A MOLECULAR EINSTEIN RING TOWARD THE z = 3.93 SUBMILLIMETER GALAXY MM18423+5938
We present high-resolution imaging of the low-order (J = 1 and 2) CO line emission from the z = 3.93 submillimeter galaxy (SMG) MM18423+5938 using the Expanded Very Large Array, and optical and near-IR imaging using the Canada-France-Hawaii Telescope. This SMG with a spectroscopic redshift was thought to be gravitationally lensed given its enormous apparent brightness. We find that the CO emission is consistent with a complete Einstein ring with a major axis diameter of ~1".4, indicative of lensing. We have also identified the lensing galaxy as a very red elliptical coincident with the geometric center of the ring and estimated its photometric redshift z ~ 1.1. A first estimate of the lens magnification factor is m ~ 12. The luminosity L'_(CO(1–0)) of the CO(1-0) emission is 2.71 ± 0.38 × 10^(11) m^(–1) K km s^(–1) pc^2, and, adopting the commonly used conversion factor for ultraluminous infrared galaxies (ULIRGs), the molecular gas mass is M(H_2) = 2.2 × 10^(11) m^(–1) M_☉, comparable to unlensed SMGs if corrected by m ~ 12. Our revised estimate of the far-IR luminosity of MM18423+5938 is 2 × 10^(13) m^(–1) < L_(FIR) < 3 × 10^(14) m^(–1) L_☉, comparable to that of ULIRGs. Further observations are required to quantify the star formation rate in MM18423+5938 and to constrain the mass model of the lens in more detail.