The mean Hα 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). ξion,？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 ξion,？0 to the faint population is important, as ultra-faint galaxies are expected to contribute the bulk of the ionizing emissivity. We probe ξion,？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 403 ？ to 2818 ？. The derived ξion,？0 is log10(ξion,0/Hz？erg？1)？=？25.36？±？0.08 over ？20.5？< ？MUV？< ？？17.5, similar to those derived for brighter galaxy samples at the same redshift and therefore suggesting that ξion shows no strong dependence on MUV. The ξion,？0 values found in our sample imply that the Lyman-continuum escape fraction for MUV？≈？？19 star-forming galaxies cannot exceed ≈8–20% in the reionization era}}