The mean H$\alpha$ EW and Lyman-continuum photon production efficiency for faint $z\approx 4-5$ galaxies

Abstract

We present the first measurements of the Lyman-continuum photon production efficiency ${\xi }_{\text{ion,0}}$ at $z\sim 4$-5 for galaxies fainter than 0.2 $L^{\ast }$ ($-$19 mag). ${\xi }_{\text{ion,0}}$ quantifies the production rate of ionizing photons with respect to the UV luminosity density assuming a fiducial escape fraction of zero. Extending previous measurements of ${\xi }_{\text{ion,0}}$ to the faint population is important, as ultra-faint galaxies are expected to contribute the bulk of the ionizing emissivity. We probe ${\xi }_{\text{ion,0}}$ to such faint magnitudes by taking advantage of 200-hour depth Spitzer/IRAC observations from the GREATS program and $\approx$300 3<$z$<6 galaxies with spectroscopic redshifts from the MUSE GTO Deep + Wide programs. Stacked IRAC [3.6]$-$[4.5] colors are derived and used to infer the H$\alpha$ rest-frame equivalent widths, which range from 403Å to 2818Å. The derived ${\xi }_{\text{ion,0}}$ is ${\log }_{10}({\xi }_{\text{ion,0}}/{\text{Hz erg}}^{-1})=25.36\pm 0.08$ over $-$20.5 < M${}_{\text{UV}}$ < $-$17.5, similar to those derived for brighter galaxy samples at the same redshift and therefore suggesting that ${\xi }_{\text{ion}}$ shows no strong dependence on $M_{UV}$. The ${\xi }_{\text{ion,0}}$ values found in our sample imply that the Lyman-continuum escape fraction for $M_{\text{UV}}\approx -19$ star-forming galaxies cannot exceed $\approx$8-20\% in the reionization era.

Published by A&A