CO (J=7->6) Observations of NGC 253: Cosmic Ray Heated Warm Molecular Gas

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.