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 26 (~40 pc). We find that significant H I column density (exceeding ~1021 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 H2 near GMCs. We focus on two possibilities to account for this scatter: either there exist spatial variations in the I(CO) to N(H2) 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 Ph is found when the data are binned in Ph . The latter correlation largely reflects the increasing likelihood of CO detection at high H I column density.