CONSTRAINTS ON THE LOW-MASS END OF THE MASS-METALLICITY RELATION AT z = 1-2 FROM LENSED GALAXIES

We present multi-wavelength imaging and near-IR spectroscopy for 10 gravitationally lensed galaxies at 0.9 < z < 2.5 selected from a new, large sample of strong lens systems in the Sloan Digital Sky Survey Data Release 7. We derive stellar masses from the rest-frame UV to near-IR spectral energy distributions, star formation rates (SFRs) from the dust-corrected H alpha flux, and metallicities from the [NII]/H alpha flux ratio. We combine the lensed galaxies with a sample of 60 star-forming galaxies from the literature in the same redshift range for which measurements of [NII]/H alpha have been published. Due to the lensing magnification, the lensed galaxies probe intrinsic stellar masses that are on average a factor of 11 lower than have been studied so far at these redshifts. They have specific SFRs that are an order of magnitude higher than seen for main-sequence star-forming galaxies at z ~ 2. We measure an evolution of 0.16 + or - 0.06 dex in the mass-metallicity relation between z ~ 1.4 and z ~ 2.2. In contrast to previous claims, the redshift evolution is smaller at low stellar masses. We do not see a correlation between metallicity and SFR at fixed stellar mass. The combined sample is in general agreement with the local fundamental relation between metallicity, stellar mass, and SFR from Mannucci et al. Using the Kennicutt-Schmidt law to infer gas fractions, we investigate the importance of gas inflows and outflows on the shape of the mass-metallicity relation using simple analytical models. This suggests that the Maiolino et al. calibration of the [NII]/H alpha flux ratio is biased high.