We report the detection of spatially extended CO ( J =1 \to 0 ) and CO ( J =5 \to 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 ( J =1 \to 0 ) components ( north-east and south-west ; previously identified in CO J =3 \to 2 emission ) with gas masses of 4.3 and 3.5 \times 10 ^ { 10 } ( \alpha _ { CO } / 0.8 ) M _ { \odot } .
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 ( J =1 \to 0 ) observations is \gtrsim 2.5 \times larger than estimated from CO ( J =3 \to 2 ) .
The two components are at approximately the same redshift , but separated by \sim 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 M _ { \odot } yr ^ { -1 } starburst in this system for at least \sim 160 Myr .
We derive line brightness temperature ratios of r _ { 31 } =0.39 \pm 0.09 and 0.37 \pm 0.10 , and r _ { 51 } =0.26 \pm 0.07 and 0.25 \pm 0.08 in the two components , respectively , suggesting that the J \geq 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 .