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C.M. Bradford (Catech), T. Nikola, G.J. Stacey (Cornell), A.D. Bolatto (UC Berkeley), J.M. Jackson (Boston University), M.L. Savage, J.A. Davidson (USRA, NASA Ames), S.J. Higdon (Cornell)
We report observations of the CO J=7-6 transition toward the nucleus of the starburst galaxy NGC~253. This is the highest-excitation CO measurement in this source to date, and allows an estimate of the molecular gas excitation conditions. Application of a large velocity gradient, escape probability model shows that the 12CO and 13CO line intensities are consistent with the full 4 x 107~Mo of molecular gas being at high excitation, T > 100 K, nH2>104 cm-3. This is in contrast with previous studies which invoke multiple molecular gas components.
The mass of warm, dense molecular gas is about 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 a substantial fraction of the gas is heated by shocks in outflows or cloud-cloud collisions, based on energetic considerations and fluorescent near-IR H2 line ratios. On the other hand, the best mechanism for heating the gas is cosmic rays, which provides 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 heat the bulk of the molecular gas to 100 K or higher.
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Bulletin of the American Astronomical Society, 34, #4
© 2002. The American Astronomical Soceity.