2015A&A...575A..88W

We investigate the physical properties of the molecular and ionized gas, and their relationship to the star formation and dust properties in M83, based on submillimeter imaging spectroscopy from within the central 3.5' (∼4kpc 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]370µm, [CI]609µm, [NII]205µm, 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, P_th, 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 [NII]205µm and the surface density of the star formation rate (SFR), Σ_SFR. This relation, when compared to integrated properties of ultra-luminous infrared galaxies (ULIRGs), exhibits a different slope, because the [NII]205µm distribution is more extended than the SFR. The spatially resolved [CI]370µm, on the other hand, shows a generally linear relationship with Σ_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-H₂ conversion factor, X_CO, when <U> increases. We estimate the gas-to-dust mass ratios to be 77±33 within the central 2kpc and 93±19 within the central 4kpc of M83, which implies a Galactic dust-to-metal mass ratio within the observed region of M83. The estimated gas-depletion time for the M83 nucleus is 1.13±0.6Gyr, which is shorter than the values for nearby spiral galaxies found in the literature (∼2.35Gyr), most likely due to the young nuclear starbursts. A linear relationship between P_th and the radiation pressure generated by <U>, P_rad, is found to be P_th≃30P_rad, which signals that the ISRF alone is insufficient to sustain the observed CO transitions. The spatial distribution of P_th 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.