Measurement of a Metallicity Gradient in a z=2 Galaxy: Implications for
  Inside-Out Assembly Histories
  We present near-infrared imaging spectroscopy of the strongly-lensed z=2.00
galaxy SDSS J120601.69+514227.8 (`the Clone arc'). Using OSIRIS on the Keck 2
telescope with laser guide star adaptive optics, we achieve resolved
spectroscopy with 0.20 arcsecond FWHM resolution in the diagnostic emission
lines [O III], Halpha, and [N II]. The lensing magnification allows us to map
the velocity and star formation from Halpha emission at a physical resolution
of ~300 pc in the galaxy source plane. With an integrated star formation rate
of ~50 Msun/yr, the galaxy is typical of sources similarly studied at this
epoch. It is dispersion-dominated with a velocity gradient of +/- 80 km/s and
average dispersion sigma = 85 km/s; the dynamical mass is 2.4 \times 10^{10}
Msun within a half-light radius of 2.9 kpc. Robust detection of [N II] emission
across the entire OSIRIS field of view enables us to trace the gas-phase
metallicity distribution with 500 pc resolution. We find a strong radial
gradient in both the [N II]/Halpha and [O III]/Halpha ratios indicating a
metallicity gradient of -0.27 +/- 0.05 dex/kpc with central metallicity close
to solar. We demonstrate that the gradient is seen independently in two
multiple images. While the physical gradient is considerably steeper than that
observed in local galaxies, in terms of the effective radius at that epoch, the
gradient is similar. This suggests that subsequent growth occurs in an
inside-out manner with the inner metallicity gradient diminished over time due
to radial mixing and enrichment from star formation.
