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

Abstract

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 H α -to-UV luminosity ratios ( L H α / L UV ) of galaxies to determine the standard log( ξ ion ). Second, we take the logarithm of the total L H α 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 ( ) 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 H α and [O iii ] λ 5007 fluxes, confirming that these equivalent widths can be used to estimate ξ ion .