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 ^ { \prime } ( \sim 4 \mathrm { 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 \mathrm { [ C \textsc { I } ] 370 } \mathrm { \mu m } , \mathrm { [ C \textsc { I } ] 609 } \mathrm { \mu m } , \mathrm { [ N \textsc { II } ] 205 } \mathrm { \mu m } , and CO transitions from \mathrm { J } = 4 - 3 to \mathrm { J } = 13 - 12 .
Combined with previously observed \mathrm { J } = 1 - 0 to \mathrm { J } = 3 - 2 transitions , the CO spectral line energy distributions are translated to spatially resolved physical parameters , column density of CO , N ( \mathrm { CO } ) , and molecular gas thermal pressure , P _ { \mathrm { 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 \mathrm { [ N \textsc { II } ] 205 } \mathrm { \mu m } and the surface density of the star formation rate ( SFR ) , \Sigma _ { \mathrm { SFR } } .
This relation , when compared to integrated properties of ultra-luminous infrared galaxies ( ULIRGs ) , exhibits a different slope , because the \mathrm { [ N \textsc { II } ] 205 } \mathrm { \mu m } distribution is more extended than the SFR .
The spatially resolved \mathrm { [ C \textsc { I } ] 370 } \mathrm { \mu m } , on the other hand , shows a generally linear relationship with \Sigma _ { \mathrm { 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 \mathrm { J } = 1 - 0 transition with the average intensity of interstellar radiation field ( ISRF ) , \langle U \rangle .
This trend implies a decrease in the CO-to-H _ { 2 } conversion factor , X _ { \mathrm { CO } } , when \langle U \rangle increases .
We estimate the gas-to-dust mass ratios to be 77 \pm 33 within the central 2 \mathrm { kpc } and 93 \pm 19 within the central 4 \mathrm { kpc } 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 \pm 0.6 \mathrm { Gyr } , which is shorter than the values for nearby spiral galaxies found in the literature ( \sim 2.35 \mathrm { Gyr } ) , most likely due to the young nuclear starbursts .
A linear relationship between P _ { \mathrm { th } } and the radiation pressure generated by \langle U \rangle , P _ { \mathrm { rad } } , is found to be P _ { \mathrm { th } } \approx 30 P _ { \mathrm { rad } } , which signals that the ISRF alone is insufficient to sustain the observed CO transitions .
The spatial distribution of P _ { \mathrm { 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 .