2020ApJ...889..180N

The ionizing photon production efficiency, ξ_ion, is a critical parameter that provides a number of physical constraints to the nature of the early universe, including the contribution of galaxies to the timely completion of the reionization of the universe. Here, we use KECK/MOSFIRE and ZFOURGE multiband photometric data to explore the ξ_ion of a population of galaxies at z ∼ 2 with log₁₀(M_*/M_☉)∼9.0–11.5. Our 130 Hα detections show a median log₁₀(ξ_ion[Hzerg^–1]) of 24.8 ± 0.5 when dust corrected using a Calzetti et al. dust prescription. Our values are typical of mass/magnitude selected ξ_ion values observed in the z ∼ 2 universe. Using BPASSv2.2.1 and Starburst99 stellar population models with simple parametric star formation histories (SFH), we find that even with models that account for effects of stellar evolution with binaries/stellar rotation, model galaxies at log₁₀(ξ_ion[Hzerg^–1])≲25.0 have low Hα equivalent widths (EWs) and redder colors compared to our z ∼ 2 observed sample. We find that introducing starbursts to the SFHs resolve the tension with the models; however, due to the rapid time evolution of ξ_ion, Hα EWs, and rest-frame optical colors, our Monte Carlo simulations of starbursts show that random distributions of starbursts in evolutionary time of galaxies are unlikely to explain the observed distribution. Thus, either our observed sample is specially selected based on their past SFH, or stellar models require additional mechanisms to reproduce the observed high UV luminosity of galaxies for a given production rate of hydrogen ionizing photons.