The ionizing photon production efficiency (ξ_ion) of lensed dwarf galaxies at z ∼ 2

We measure the ionizing photon production efficiency (ξ_ion) for a sample of faint, low-mass galaxies (10^7.8-10^9.8 M☉) at an intermediate redshift of 1.4 < z < 2.7. This allows us to constrain the total ionizing emissivity and, thus, the contribution of dwarf galaxies to the ionizing background and cosmic reionization. In order to study these faint systems, we target galaxies that are highly magnified by strong lensing galaxy clusters (Abell 1689, MACS J0717, and MACS J1149). We utilize Keck/MOSFIRE rest-optical spectra to measure nebular emission line fluxes and HST rest-UV and rest-optical imaging to measure the photometry. We present two ways of flux stacking. First, we take the average of the log(L_Hα/L_UV ) of galaxies in our sample in order to determine the typical log(ξ_ion). Second, we take the logarithm of the total L_Hα over the total L_UV . We prefer the latter as it provides the total ionizing UV luminosity density of galaxies once it is multiplied by the total non-ionizing UV luminosity density derived from the UV luminosity function. For dwarf galaxies in our sample, log(ξ_ion) calculated from the second method is ∼ 0.2 dex higher than the first method. We do not find any strong dependence between log(ξ_ion) and stellar mass, M_UV or UV spectral slope (β). We report a value of log(ξ_ion) ∼ 25.47 ± 0.09 for our UV-complete sample (−22 < M_UV < −17.3) and ∼ 25.37 ± 0.11 for our mass-complete sample (7.8 < log(M_∗) < 9.8). The measured ξ_ion of our low-mass galaxies is consistent with measurements of more massive and more luminous galaxies in other high-redshift studies that used the same stacking technique. We report a 0.2 − 0.3 dex increase in log(ξ_ion) of our sample relative to low-redshift galaxies of similar mass, indicating an evolution in the stellar properties, possibly due to effects of metallicity, age, or the prevalance of binary stars. We also find a correlation between log(ξ_ion) and the equivalent widths of Hα and [OIII]λ5007 fluxes, confirming that these equivalent widths can be used to estimate ξ_ion.