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

¹ Leiden Observatory, Leiden University, 2300 RA Leiden, The Netherlands
e-mail: daniellam@strw.leidenuniv.nl
² Centre for Astrophysics and SuperComputing, Swinburne, University of Technology, Hawthorn, Victoria 3122, Australia
³ Leibniz-Institut für Astrophysik Postdam (AIP), An der Sternwarte 16, 14482 Potsdam, Germany
⁴ Univ. Lyon, Univ. Lyon1, ENS de Lyon, CNRS, Centre de Recherche Astrophysique de Lyon UMR5574, 69230 Saint-Genis-Laval, France
⁵ Department of Astronomy, University of Michigan, 1085 South University Ave, Ann Arbor, MI 48109, USA

Received: 7 February 2019
Accepted: 20 June 2019

Abstract

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 Hα rest-frame equivalent widths, which range from 403 Å to 2818 Å. The derived ξ_ion, 0 is log₁₀(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.