Discovery of a Multiply-Lensed Submillimeter Galaxy in Early HerMES Herschel/SPIRE Data

We report the discovery of a bright ($f(250\text{mum})>400$ mJy), multiply-lensed submillimeter galaxy \obj\ in {\it Herschel}/SPIRE Science Demonstration Phase data from the HerMES project. Interferometric 880\mum\ Submillimeter Array observations resolve at least four images with a large separation of $\sim 9\text{arcsec}$. A high-resolution adaptive optics $K_p$ image with Keck/NIRC2 clearly shows strong lensing arcs. Follow-up spectroscopy gives a redshift of $z=2.9575$, and the lensing model gives a total magnification of $\mu \sim 11\pm 1$. The large image separation allows us to study the multi-wavelength spectral energy distribution (SED) of the lensed source unobscured by the central lensing mass. The far-IR/millimeter-wave SED is well described by a modified blackbody fit with an unusually warm dust temperature, $88\pm 3$ K. We derive a lensing-corrected total IR luminosity of $(1.43\pm 0.09)\times {10}^{13}{\mathrm{L}}_{\odot }$, implying a star formation rate of $\sim 2500, $\mathrm{M}$_{\odot}, $\mathrm{y}\mathrm{r}$^{-1}$. However, models primarily developed from brighter galaxies selected at longer wavelengths are a poor fit to the full optical-to-millimeter SED. A number of other strongly lensed systems have already been discovered in early {\it Herschel} data, and many more are expected as additional data are collected.