Metallicity Gradient of a Lensed Face-on Spiral Galaxy at Redshift 1.49
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We present the first metallicity gradient measurement for a grand-design face-on spiral galaxy at z$\sim$1.5. This galaxy has been magnified by a factor of 22$\times$ by a massive, X-ray luminous galaxy cluster MACS\,J1149.5+2223 at z=0.544. Using the Laser Guide Star Adaptive Optics aided integral field spectrograph OSIRIS on KECK II, we target the \ha\ emission and achieve a spatial resolution of 0\farcs1, corresponding to a source plane resolution of 170 parsec. The galaxy has well-developed spiral arms and the nebular emission line dynamics clearly indicate a rotationally supported disk with V$_{rot}$/ $\sigma$ $\sim$ 4. The best-fit disk velocity field model yields a maximum rotation of V$_{rot}$${\it \sin{i}}$=150$\pm$15 km s$^{-1}$, and a dynamical mass of M$_{dyn}$=1.3$\pm0.2\times10^{10}${\it csc$^2$(i)}\,M$_{\odot}$ (within 2.5\,kpc), where the inclination angle i=45$\pm10^{\circ}$. Based on the \nii\ and \ha\ ratios, we measured the radial chemical abundance gradient from the inner hundreds of parsecs out to $\sim$5 kilo-parsecs. The slope of the gradient is -0.16$\pm$0.02 dex kpc$^{-1}$, significantly steeper than the gradient of late-type or early-type galaxies in the local universe. If representative of disk galaxies at z$\sim$1.5, our results support an “inside-out” disk formation scenario in which early infall/collapse in the galaxy center builds a chemically enriched nucleus, followed by slow enrichment of the disk over the next 9 Gyrs.
