We present the results of combined deep Keck/NIRC2 , HST /WFC3 near-infrared and Herschel far infrared observations of an extremely star forming dusty lensed galaxy identified from the Herschel Astrophysical Terahertz Large Area Survey ( H -ATLAS J133542.9+300401 ) .
The galaxy is gravitationally lensed by a massive WISE identified galaxy cluster at z \sim 1 .
The lensed galaxy is spectroscopically confirmed at z = 2.685 from detection of CO ( 1 \rightarrow 0 ) by GBT and from detection of CO ( 3 \rightarrow 2 ) obtained with CARMA .
We use the combined spectroscopic and imaging observations to construct a detailed model of the background dusty lensed sub-millemter galaxy ( SMG ) which allows us to study the source plane properties of the target .
The best-fit lens model provide magnification of \mu _ { star } = 2.10 \pm 0.11 and \mu _ { dust } = 2.02 \pm 0.06 for the stellar and dust components respectively .
Multi-band data yields a magnification corrected star formation rate of 1900 ( \pm 200 ) M _ { \odot } { yr ^ { -1 } } and stellar mass of 6.8 _ { -2.7 } ^ { +0.9 } \times 10 ^ { 11 } M _ { \odot } consistent with a main sequence of star formation at z \sim 2.6 .
The CO observations yield a molecular gas mass of 8.3 ( \pm 1.0 ) \times 10 ^ { 10 } M _ { \odot } , similar to the most massive star-forming galaxies , which together with the high star-formation efficiency are responsible for the intense observed star formation rates .
The lensed SMG has a very short gas depletion time scale of \sim 40 Myr .
The high stellar mass and small gas fractions observed indicate that the lensed SMG likely has already formed most of its stellar mass and could be a progenitor of the most massive elliptical galaxies found in the local Universe .