Gas and dust in the active spiral galaxy NGC 3079.

Details Gas and dust in the active spiral galaxy NGC 3079. Gas and dust in the active spiral galaxy NGC 3079.

Oct 1997

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1997a%26a...326..963b&link_type=abstract

Astronomy and Astrophysics, v.326, p.963-975

Astronomy and Astrophysics

Astrophysics

29

Galaxies: Individual: Ngc 3079, Galaxies: Starburst, Galaxies: Ism, Galaxies, Nuclei, Ism: Clouds

Scientific paper

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)_=~3x10^19^cm^-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 ^13^CO 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.

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