A12COJ = 6–5 Map of M82: The Significance of Warm Molecular Gas

We present a 12 CO J ¼ 6–5 map of the nuclear regions of the starburst galaxy M82 at a resolution of 14 00 taken at the Caltech Submillimeter Observatory (CSO). Hot spots were found on either side of the dynamical center. We compare our results with a high-resolution 12 CO J ¼ 2–1 interferometer map, and present a 12 CO J ¼ 6–5/ 12 CO J ¼ 2–1 line ratio map obtained using a novel deconvolution technique. This line ratio is highest at the two J ¼ 6–5 integrated intensity peaks, reaching 0.4 and 0.5 in the northeast and southwest peaks, respectively, and is typically 0.2 elsewhere in the nuclear region. We also present measurements of 12 CO J ¼ 4–3, 12 CO J ¼ 3–2, and 13 CO J ¼ 3–2, and an upper limit for 13 CO J ¼ 6–5. We analyze these observations in the context of a two-component large velocity gradient (LVG) excitation model. Likelihood density curves were calculated for each of the model parameters and a variety of related physical quantities for the northeast and southwest peaks based on the measured line intensities and their associated uncertainties. This approach reveals in an unbiased way how well various quantities can be constrained by the CO observations. We find that the beam-averaged 12 CO and 13 CO column densities, the isotopomer abundance ratio, and the area filling factors are among the best constrained quantities, while the cool component H2 density and pressure are less well constrained. The results of this analysis suggest that the warm gas is less dense than the cool gas, and that over half of the total molecular gas mass in these nuclear regions is warmer than 50 K. Subject headings: galaxies: individual (M82) — galaxies: ISM — galaxies: starburst — submillimeter