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 ten
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 (SDSS) DR7. We derive
stellar masses from the rest-frame UV to near-IR spectral energy distributions,
star formation rates (SFR) from the dust-corrected Ha flux, and metallicities
from the [N II]/Ha flux ratio. We combine the lensed galaxies with a sample of
sixty star-forming galaxies from the literature in the same redshift range for
which measurements of [N II]/Ha 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 star formation rates 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+/-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. (2010, 2011). 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.(2008) calibration of the [N II]/Ha flux ratio is biased
high. We conclude that both an absolute metallicity calibration and direct
measurements of the gas mass are needed to use the observed mass-metallicity
relation to gain insight into the impact of gas flows on the chemical evolution
of galaxies.
