Spatially resolved physical conditions of molecular gas and potential star formation tracers in M83, revealed by the Herschel SPIRE FTS

We investigate the physical properties of the molecular and ionized gas, and their relationship to the star formation and dust properties in M 83, based on submillimeter imaging spectroscopy from within the central 3.5' (similar to 4 kpc in diameter) around the starburst nucleus. The observations use the Fourier Transform Spectrometer (FTS) of the Spectral and Photometric Imaging REceiver (SPIRE) onboard the Herschel Space Observatory. The newly observed spectral lines include [CI] 370 mu m, [CI] 609 mu m, [NII] 205 mu m, and CO transitions from J = 4-3 to J = 13-12. Combined with previously observed J = 1-0 to J = 3-2 transitions, the CO spectral line energy distributions are translated to spatially resolved physical parameters, column density of CO, N(CO), and molecular gas thermal pressure, Pth, with a non-local thermal equilibrium (non-LTE) radiative transfer model, RADEX. Our results show that there is a relationship between the spatially resolved intensities of [NIT] 205 pm and the surface density of the star formation rate (SFR), Sigma(SFR). This relation, when compared to integrated properties of ultra-luminous infrared galaxies (ULIRGs), exhibits a different slope, because the [NII] 205 mu m distribution is more extended than the SFR. The spatially resolved [CI] 370 pm, on the other hand, shows a generally linear relationship with Sigma(SFR) and can potentially be a good SFR tracer. Compared with the dust properties derived from broad-band images, we find a positive trend between the emissivity of CO in the J = 1-0 transition with the average intensity of interstellar radiation field (ISRF), < U >. This trend implies a decrease in the CO-to-H-2 conversion factor, X-CO, when < U > increases. We estimate the gas-to-dust mass ratios to be 77 +/- 33 within the central 2 kpc and 93 +/- 19 within the central 4 kpc of M 83, which implies a Galactic dust-to-metal mass ratio within the observed region of M 83. The estimated gas-depletion time for the M 83 nucleus is 1.13 +/- 0.6 Gyr, which is shorter than the values for nearby spiral galaxies found in the literature (similar to 2.35 Gyr), most likely due to the young nuclear starbursts. A linear relationship between Pth and the radiation pressure generated by < U >, P-rad, is found to be P-th approximate to 30 P-rad which signals that the ISRF alone is insufficient to sustain the observed CO transitions. The spatial distribution of Pth reveals a pressure gradient, which coincides with the observed propagation of starburst activities and the alignment of (possibly background) radio sources. We discover that the off-centered (from the optical nucleus) peak of the molecular gas volume density coincides well with a minimum in the relative aromatic feature strength, indicating a possible destruction of their carriers. We conclude that the observed CO transitions are most likely associated with mechanical heating processes that are directly or indirectly related to very recent nuclear starbursts.