VLA Mapping of the CO(1-0) Line in SMM J14011+0252
  We present high-resolution CO(1-0) observations of the lensed submillimeter
galaxy (SMG) SMM J14011+0252 at z=2.6. Comparison to the previously-detected
CO(3-2) line gives an intensity ratio of r_3,1=0.97+/-0.16 in temperature
units, larger than is typical for SMGs but within the range seen in the low-z
ultraluminous infrared galaxy population. Combining our new data with previous
mid-J CO observations, we perform a single-phase large velocity gradient (LVG)
analysis to constrain the physical conditions of the molecular gas. Acceptable
models have significant degeneracies between parameters, even when we rule out
all models that produce optically thin emission, but we find that the bulk of
the molecular gas has T_kin=20-60 K, n_{H_2}~10^4-10^5 cm^-3, and
N_CO/Delta-v=10^{17.00+/-0.25} cm^-2 km^-1 s. For our best-fit models to
self-consistently recover a typical CO-to-H_2 abundance and a plausible degree
of virialization, the local velocity gradient in the molecular gas must be
substantially larger than its galaxy-wide average. This conclusion is
consistent with a scenario in which SMM J14011+0252 has a fairly face-on
orientation and a molecular ISM composed of many unresolved clouds. Using
previous H-alpha observations, we find that SMM J14011+0252 has a spatially
resolved star formation rate vs. molecular gas surface density relation
inconsistent with those of "normal" local star-forming galaxies, even if we
adopt a local "disk-like" CO-to-H_2 conversion factor as motivated by our LVG
analysis. This discrepancy supports the inference of a star formation relation
for high-z starbursts distinct from the local relation that is not solely due
to differing choices of gas mass conversion factor.
