Imaging the Molecular Gas Properties of a Major Merger Driving the
  Evolution of a z=2.5 Submillimeter Galaxy
  We report the detection of spatially extended CO 1-0 and 5-4 emission in the
z=2.49 submillimeter galaxy (SMG) J123707+6214, using the Expanded Very Large
Array and the Plateau de Bure Interferometer. The large molecular gas reservoir
is spatially resolved into two CO(1-0) components (north-east and south-west;
previously identified in CO 3-2 emission) with gas masses of 4.3 and 3.5 x
10^10 (alpha_CO/0.8) Msun. We thus find that the optically invisible north-east
component slightly dominates the gas mass in this system. The total molecular
gas mass derived from the CO(1-0) observations is ~2.5 times larger than
estimated from CO(3-2). The two components are at approximately the same
redshift, but separated by ~20 kpc in projection. The morphology is consistent
with that of an early-stage merger. The total amount of molecular gas is
sufficient to maintain the intense 500 Msun/yr starburst in this system for at
least ~160 Myr. We derive line brightness temperature ratios of
r_31=0.39+/-0.09 and 0.37+/-0.10, and r_51=0.26+/-0.07 and 0.25+/-0.08 in the
two components, respectively, suggesting that the J>=3 lines are substantially
subthermally excited. This also suggests comparable conditions for star
formation in both components. Given the similar gas masses of both components,
this is consistent with the comparable starburst strengths observed in the
radio continuum emission. Our findings are consistent with other recent studies
that find evidence for lower CO excitation in SMGs than in high-z quasar host
galaxies with comparable gas masses. This may provide supporting evidence that
both populations correspond to different evolutionary stages in the formation
of massive galaxies.
