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 of ∼1–1.5, using far-UV imaging from the Ultra-Violet Imaging Telescope on board AstroSat. The sample spans a wide range of stellar masses ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mn>8</mml:mn> <mml:mo>.</mml:mo> <mml:mn>3</mml:mn> <mml:mo>≲</mml:mo> <mml:msub> <mml:mi>log</mml:mi> <mml:mn>10</mml:mn> </mml:msub> <mml:mo stretchy="false">(</mml:mo> <mml:msub> <mml:mi>M</mml:mi> <mml:mo>*</mml:mo> </mml:msub> <mml:mo>/</mml:mo> <mml:msub> <mml:mi>M</mml:mi> <mml:mo>⊙</mml:mo> </mml:msub> <mml:mo stretchy="false">)</mml:mo> <mml:mo>≲</mml:mo> <mml:mspace width="0.25em"/> <mml:mn>10</mml:mn> <mml:mo>.</mml:mo> <mml:mn>1</mml:mn> </mml:math> ) and UV luminosities (−20.6 ≲ M UV ≲ −17.5). LyC emission is undetected in most galaxies (42/49), and stacking these galaxies only yields an upper limit on the observed LyC-to-nonionizing UV flux density ratio ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msub> <mml:mrow> <mml:mo stretchy="false">(</mml:mo> <mml:msub> <mml:mrow> <mml:mi>F</mml:mi> </mml:mrow> <mml:mrow> <mml:mi>λ</mml:mi> <mml:mo>,</mml:mo> <mml:mi mathvariant="normal">LyC</mml:mi> </mml:mrow> </mml:msub> <mml:mo>/</mml:mo> <mml:msub> <mml:mrow> <mml:mi>F</mml:mi> </mml:mrow> <mml:mrow> <mml:mi>λ</mml:mi> <mml:mo>,</mml:mo> <mml:mi mathvariant="normal">UV</mml:mi> </mml:mrow> </mml:msub> <mml:mo stretchy="false">)</mml:mo> </mml:mrow> <mml:mrow> <mml:mi mathvariant="normal">obs</mml:mi> </mml:mrow> </mml:msub> <mml:mo>&lt;</mml:mo> <mml:mn>0.12</mml:mn> </mml:math> ). This corresponds to an absolute escape fraction upper limit of a few percent (〈 f esc,abs 〉 &lt;0.03), under a range of stellar population assumptions. Including all galaxies (with seven LyC-leaker candidates) produces a marginal 2.4 σ 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 ∼3 σ 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 ∼ 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 ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msub> <mml:mrow> <mml:mi>log</mml:mi> </mml:mrow> <mml:mrow> <mml:mn>10</mml:mn> </mml:mrow> </mml:msub> <mml:mo stretchy="false">(</mml:mo> <mml:msub> <mml:mrow> <mml:mi>M</mml:mi> </mml:mrow> <mml:mrow> <mml:mo>*</mml:mo> </mml:mrow> </mml:msub> <mml:mo>/</mml:mo> <mml:msub> <mml:mrow> <mml:mi>M</mml:mi> </mml:mrow> <mml:mrow> <mml:mo>⊙</mml:mo> </mml:mrow> </mml:msub> <mml:mo stretchy="false">)</mml:mo> <mml:mo>&lt;</mml:mo> <mml:mn>9.5</mml:mn> </mml:math> ), with their average absolute escape fraction ranging from 〈 f esc,abs 〉 ∼ 0.1–0.2 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.