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

Abstract

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 H 2 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.

Excitation of the molecular gas in the nuclear region of M 82

“…Also the predicted [C ii]/[C i] ratios are an order of magnitude too high and, therefore, a two component interpretation, such as done by Panuzzo et al (2010), is insufficient for modelling the CO excitation in M82. This conclusion is reinforced by earlier analyses by e.g., based on large velocity gradient analysis of multiple CO lines (e.g., Wild et al 1992;Güsten et al 1993;Weiß et al 2001;Ward et al 2003;Weiß et al 2005) …”

“…The 12 CO/ 13 CO abundance ratio is fixed at a value of 40 (similar to the value found by Ward et al 2003). The comparison with the CO lines is shown in Fig.…”

“…Ward et al (2003) present separate observations of the two lobes. Since both fit within our largest beam, we added the fluxes.…”

“…The ground-based data have been scaled to match our observations, using the CO (6−5) and CO (7−6) lines. Both the Ward et al (2003) and Panuzzo et al (2010) data have already been corrected for source-beam coupling factors. This resulted in the two CO excitation diagrams shown in Fig.…”

Probing the molecular interstellar medium of M82 withHerschel-SPIRE spectroscopy

“…Its infrared luminosity (5.6 × 10 10 L , Sanders et al 2003) corresponds to a star-formation rate of 9.8 M yr −1 , which has almost certainly been enhanced by its interaction with M81 and NGC 3077 (Yun et al 1993). With a reported molecular gas content of 1.3 ×10 9 M (Walter et al 2002), its bright emission lines of CO and other molecules allow us to study its ISM in great detail (Shen & Lo 1995;Walter et al 2002;Ward et al 2003).…”