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

Abstract

We measure the ionizing photon production efficiency (xi(ion)) of low-mass galaxies (10(7.8)-10(9.8) M<sub) 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 alpha-to-UV luminosity ratios (L-H alpha/L-UV) of galaxies to determine the standard log(xi(ion)). Second, we take the logarithm of the total L-H alpha 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(xi(ion)) calculated from the second method is similar to 0.2 dex higher than the first method. We do not find any strong dependence between log(xi(ion)) and stellar mass, far-UV magnitude (M-UV), or UV spectral slope (beta). We report a value of log(xi(ion)) similar to 25.47 +/- 0.09 for our UV-complete sample (-22 M-UV < -17.3) and similar to 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(xi(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(xi(ion)) and the equivalent widths of H alpha and [O III] lambda 5007 fluxes, confirming that these equivalent widths can be used to estimate xi(ion).