A Molecular Einstein Ring Toward the z=3.93 Submillimeter Galaxy
  MM18423+593
  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 lens 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 x10^11/m K km s^-1 pc^2, and, adopting the commonly
used conversion factor for ULIRGs, the molecular gas mass is M(H_2) = 2.2
x10^11/m M_sol, comparable to unlensed SMGs if corrected by m ~ 12. Our revised
estimate of the far-IR luminosity of MM18423+5938 is 2 x10^13/m < L_FIR < 3
x10^14/m L_sol, 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.
