Galaxies under the Cosmic Microscope: A Gemini Multiobject Spectrograph Study of Lensed Disk Galaxy 289 in A2218

In this paper, we exploit the gravitational potential of the rich cluster A2218 as a magnifying glass. We demonstrate that the magnification due to the cluster allows us to observe distant background galaxies at a comparable level of detail to galaxies at z ~0.1. Using the Gemini Multiobject Spectrograph (GMOS) integral field unit (IFU) on Gemini North, we observed the spatially resolved [O II] (lambda)3727 emission line spectrum for a lensed disk galaxy at z = 1.034. Using a detailed model for the cluster mass distribution, we are able to correct for the lensing by the cluster and reconstruct the source morphology. We find that the overall magnification is a factor of 4.92 ± 0.15, and the rest-frame absolute I-band magnitude is M(rest)I = -22.4 ± 0.2, where the error bars include conservative estimates of the uncertainty in the source-plane reconstruction. The inclination-corrected circular velocity is 206 ± 18 km s^ -1. The galaxy lies very close to the mean Tully-Fisher relation of present-day spirals. Although our results are based on a single object, they demonstrate that gravitational lensing can be viably used to make detailed studies of the evolution of the structure of distant field galaxies.