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Astron. Astrophys. 326, 963-975


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Gas and dust in the active spiral galaxy NGC 3079

J. Braine1,2, M. Guélin2, M. Dumke3, N. Brouillet1, F. Herpin1, and R. Wielebinski3

1Observatoire de Bordeaux, URA 352, CNRS/INSU, B.P. 89, F-33270 Floirac, France
2Institut de Radioastronomie Millimétrique, 300 rue de la Piscine, F-38406 St. Martin d'Hères, France
3Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, D-53121 Bonn, Germany

Received 5 May 1997 / Accepted 16 June 1997

Abstract

We present detailed observations of the 1.2 mm continuum and the CO line emission in the exceptional starburst/active galaxy NGC 3079 with the IRAM 30-m telescope.

The 1.2 mm thermal dust emission is much less centrally concentrated than the CO or radio continuum. Roughly 10% of the total 1.2 mm flux comes from the central 11'' as opposed to FORMULA % for the CO or radio continuum.

We find no evidence for cool dust in the center of NGC 3079. A dust temperature of FORMULA fits the data well. We then determine N(H2)/ICO(2-1) 3 1019 cm-2 (K km s-1)-1, roughly an order of magnitude below most estimates.

In the disk, cool dust is present. For FORMULA , the cross-section (or absorption coefficient) we derive for dust in the atomic medium is very close to the value of Draine & Lee (1984). The combination of the errors in the HI column density and the FORMULA thermal dust emission are probably less than a factor 2 in these and other recent observations. Unless the dust temperature has been severely overestimated, the dependence of the dust opacity on wavelength ( FORMULA ) is closer to FORMULA than FORMULA between 100 µm and 1.2 mm.

Based on this cross-section, we estimate an average conversion factor of N(H2)/ICO(2-1) 1 2 1020 cm-2 (K km s-1)-1 in the disk. This is within the range of estimates for our galactic disk, although lower than early estimates.

Given the very low N(H2)/ICO(2-1) value found for the nuclear region, we compare it with ratios derived from radiative transfer equations and FORMULACO measurements. These estimates are in good agreement with our dust-based N(H2)/ICO(2-1) factor. It appears unlikely that grain emission is enhanced (with respect to that in the atomic medium) through grain-grain agglomeration or through condensation of molecules onto grains.

The dust mantles may be partially evaporated in the center of NGC 3079. The dust temperature may allow evaporation of CO-rich mantles and C-shocks are another plausible mechanism capable of disrupting mantles while leaving CO molecules intact. The effect would be to raise the N(H2)/ICO(2-1) ratio slightly and to increase the gas-phase CO abundance.

Key words: galaxies: individual: NGC 3079 -- galaxies: starburst -- galaxies: ISM -- galaxies, nuclei -- ISM: clouds

Send offprint requests to: Jonathan Braine in Bordeaux,
braine@observ.u-bordeaux.fr


© European Southern Observatory (ESO) 1997

Online publication: October 1, 1997
Last change: April 8, 1998
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