The MOSDEF Survey: Direct Observational Constraints on the Ionizing Photon Production Efficiency, ξion, at z ∼ 2

We combine H α and H β spectroscopic measurements and UV photometry for a sample of 673 galaxies from the MOSDEF survey to constrain hydrogen-ionizing photon production efficiencies ( ) at z = 1.4–2.6. We find , assuming the Calzetti (SMC) curve for the UV dust correction and a scatter of 0.28 dex in the distribution. After accounting for observational uncertainties and variations in dust attenuation, we conclude that the remaining scatter in is likely dominated by galaxy-to-galaxy variations in stellar populations, including the slope and upper-mass cutoff of the initial mass function, stellar metallicity, star formation burstiness, and stellar evolution (e.g., single/binary star evolution). Moreover, is elevated in galaxies with high ionization states (high [O iii ]/[O ii ]) and low oxygen abundances (low [N ii ]/H α and high [O iii ]/H β ) in the ionized ISM. However, does not correlate with the offset from the z ∼ 0 star-forming locus in the BPT diagram, suggesting no change in the hardness of the ionizing radiation accompanying the offset from the z ∼ 0 sequence. We also find that galaxies with blue UV spectral slopes ( ) have elevated by a factor of ∼2 relative to the average of the sample ( ). If these blue galaxies are similar to those at z > 6, our results suggest that a lower Lyman-continuum escape fraction is required for galaxies to maintain reionization, compared to the canonical predictions from stellar population models. Furthermore, we demonstrate that even with robustly dust-corrected H α , the UV dust attenuation can cause on average a ∼0.3 dex systematic uncertainty in calculations.