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Issue A&A
Volume 441, Number 3, October III 2005
Page(s) 961 - 973
Section Extragalactic astronomy
DOI 10.1051/0004-6361:20053358



A&A 441, 961-973 (2005)
DOI: 10.1051/0004-6361:20053358

Photon dominated regions in the spiral arms of M 83 and M 51

C. Kramer1, B. Mookerjea1, E. Bayet2, S. Garcia-Burillo3, M. Gerin2, F. P. Israel4, J. Stutzki1 and J. G. A. Wouterloot5

1  KOSMA, I. Physikalisches Institut, Universität zu Köln, Zülpicher Straße 77, 50937 Köln, Germany
    e-mail: kramer@ph1.uni-koeln.de
2  Radioastronomie Millimétrique: UMR 8540 du CNRS, Laboratoire de Physique de l'ENS, 24 rue Lhomond, 75231 Paris Cedex 05, France
3  Centro Astronomico de Yebes, IGN, 19080 Guadalajara, Spain
4  Sterrewacht Leiden, PO Box 9513, 2300 RA Leiden, The Netherlands
5  Joint Astronomy Centre, 660 N. A'ohoku Place, 96720 Hilo, HI, USA

(Received 3 May 2005 / Accepted 2 June 2005)

Abstract
We present [ $\ion$] 3P1-3P0 spectra at four spiral arm positions and the nuclei of the nearby galaxies M 83 and M 51 obtained at the JCMT. The spiral arm positions lie at galacto-centric distances of between 2 kpc and 6 kpc. This data is complemented with maps of CO 1-0, 2-1, and 3-2, and ISO/LWS far-infrared data of [ $\ion{C}{ii}$] (158 $\mu$m), [ $\ion$] (63 $\mu$m), and [ $\ion{N}{ii}$] (122 $\mu$m) allowing for the investigation of a complete set of all major gas cooling lines. From the intensity of the [ $\ion{N}{ii}$] line, we estimate that between 15% and 30% of the observed [ $\ion{C}{ii}$] emission originates from the dense ionized phase of the ISM. The analysis indicates that emission from the diffuse ionized medium is negligible. In combination with the FIR dust continuum, we find gas heating efficiencies below ~$0.21\%$ in the nuclei, and between 0.25 and 0.36% at the outer positions. Comparison with models of photon-dominated regions (PDRs) with the standard ratios [ $\ion$](63)/[ $\ion{C}{ii}$] $_{\rm PDR}$ and ([ $\ion$](63)+[ $\ion{C}{ii}$] $_{\rm PDR}$) vs. TIR, the total infrared intensity, yields two solutions. The physically most plausible solution exhibits slightly lower densities and higher FUV fields than found when using a full set of line ratios, [ $\ion{C}{ii}$] $_{\rm PDR}$/[ $\ion$](1-0), [ $\ion$](1-0)/CO(3-2), CO(3-2)/CO(1-0), [ $\ion{C}{ii}$]/CO(3-2), and, [ $\ion$](63)/[ $\ion{C}{ii}$] $_{\rm PDR}$. The best fits to the latter ratios yield densities of 104 cm-3 and FUV fields of ~G0=20-30 times the average interstellar field without much variation. At the outer positions, the observed total infrared intensities are in agreement with the derived best fitting FUV intensities. The ratio of the two intensities lies at 4-5 at the nuclei, indicating the presence of other mechanisms heating the dust. The [ $\ion$] area filling factors lie below 2% at all positions, consistent with low volume filling factors of the emitting gas. The fit of the model to the line ratios improves significantly if we assume that [ $\ion$] stems from a larger region than CO 2-1. Improved modelling would need to address the filling factors of the various submm and FIR tracers, taking into consideration the presence of density gradients of the emitting gas by including cloud mass and size distributions within the beam.


Key words: galaxies: ISM -- galaxies: structure -- galaxies: individual: M 83, M 51 -- ISM: structure -- infrared: galaxies -- submillimeter

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