The production and escape of Lyman-Continuum radiation from star-forming galaxies at z~2 and their redshift evolution

Abstract

We study the production rate of ionizing photons of a sample of 588 H$\alpha$ emitters (HAEs) and 160 Lyman-$\alpha$ emitters (LAEs) at $z=2.2$ in the COSMOS field in order to assess the implied emissivity from galaxies, based on their UV luminosity. By exploring the rest-frame Lyman Continuum (LyC) with GALEX/$NUV$ data, we find f${}_{\mathrm{esc}}<2.8(6.4)$% through median (mean) stacking. By combining the H$\alpha$ luminosity density with IGM emissivity measurements from absorption studies, we find a globally averaged $⟨$f${}_{\mathrm{esc}}⟩$ of $5.9_{-4.2}^{+14.5}$ % at $z=2.2$ if we assume HAEs are the only source of ionizing photons. We find similarly low values of the global $⟨$f${}_{\mathrm{esc}}⟩$ at $z\approx 3-5$, also ruling out a high $⟨$f${}_{\mathrm{esc}}⟩$ at $z<5$. These low escape fractions allow us to measure ${\xi }_{ion}$, the number of produced ionizing photons per unit UV luminosity, and investigate how this depends on galaxy properties. We find a typical ${\xi }_{ion}\approx 10^{24.77\pm 0.04}$ Hz erg${}^{-1}$ for HAEs and ${\xi }_{ion}\approx 10^{25.14\pm 0.09}$ Hz erg${}^{-1}$ for LAEs. LAEs and low mass HAEs at $z=2.2$ show similar values of ${\xi }_{ion}$ as typically assumed in the reionization era, while the typical HAE is three times less ionizing. Due to an increasing ${\xi }_{ion}$ with increasing EW(H$\alpha$), ${\xi }_{ion}$ likely increases with redshift. This evolution alone is fully in line with the observed evolution of ${\xi }_{ion}$ between $z\approx 2-5$, indicating a typical value of ${\xi }_{ion}\approx 10^{25.4}$ Hz erg${}^{-1}$ in the reionization era.

Accepted for publication in MNRAS. Main text 15 pages, 8 figures

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