Molecular and Atomic Gas in the Large Magellanic Cloud - I. Conditions
  for CO Detection
  We analyze the conditions for detection of CO(1-0) emission in the Large
Magellanic Cloud (LMC), using the recently completed second NANTEN CO survey.
In particular, we investigate correlations between CO integrated intensity and
HI integrated intensity, peak brightness temperature, and line width at a
resolution of 2.6' (~40 pc). We find that significant HI column density and
peak brightness temperature are necessary but not sufficient conditions for CO
detection, with many regions of strong HI emission not associated with
molecular clouds. The large scatter in CO intensities for a given HI intensity
persists even when averaging on scales of >200 pc, indicating that the scatter
is not solely due to local conversion of HI into H_2 near GMCs. We focus on two
possibilities to account for this scatter: either there exist spatial
variations in the I(CO) to N(H_2) conversion factor, or a significant fraction
of the atomic gas is not involved in molecular cloud formation. A weak tendency
for CO emission to be suppressed for large HI linewidths supports the second
hypothesis, insofar as large linewidths may be indicative of warm HI, and calls
into question the likelihood of forming molecular clouds from colliding HI
flows. We also find that the ratio of molecular to atomic gas shows no
significant correlation (or anti-correlation) with the stellar surface density,
though a correlation with midplane hydrostatic pressure P_h is found when the
data are binned in P_h. The latter correlation largely reflects the increasing
likelihood of CO detection at high HI column density.
