Spatially resolved physical conditions of molecular gas and potential star formation tracers in M 83, revealed by the Herschel SPIRE FTS^⋆

¹ Laboratoire AIM, CEA/Saclay, L’Orme des Merisiers, 91191 Gif-sur-Yvette, France
e-mail: ronin.wu@cea.fr
² International Research Fellow of the Japan Society for the Promotion of Science (JSPS), Department of Astronomy, the University of Tokyo, Bunkyo-ku, 113-0033 Tokyo, Japan
e-mail: ronin.wu@astron.s.u-tokyo.ac.jp
³ McMaster University, Department of Physics and Astronomy, Hamilton, Ontario, L8S 4M1, Canada
⁴ Center for Astrophysics and Space Astronomy, 389-UCB, University of Colorado, Boulder, CO 80303, USA
⁵ Istituto di Astrofisica e Planetologia Spaziali, INAF, via Fosso del Cavaliere 100, 00133 Roma, Italy
⁶ Centro de Astrobiología (CSIC), Ctra de Torrejn a Ajalvir, km 4, 28850 Torrejón de Ardoz, Madrid, Spain
⁷ Zentrum für Astronomie der Universität Heidelberg, Institut für Theoretische Astrophysik, Albert-Ueberle-Str. 2, 69120 Heidelberg, Germany
⁸ Sterrenkundig Observatorium, Universiteit Gent, Krijgslaan 281 S9, 9000 Gent, Belgium
⁹ Laboratoire d’Astrophysique de Marseille, Université d’Aix-Marseille & CNRS, UMR 7326, 38 rue F. Joliot-Curie, 13388 Marseille Cedex 13, France
¹⁰ Observatoire de Paris – Lab. GEPI, Bât. 11, 5 place Jules Janssen, 92195 Meudon Cedex, France

Received: 19 March 2014
Accepted: 3 December 2014

Abstract

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′ (~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 μm, [CI] 609 μm, [NII] 205 μm, 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] 205 μm 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] 205 μm distribution is more extended than the SFR. The spatially resolved [CI] 370 μm, 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 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 (~2.35 Gyr), 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 ≈ 30 P_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 propagationof 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.