2019A&A...627A.164L

We present the first measurements of the Lyman-continuum photon production efficiency ξ_ion,0 at z∼4-5 for galaxies fainter than 0.2L* (-19mag). ξ_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 ξ_ion,0 to the faint population is important, as ultra-faint galaxies are expected to contribute the bulk of the ionizing emissivity. We probe ξ_ion,0 to such faint magnitudes by taking advantage of 200-h depth Spitzer/IRAC observations from the GREATS program and ~=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 Hα rest-frame equivalent widths, which range from 403Å to 2818Å. The derived ξ_ion,0 is log₁₀(ξ_ion,0/(Hz/erg))=25.36+.-0.08 over -20.5<M_UV< -17.5, similar to those derived for brighter galaxy samples at the same redshift and therefore suggesting that ξ_ion shows no strong dependence on M_UV. The ξ_ion,0 values found in our sample imply that the Lyman-continuum escape fraction for M_UV~=-19 star-forming galaxies cannot exceed ~=8-20% in the reionization era.