MEASUREMENT OF A METALLICITY GRADIENT IN A z = 2 GALAXY: IMPLICATIONS FOR INSIDE-OUT ASSEMBLY HISTORIES

Abstract

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.