MOLECULAR AND ATOMIC GAS IN THE LARGE MAGELLANIC CLOUD. I. CONDITIONS FOR CO DETECTION
- Astronomy Department, University of Illinois, 1002 W. Green St, Urbana, IL 61801 (United States)
- CSIRO Australia Telescope National Facility, P.O. Box 76, Epping, NSW 1710 (Australia)
- Department of Physics, Nagoya University, Chikusa-ku, Nagoya 464-8602 (Japan)
- National Radio Astronomy Observatory, 520 Edgemont Rd, Charlottesville, VA 22903 (United States)
- Jet Propulsion Laboratory, California Institute of Technology, 4800 Oak Grove Drive, Pasadena, CA 91109 (United States)
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 ({approx}40 pc). We find that significant H I column density (exceeding {approx}10{sup 21} cm{sup -2}) and peak brightness temperature (exceeding {approx}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{sub 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{sub 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{sub h} is found when the data are binned in P{sub h} . The latter correlation largely reflects the increasing likelihood of CO detection at high H I column density.
- OSTI ID:
- 21296221
- Journal Information:
- Astrophysical Journal, Vol. 696, Issue 1; Other Information: DOI: 10.1088/0004-637X/696/1/370; Country of input: International Atomic Energy Agency (IAEA); ISSN 0004-637X
- Country of Publication:
- United States
- Language:
- English
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