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CO (J = 7→6) Observations of NGC 253: Cosmic-Ray-heated Warm Molecular Gas
The Astrophysical Journal (2003)
  • C. M. Bradford, Cornell University
  • T. Nikola, Cornell University
  • G. J. Stacey, Cornell University
  • A. D. Bolatto, Boston University
  • J. M. Jackson, Boston University
  • M. L. Savage, Universities of Space Research Association/National Aeronautics and Space Administration (USRA/NASA) Ames Research Center
  • J. A. Davidson, Universities of Space Research Association/National Aeronautics and Space Administration (USRA/NASA) Ames Research Center
  • Sarah J.U. Higdon, University of London
Abstract
We report observations of the CO J ¼ 7 ! 6 transition toward the starburst nucleus of NGC 253. This is the highest excitation CO measurement in this source to date and allows an estimate of the molecular gas excitation conditions. Comparison of the CO line intensities with a large velocity gradient, escape probability model indicates that the bulk of the 2ð Þ 5 107 M of molecular gas in the central 180 pc is highly excited. A model with T 120 K, nH2 4:5  104 cm3, is consistent with the observed CO intensities, as well as the rotational H2 lines observed with the Infrared Space Observatory. The inferred mass of warm, dense molecular gas is 10–30 times the atomic gas mass as traced through its [C ii] and [O i] line emission. This large mass ratio is inconsistent with photodissociation region models in which the gas is heated by far-UV starlight. It is also not likely that the gas is heated by shocks in outflows or cloud-cloud collisions. We conclude that the best mechanism for heating the gas is cosmic rays, which provide a natural means of uniformly heating the full volume of molecular clouds. With the tremendous supernova rate in the nucleus of NGC 253, the cosmic-ray heating rate is at least 800 times greater than that in the Galaxy, more than sufficient to match the cooling observed in the CO lines.
Keywords
  • Galaxies,
  • Individual,
  • NGC 253,
  • ISM,
  • Nuclei,
  • Starburst,
  • Molecules,
  • Submillimeter
Publication Date
April 1, 2003
DOI
10.1086/367854
Publisher Statement
This version of the paper was obtained from arXIV.org. In order for the work to be deposited in arXIV.org, the author must have copyright or it must be available under the Creative Commons Attribution license, Creative Commons Attribution-Noncommercial-ShareAlike license, orCreate Commons Public Domain Declaration. The publisher's final edited version of this article will be available at The Astrophysical Journal.
Citation Information
C. M. Bradford, T. Nikola, G. J. Stacey, A. D. Bolatto, et al.. "CO (J = 7→6) Observations of NGC 253: Cosmic-Ray-heated Warm Molecular Gas" The Astrophysical Journal Vol. 586 Iss. 2 (2003) p. 891 - 901
Available at: http://works.bepress.com/sarah_higdon/35/