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 similar to 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 similar to 1.1. A first estimate of the lens magnification factor is m similar to 12. The luminosity L'(CO(1-0)) of the CO(1 - 0) emission is 2.71 +/- 0.38 x 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 x 10(11) m(-1)M(circle dot), comparable to unlensed SMGs if corrected by m similar to 12. Our revised estimate of the far-IR luminosity of MM18423+5938 is 2 x 10(13) m(-1) < LFIR < 3 x 10(14) m(-1)L(circle dot), 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.