Gravitationally Lensed Galaxies at 2<z<3.5: Direct Abundance
  Measurements of Lya Emitters
  Strong gravitational lensing magnifies the flux from distant galaxies,
allowing us to detect emission lines that would otherwise fall below the
detection threshold for medium-resolution spectroscopy. Here we present the
detection of temperature-sensitive oxygen emission lines from three galaxies at
2<z<3.5, which enables us to directly determine the oxygen abundances and
thereby double the number of galaxies at z>2 for which this has been possible.
The three galaxies have ~10% solar oxygen abundances in agreement with strong
emission line diagnostics. Carbon and nitrogen ratios relative to oxygen are
sub-solar as expected for young metal-poor galaxies. Two of the galaxies are
Lya emitters with rest-frame equivalent widths of 20 A and 40 A, respectively,
and their high magnification factors allow us for the first time to gain
insight into the physical characteristics of high-redshift Lya emitters. Using
constraints from the physical properties of the galaxies, we accurately
reproduce their line profiles with radiative transfer models. The models show a
relatively small outflow in agreement with the observed small velocity offsets
between nebular emission and interstellar absorption lines.
