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, using the recently completed second NANTEN CO survey. In particular, we investigate correlations between CO integrated intensity and H i integrated intensity, peak brightness temperature, and line width at a resolution of 2 6 (∼40 pc). We find that significant H i column density (exceeding ∼10 21 cm −2 ) and peak brightness temperature (exceeding ∼20 K) are necessary but not sufficient conditions for CO detection, with many regions of strong H i emission not associated with molecular clouds. The large scatter in CO intensities for a given H i intensity persists even when averaging on scales of >200 pc, indicating that the scatter is not solely due to local conversion of H i 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 H i linewidths supports the second hypothesis, insofar as large linewidths may be indicative of warm H i , and calls into question the likelihood of forming molecular clouds from colliding H i flows. We also find that the ratio of molecular to atomic gas shows no significant correlation (or anticorrelation) 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 H i column density.