Herschel-SPIRE Imaging Spectroscopy of Molecular Gas in M82
  We present new Herschel-SPIRE imaging spectroscopy (194-671 microns) of the
bright starburst galaxy M82. Covering the CO ladder from J=4-3 to J=13-12,
spectra were obtained at multiple positions for a fully sampled ~ 3 x 3
arcminute map, including a longer exposure at the central position. We present
measurements of 12CO, 13CO, [CI], [NII], HCN, and HCO+ in emission, along with
OH+, H2O+ and HF in absorption and H2O in both emission and absorption, with
discussion. We use a radiative transfer code and Bayesian likelihood analysis
to model the temperature, density, column density, and filling factor of
multiple components of molecular gas traced by 12CO and 13CO, adding further
evidence to the high-J lines tracing a much warmer (~ 500 K), less massive
component than the low-J lines. The addition of 13CO (and [CI]) is new and
indicates that [CI] may be tracing different gas than 12CO. No
temperature/density gradients can be inferred from the map, indicating that the
single-pointing spectrum is descriptive of the bulk properties of the galaxy.
At such a high temperature, cooling is dominated by molecular hydrogen.
Photon-dominated region (PDR) models require higher densities than those
indicated by our Bayesian likelihood analysis in order to explain the high-J CO
line ratios, though cosmic-ray enhanced PDR models can do a better job
reproducing the emission at lower densities. Shocks and turbulent heating are
likely required to explain the bright high-J emission.
