1997A&A...326..963B -
Astronomy and Astrophysics, volume 326, 963-975 (1997/10-3)
Gas and dust in the active spiral galaxy NGC 3079.
BRAINE J., GUELIN M., DUMKE M., BROUILLET N., HERPIN F. and WIELEBINSKI R.
Abstract (from CDS):
We present detailed observations of the 1.2mm continuum and the CO line emission in the exceptional starburst/active galaxy NGC 3079 with the IRAM 30-m telescope. The 1.2mm thermal dust emission is much less centrally concentrated than the CO or radio continuum. Roughly 10% of the total 1.2mm flux comes from the central 11" as opposed to ∼40% for the CO or radio continuum. We find no evidence for cool dust in the center of NGC 3079. A dust temperature of 32K fits the data well. We then determine N(H
2)/I
CO(2–1)≃3x10
19cm
–2.(K.km/s)
–1, roughly an order of magnitude below most estimates. In the disk, cool dust is present. For T
dust=17K, the cross-section (or absorption coefficient) we derive for dust in the atomic medium is very close to the value of Draine & Lee (
1984ApJ...285...89D). The combination of the errors in the HI column density and the 1.2mm 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 (λ) is closer to λ
–2 than λ
–1.5 between 100 µm and 1.2mm. Based on this cross-section, we estimate an average conversion factor of N(H
2)/I
CO(2–1)≃1-2x10
20 cm
–2.(K.km/s)
–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(H
2)/ I
CO(2–1) value found for the nuclear region, we compare it with ratios derived from radiative transfer equations and
13CO measurements. These estimates are in good agreement with our dust-based N(H
2)/ I
CO 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(H
2)/I
CO(2–1) ratio slightly and to increase the gas-phase CO abundance.
Abstract Copyright:
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Journal keyword(s):
galaxies: individual: NGC 3079 - galaxies: starburst - galaxies: ISM - galaxies, nuclei - ISM: clouds
Simbad objects:
4
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