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Title:
Molecular and Atomic Gas in the Large Magellanic Cloud. I. Conditions for CO Detection
Authors:
Wong, T.; Hughes, A.; Fukui, Y.; Kawamura, A.; Mizuno, N.; Ott, J.; Muller, E.; Pineda, J. L.; Welty, D. E.; Kim, S.; Mizuno, Y.; Murai, M.; Onishi, T.
Affiliation:
AA(Astronomy Department, University of Illinois, 1002 W. Green St, Urbana, IL 61801, USA ; CSIRO Australia Telescope National Facility, P.O. Box 76, Epping, NSW 1710, Australia ; ), AB(CSIRO Australia Telescope National Facility, P.O. Box 76, Epping, NSW 1710, Australia ; Centre for Astrophysics and Supercomputing, Swinburne University of Technology, P.O. Box 218, Hawthorn, VIC 3122, Australia ), AC(Department of Physics, Nagoya University, Chikusa-ku, Nagoya 464-8602, Japan ), AD(Department of Physics, Nagoya University, Chikusa-ku, Nagoya 464-8602, Japan ), AE(Department of Physics, Nagoya University, Chikusa-ku, Nagoya 464-8602, Japan ; ALMA-J Project Office, National Astronomical Observatory of Japan, 2-21-1 Osawa, Mitaka, 181-8588 Tokyo, Japan ), AF(National Radio Astronomy Observatory, 520 Edgemont Rd, Charlottesville, VA 22903, USA ; California Institute of Technology, 1200 E. California Blvd, Caltech Astronomy, 105-24, Pasadena, CA 91125-2400, USA ; Jansky Fellow of the National Radio Astronomy Observatory. ), AG(CSIRO Australia Telescope National Facility, P.O. Box 76, Epping, NSW 1710, Australia ; Department of Physics, Nagoya University, Chikusa-ku, Nagoya 464-8602, Japan ), AH(Jet Propulsion Laboratory, California Institute of Technology, 4800 Oak Grove Drive, Pasadena, CA 91109, USA ; NASA Postdoctoral Program Fellow.), AI(Astronomy Department, University of Illinois, 1002 W. Green St, Urbana, IL 61801, USA ), AJ(Department of Astronomy and Space Science, Sejong University, KwangJin-gu, KunJa-dong 98, Seoul 143-747, Korea ), AK(Department of Physics, Nagoya University, Chikusa-ku, Nagoya 464-8602, Japan ), AL(Department of Physics, Nagoya University, Chikusa-ku, Nagoya 464-8602, Japan ), AM(Department of Physics, Nagoya University, Chikusa-ku, Nagoya 464-8602, Japan )
Publication:
The Astrophysical Journal, Volume 696, Issue 1, pp. 370-384 (2009). (ApJ Homepage)
Publication Date:
05/2009
Origin:
IOP
ApJ Keywords:
galaxies: ISM, ISM: molecules, Magellanic Clouds
DOI:
10.1088/0004-637X/696/1/370
Bibliographic Code:
2009ApJ...696..370W

Abstract

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 2farcm6 (~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.
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