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 _o&dot;) 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 &lt 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.