CO (J = 7→6) Observations of NGC 253: Cosmic‐Ray–heated Warm Molecular Gas
| Autor: | Sarah J.U. Higdon, Thomas Nikola, Charles M. Bradford, Alberto D. Bolatto, Gordon J. Stacey, Jacqueline A. Davidson, James M. Jackson, Maureen L. Savage |
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| Rok vydání: | 2003 |
| Předmět: |
Physics
Solar mass 010308 nuclear & particles physics Molecular cloud Astronomy and Astrophysics Cosmic ray Astrophysics::Cosmology and Extragalactic Astrophysics Astrophysics Photodissociation region Mass ratio 01 natural sciences Galaxy Supernova Space and Planetary Science 0103 physical sciences 010303 astronomy & astrophysics Astrophysics::Galaxy Astrophysics Line (formation) |
| Zdroj: | The Astrophysical Journal. 586:891-901 |
| ISSN: | 1538-4357 0004-637X |
| Popis: | We report observations of the CO J=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 2-5 x 10^7 solar masses of molecular gas in the central 180 pc is highly excited. A model with T ~ 120 K, n_H_2 ~ 4.5 x 10^4 cm^-3 is consistent with the observed CO intensities as well as the rotational H2 lines observed with ISO. The inferred mass of warm, dense molecular gas is 10--30 times the atomic gas mass as traced through its [CII] and [OI] line emission. This large mass ratio is inconsistent with photodissociation region models where 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 CR heating rate is at least ~800 times greater than in the Galaxy, more than sufficient to match the cooling observed in the CO lines. Comment: 12+ pages, 4 figures, to appear in The Astrophysical Journal (now includes tables) |
| Databáze: | OpenAIRE |
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