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We present multi-wavelength imaging and near-IR spectroscopy for tengravitationally lensed galaxies at 0.9<z<2.5 selected from a new, large sampleof strong lens systems in the Sloan Digital Sky Survey (SDSS) DR7. We derivestellar masses from the rest-frame UV to near-IR spectral energy distributions,star formation rates (SFR) from the dust-corrected Ha flux, and metallicitiesfrom the [N II]/Ha flux ratio. We combine the lensed galaxies with a sample ofsixty star-forming galaxies from the literature in the same redshift range forwhich measurements of [N II]/Ha have been published. Due to the lensingmagnification, the lensed galaxies probe intrinsic stellar masses that are onaverage a factor of 11 lower than have been studied so far at these redshifts.They have specific star formation rates that are an order of magnitude higherthan seen for main-sequence star-forming galaxies at z~2. We measure anevolution of 0.16+/-0.06 dex in the mass-metallicity relation between z~1.4 andz~2.2. In contrast to previous claims, the redshift evolution is smaller at lowstellar masses. We do not see a correlation between metallicity and SFR atfixed stellar mass. The combined sample is in general agreement with the localfundamental relation between metallicity, stellar mass and SFR from Mannucci etal. (2010, 2011). Using the Kennicutt-Schmidt law to infer gas fractions, weinvestigate the importance of gas inflows and outflows on the shape of themass-metallicity relation using simple analytical models. This suggests thatthe Maiolino et al.(2008) calibration of the [N II]/Ha flux ratio is biasedhigh. We conclude that both an absolute metallicity calibration and directmeasurements of the gas mass are needed to use the observed mass-metallicityrelation to gain insight into the impact of gas flows on the chemical evolutionof galaxies.

Constraints on the low-mass end of the mass-metallicity relation at z=1-2 from lensed galaxies