Abstract
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.
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. The local fundamental relation between
metallicity, stellar mass and SFR from Mannucci et al.(2010) underestimates the
metallicity by 0.5+/-0.3 dex for stellar masses below 10^9.8 Msun. We see no
correlation between SFR and metallicity at fixed stellar mass and thus there is
no evidence for the existence of a fundamental relation for the high specific
star formation rates at z=1-2 probed by this sample. 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.
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