A BRIGHT, SPATIALLY EXTENDED LENSED GALAXY AT z=1.7 BEHIND THE CLUSTER RCS2 032727-132623

We present the discovery of an extremely bright and extended lensed source from the second Red Sequence Cluster Survey (RCS2). RCSGA 032727-132609 is spectroscopically confirmed as a giant arc and counterimage of a background galaxy at z = 1.701, strongly lensed by the foreground galaxy cluster RCS2 032727-132623 at z = 0.564. The giant arc extends over similar to 38 '' and has an integrated r-band magnitude of 19.1, making it similar to 20 times larger and similar to 3.5 times brighter than the prototypical lensed galaxy MS1512-cB58. This is the brightest distant lensed galaxy in the universe known to date. We have collected photometry in nine bands, ranging from u to K(s), which densely sample the rest-frame UV and optical light, including the age-sensitive 4000 angstrom break. A lens model is constructed for the system and results in a robust total magnification of 2.04 +/- 0.16 for the counterimage; we estimate an average magnification of 17.2 +/- 1.4 for the giant arc based on the relative physical scales of the arc and counterimage on the sky. Fits of single-component spectral energy distribution models to the photometry result in a moderately young age, t = 80 +/- 40 Myr, small amounts of dust, E(B - V) <= 0.11, and an exponentially declining star formation history with e-folding time tau = 10-50 Myr. After correcting for the lensing magnification, we find a stellar mass of M(*) similar to 10(10) M(circle dot) and a current star formation rate (SFR) <= 77M(circle dot) yr(-1). Allowing for episodic star formation, an underlying old burst could contain up to twice the mass inferred from single-component modeling. RCSGA 032727-132609 is typical of the known population of star-forming galaxies near this redshift in terms of its age and stellar mass. Its large magnification and spatial extent provide a unique opportunity to study the physical properties of an individual high-redshift star-forming galaxy in great detail, opening up a new window to the process of galaxy evolution between z = 1.7 and our local universe.