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Molecular line intensities as measures of cloud masses – II. Conversion factors for specific galaxy types

Bell, T. A. and Viti, S. and Williams, D. A. (2007) Molecular line intensities as measures of cloud masses – II. Conversion factors for specific galaxy types. Monthly Notices of the Royal Astronomical Society, 378 (3). pp. 983-994. ISSN 0035-8711. doi:10.1111/j.1365-2966.2007.11830.x.

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We present theoretically established values of the CO-to-H_2 and C-to-H_2 conversion factors that may be used to estimate the gas masses of external galaxies. We consider four distinct galaxy types, represented by M51, NGC 6946, M82 and SMC N27. The physical parameters that best represent the conditions within the molecular clouds in each of the galaxy types are estimated using a χ^2 analysis of several observed atomic fine structure and CO rotational lines. This analysis is explored over a wide range of density, radiation field, extinction and other relevant parameters. Using these estimated physical conditions in methods that we have previously established, CO-to-H_2 conversion factors are then computed for CO transitions up to J = 9 → 8. For the conventional CO(1–0) transition, the computed conversion factor varies significantly below and above the canonical value for the Milky Way in the four galaxy types considered. Since atomic carbon emission is now frequently used as a probe of external galaxies, we also present, for the first time, the C-to-H_2 conversion factor for this emission in the four galaxy types considered.

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Additional Information:© 2007 The Authors. Journal compilation © 2007 RAS. Accepted 2007 April 4. Received 2007 March 28; in original form 2006 November 29. We wish to thank Profs Mike Barlow and Tom Millar for helpful comments and discussions, and the referee for constructive comments which helped to improve an earlier draft of this paper. TAB is supported by a PPARC studentship. SV acknowledges individual financial support from a PPARC Advanced Fellowship.
Funding AgencyGrant Number
Particle Physics and Astronomy Research Council (PPARC) Advanced FellowshipUNSPECIFIED
Subject Keywords:ISM : clouds; ISM : molecules; galaxies : ISM; radio lines : galaxies; radio lines : ISM
Issue or Number:3
Record Number:CaltechAUTHORS:20091016-110950345
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Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:16361
Deposited By: Tony Diaz
Deposited On:26 Oct 2009 19:33
Last Modified:08 Nov 2021 23:26

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