The Ionizing Photon Production Efficiency (ξ_(ion)) of Lensed Dwarf Galaxies at z ∼ 2

We measure the ionizing photon production efficiency (ξ_(ion)) of low-mass galaxies (10^(7.8)–10^(9.8) M⊙) at 1.4 < z < 2.7 to better understand the contribution of dwarf galaxies to the ionizing background and reionization. We target galaxies that are magnified by strong-lensing galaxy clusters and use Keck/MOSFIRE to measure nebular emission-line fluxes and Hubble Space Telescope to measure the rest-UV and rest-optical photometry. We present two methods of stacking. First, we take the average of the log of Hα-to-UV luminosity ratios (L_(Hα) /L_(UV)) of galaxies to determine the standard 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 when multiplied by the nonionizing UV luminosity density. 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, far-UV magnitude (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). These values are consistent with measurements of more massive, more luminous galaxies in other high-redshift studies that use the same stacking technique. Our log(ξ ion) is 0.2–0.3 dex higher than low-redshift galaxies of similar mass, indicating an evolution in the stellar properties, possibly due to metallicity or age. We also find a correlation between log(ξ_(ion)) and the equivalent widths of Hα and [O III] λ5007 fluxes, confirming that these equivalent widths can be used to estimate ξ_(ion)