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Astron. Astrophys. 326, 963-975 (1997)

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Gas and dust in the active spiral galaxy NGC 3079

J. Braine 1, 2, M. Guélin 2, M. Dumke 3, N. Brouillet 1, F. Herpin 1 and R. Wielebinski 3

1 Observatoire de Bordeaux, URA 352, CNRS/INSU, B.P. 89, F-33270 Floirac, France
2 Institut de Radioastronomie Millimétrique, 300 rue de la Piscine, F-38406 St. Martin d'Hères, France
3 Max-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 [FORMULA] continuum and the CO line emission in the exceptional starburst/active galaxy NGC 3079 with the IRAM 30-m telescope.

The [FORMULA] thermal dust emission is much less centrally concentrated than the CO or radio continuum. Roughly 10% of the total [FORMULA] flux comes from the central [FORMULA] 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 [FORMULA], 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 [FORMULA] and [FORMULA].

Based on this cross-section, we estimate an average conversion factor of [FORMULA] in the disk. This is within the range of estimates for our galactic disk, although lower than early estimates.

Given the very low [FORMULA] value found for the nuclear region, we compare it with ratios derived from radiative transfer equations and [FORMULA] CO measurements. These estimates are in good agreement with our dust-based [FORMULA] 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 [FORMULA] 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

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© European Southern Observatory (ESO) 1997

Online publication: April 8, 1998
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