bluest. Square points are observed photometry, colored red if used in power-law fitting (red line) and blue otherwise. The model SEDs are each shown in their rest-frame as gray curves with the predicted photometry as empty black circles. Most models can reproduce both the UV continuum and optical emission features, except for sources with extremely blue $\beta_{\rm PL}$ (bottom row).
• Figure 2: Comparison of UV slope measurements from power-law fits to photometry ($\beta_{\rm PL}$) and to model SEDs with $f_{\rm esc}$=0 ($\beta_{\rm SED}$) and with varying $f_{\rm esc}$ ($\beta_{\rm fesc}$). Upper panel: Histograms for each $\beta_{\rm PL}$ (red), $\beta_{\rm SED}$ (blue), and $\beta_{\rm fesc}$ (gray), with $\beta_{\rm PL}$ showing the widest distribution. Lower panel: Comparing the agreement of $\beta_{\rm PL}$ to the UV slope inferred from model SEDs for both $\beta_{\rm SED}$ (blue contours) and $\beta_{\rm fesc}$ (gray contours). Outer and inner contours enclose the 68th and 95th percentiles. The dashed black line indicates the one-to-one relation, from which contours deviate at very blue UV slopes not reproduced by models.
• Figure 3: Mild evolution of $\beta$ with redshift driven by a lack of red galaxies at high redshift. Red diamonds are median bins for galaxies with robust (S/N $\geq$ 5) $\beta$ measurements, plotted as blue diamonds. The two highest redshift sources (blue stars) are the high-z galaxy candidates of Kokorev2025.
• Figure 4: UV continuum slope versus M$_{\mathrm {UV}}$, showcasing the redder UV slopes of the faintest galaxies. We plot the full GLIMPSE sample (blue circles) as well as the JADES photometric sample of Endsley2024 (gray circles). Red diamonds are median bins for robust UV slopes with S/N $\geq$ 5 and the red curve is the best-fit of all robust points. Best-fit relations from Cullen2023Topping2024 are plotted as solid and dashed black lines, respectively. Yellow lines show the combined HST + JWST sample of Zhao2024, extrapolated to fainter magnitudes for redshifts z=6,8,10.
• Figure 5: Notable faint (M$_{\mathrm {UV}}$$> -16$) galaxies observed with GLIMPSE, selected to emphasize the diversity of this subsample. Each column showcases galaxies whose faint magnitudes are primarily driven by dust attenuation (left), older stellar populations (middle), and low stellar masses (right). For each galaxy, we compare the observed photometry (squares) to Bagpipes model photometry (black circles) and show the best-fit SED as gray curves. We list A$_{\rm V}$ and intrinsic total stellar mass from the maximum-likelihood model. We also report the logarithmic ratio of the stellar mass formed in the recent burst to that formed in the older, constant star-formation component (M$_{\rm b}$/M$_{\rm c}$), indicating the relative mass-fraction of the stellar population.