Star Formation at z = 2.481 in the Lensed Galaxy SDSS J1110 = 6459. I. Lens Modeling and Source Reconstruction*

Using the combined resolving power of the Hubble Space Telescope and gravitational lensing, we resolve starforming structures in a z similar to 2.5 galaxy on scales much smaller than the usual kiloparsec diffraction limit of HST. SGAS. J111020.0+ 645950.8 is a clumpy, star-forming galaxy lensed by the galaxy cluster SDSS. J1110+ 6459 at z = 0.659, with a total magnification similar to 30 x across the entire arc. We use a hybrid parametric/non-parametric strong lensing mass model to compute the deflection and magnification of this giant arc, reconstruct the light distribution of the lensed galaxy in the source plane, and resolve the star formation into two dozen clumps. We develop a forward-modeling technique to model each clump in the source plane. We ray-trace the model to the image plane, convolve with the instrumental point-spread function (PSF), and compare with the GALFIT model of the clumps in the image plane, which decomposes clump structure from more extended emission. This technique has the advantage, over ray-tracing, of accounting for the asymmetric lensing shear of the galaxy in the image plane and the instrument PSF. At this resolution, we can begin to study star formation on a clump-by-clump basis, toward the goal of understanding feedback mechanisms and the buildup of exponential disks at high redshift.