Modeling of the Hermes submillimeter source lensed by a dark matter dominated foreground group of galaxies

We present the results of a gravitational lensing analysis of the bright z(s) = 2.957 submillimeter galaxy (SMG) HERMES found in the Herschel/SPIRE science demonstration phase data from the Herschel Multi-tiered Extragalactic Survey (HerMES) project. The high-resolution imaging available in optical and near-IR channels, along with CO emission obtained with the Plateau de Bure Interferometer, allows us to precisely estimate the intrinsic source extension and hence estimate the total lensing magnification to be mu = 10.9 +/- 0.7. We measure the half-light radius R-eff of the source in the rest-frame near-UV and V bands that characterize the unobscured light coming from stars and find R-eff,R-* = [2.0 +/- 0.1] kpc, in good agreement with recent studies on the SMG population. This lens model is also used to estimate the size of the gas distribution (R-eff,R-gas = [1.1 +/- 0.5] kpc) by mapping back in the source plane the CO (J = 5 -> 4) transition line emission. The lens modeling yields a relatively large Einstein radius R-Ein = 4 ''.10 +/- 0 ''.02, corresponding to a deflector velocity dispersion of [483 +/- 16] km s(-1). This shows that HERMES is lensed by a galaxy group-size dark matter halo at redshift z(l) similar to 0.6. The projected dark matter contribution largely dominates the mass budget within the Einstein radius with f(dm)(<R-Ein) similar to 80%. This fraction reduces to f(dm)(< R-eff,R-G1 similar or equal to 4.5 kpc) similar to 47% within the effective radius of the main deflecting galaxy of stellar mass M-*,M-G1 = [8.5 +/- 1.6] x 10(11) M-circle dot. At this smaller scale the dark matter fraction is consistent with results already found for massive lensing ellipticals at z similar to 0.2 from the Sloan Lens ACS Survey.