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Molecular and Atomic Gas in the Large Magellanic Cloud. I. Conditions for CO Detection

Wong, T. and Hughes, A. and Fukui, Y. and Kawamura, A. and Mizuno, N. and Ott, J. and Muller, E. and Pineda, J. L. and Welty, D. E. and Kim, S. and Mizuno, Y. and Murai, M. and Onishi, T. (2009) Molecular and Atomic Gas in the Large Magellanic Cloud. I. Conditions for CO Detection. Astrophysical Journal, 696 (1). pp. 370-384. ISSN 0004-637X.

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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(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 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.

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Ott, J.0000-0001-8224-1956
Additional Information:© 2009 The American Astronomical Society. Received 2008 November 17; accepted 2009 February 13; published 2009 April 14. We thank Robert Gruendl for providing the stellar surface density image and Lister Staveley-Smith for providing the Hi data cube for analysis. The survival analysis used in this study was performed with the ASURV software provided by the Penn State Center for Astrostatistics. T.W. acknowledges support from the ATNF and Nagoya University during the early stages of this project, and support from the University of Illinois during its completion. J.O. acknowledges support from NRAO, which is operated by Associated Universities, Inc., under cooperative agreement with the National Science Foundation. J.L.P. was supported by an appointment to the NASA Postdoctoral Program at the Jet Propulsion Laboratory, administered by Oak Ridge Associated Universities through a contract with NASA. S.K. was supported in part by the Korea Science and Engineering Foundation (KOSEF) under a cooperative agreement with the Astrophysical Research Center of the Structure and Evolution of the Cosmos (ARCSEC). We thank the referee for a detailed report which led to significant improvement and clarification of the text and figures.
Funding AgencyGrant Number
Australia Telescope National FacilityUNSPECIFIED
Nagoya UniversityUNSPECIFIED
University of IllinoisUNSPECIFIED
National Radio Astronomy ObservatoryUNSPECIFIED
Korea Science and Engineering FoundationUNSPECIFIED
Astrophysical Research Center of the Structure and Evolution of the Cosmos (ARCSEC)UNSPECIFIED
Subject Keywords:galaxies: ISM; ISM: molecules; Magellanic Clouds
Issue or Number:1
Record Number:CaltechAUTHORS:20090827-105757070
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Official Citation:Molecular and Atomic Gas in the Large Magellanic Cloud. I. Conditions for CO Detection T. Wong, A. Hughes, Y. Fukui, A. Kawamura, N. Mizuno, J. Ott, E. Muller, J. L. Pineda, D. E. Welty, S. Kim, Y. Mizuno, M. Murai, and T. Onishi 2009 ApJ 696 370-384 doi: 10.1088/0004-637X/696/1/370.
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:15353
Deposited By: Tony Diaz
Deposited On:11 Sep 2009 22:26
Last Modified:03 Oct 2019 00:57

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