Probing the molecular interstellar medium of M82 with Herschel-SPIRE spectroscopy*

We present the observations of the starburst galaxy M82 taken with the HerschelSPIRE Fourier-transform spectrometer. The spectrum (194–671 μm) shows a prominent CO rotational ladder from J= 4–3 to 13–12 emitted by the central region of M82. The fundamental properties of the gas are well constrained by the high Jlines observed for the first time. Radiative transfer modeling of these high-S/N 12CO and 13CO lines strongly indicates a very warm molecular gas component at ~500 K and pressure of ~3×106K cm-3, in good agreement with the H2rotational lines measurements from Spitzerand ISO. We suggest that this warm gas is heated by dissipation of turbulence in the interstellar medium (ISM) rather than X-rays or UV flux from the straburst. This paper illustrates the promise of the SPIRE FTS for the study of the ISM of nearby galaxies.