Tracing the molecular gas in distant submillimetre galaxies via CO(1-0)
  imaging with the EVLA
  We report the results of a pilot study with the EVLA of 12CO J=1-0 emission
from four SMGs at z=2.2-2.5, each with an existing detection of CO J=3-2. Using
the EVLA's most compact configuration we detect strong, broad J=1-0 line
emission from all of our targets. The median line width ratio,
sigma(1-0)/sigma(3-2) = 1.15 +/- 0.06, suggests that the J=1-0 is more
spatially extended than the J=3-2 emission, a situation confirmed by our maps
which reveal velocity structure in several cases and typical sizes of ~16 kpc
FWHM. The median Tb ratio is r(3-2/1-0) = 0.55 +/- 0.05, noting that our value
may be biased high because of the J=3-2-based sample selection. Naively, this
suggests gas masses ~2x higher than estimates made using higher-J transitions
of CO, with the discrepency due to the difference in assumed Tb ratio. We also
estimate masses using the 12CO J=1-0 line and the observed global Tb ratios,
assuming standard underlying Tb ratios as well as a limiting SFE, i.e. without
calling upon X(CO). Using this new method, we find a median molecular gas mass
of (2.5 +/- 0.8) x 10^10 Msun, with a plausible range stretching 3x higher.
Even larger masses cannot be ruled out, but are not favoured by dynamical
constraints: the median dynamical mass for our sample is (2.3 +/- 1.4) x 10^11
Msun. We examine the Schmidt-Kennicutt relation for all the distant galaxy
populations for which CO J=1-0 or J=2-1 data are available, finding small
systematic differences. These have previously been interpreted as evidence for
different modes of star formation, but we argue that these differences are to
be expected, given the still considerable uncertainties. Finally, we discuss
the morass of degeneracies surrounding molecular gas mass estimates, the
possibilities for breaking them, and the future prospects for imaging and
studying cold, quiescent molecular gas at high redshifts [abridged].
