The mean H α EW and Lyman-continuum photon production efficiency for faint z ≈ 4−5 galaxies

We present the first measurements of the Lyman-continuum photon production efficiency ξ ion, 0 at z ∼ 4–5 for galaxies fainter than 0.2 L * (−19 mag). 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 H α rest-frame equivalent widths, which range from 403 Å to 2818 Å. The derived ξ ion, 0 is log 10 ( ξ ion,0 /Hz erg −1 ) = 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.