The AstroSat UV Deep Field South-V: Constraints on the average escape of ionizing photons in the cosmic dusk

Abstract

We investigate the escape of ionizing (Lyman-continuum; LyC) photons from 49 star-forming galaxies at redshifts $\sim 1-1.5$, using far-ultraviolet (FUV) imaging from the Ultra-Violet Imaging Telescope (UVIT) onboard AstroSat. The sample spans a wide range of stellar masses and UV luminosities. LyC emission is undetected in most galaxies (42/49), and stacking these galaxies yields only an upper limit on the observed LyC-to-nonionizing UV flux density ratio ($({F}_{\lambda,\rm{LyC}}/{F}_{\lambda,\rm{UV}})_{\rm{obs}}<0.12$). Including all galaxies (with 7 LyC-leaker candidates) produces a marginal $2.4\sigma$ detection, suggesting that the average LyC signal is driven by a small number of sources. To identify the conditions favorable for LyC escape, we perform stacking analyses in bins of stellar mass, UV slope, compactness, inclination, and star formation rate surface density. A stacked LyC signal is detected at a significance of $\sim3\sigma$ from a subset of galaxies that are characterized as being compact, have high star formation rate surface densities, and blue UV continuum slopes, despite each of these being individually undetected in LyC. This provides the first systematic evidence at $z\sim 1-1.5$ linking these properties to LyC escape, consistent with trends observed in the lower redshift universe. Additionally, LyC leakage appears more efficient in low-mass galaxies ($\log_{10}(M_{*}/M_{\odot})<9.5$), with their average absolute escape fraction ranging from $\langle f_{\text{esc,abs}} \rangle\sim0.1-0.2$ depending on stellar population assumptions. These results support the scenario that compact, low-mass starbursts were key contributors to the ionizing photon budget during cosmic reionization.