GAS AND DUST IN THE ACTIVE SPIRAL GALAXY NGC-3079

We present detailed observations of the 1.2 mm continuum and the CO li ne emission in the exceptional starburst/active galaxy NGC 3079 with t he 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 simil ar 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 32 K fits the da ta well. We then determine N(H-2)ICO(2-1) approximate to 3 X 10(19) cm (-2) (K km s(-1))(-1), roughly an order of magnitude below most estima tes. In the disk, cool dust is present. For T-dust = 17 K, the cross-s ection (or absorption coefficient) we derive for dust in the atomic me dium is very close to the value of Draine & Lee (1984). The combinatio n of the errors in the HI column density and the 1.2 mm thermal dust e mission are probably less than a factor 2 in these and other recent ob servations. Unless the dust temperature has been severely overestimate d, the dependence of the dust opacity on wavelength (lambda) is closer to lambda(-2) than lambda(-1.5) between 100 mu m and 1.2 mm.Based on this cross-section, we estimate an average conversion factor of N(H-2) /ICO(2-1) approximate to 1 - 2 X 10(20) cm(-2)(K km s(-1))(-1) in the disk. This is within the range of estimates for our galactic disk, alt hough lower than early estimates. Given the very low N(H-2)/ICO(2-1) v alue found for the nuclear region, we compare it with ratios derived f rom radiative transfer equations and (CO)-C-13 measurements. These est imates are in good agreement with our dust-based N(H-2)/ICO(2-1) facto r. It appears unlikely that grain emission is enhanced (with respect t o that in the atomic medium) through grain-grain agglomeration or thro ugh condensation of molecules onto grains. The dust mantles may be par tially evaporated in the center of NGC 3079. The dust temperature may allow evaporation of CO-rich mantles and C-shocks are another plausibl e mechanism capable of disrupting mantles while leaving CO molecules i ntact. The effect would be to raise the N(H-2)/ICO(2-1) ratio slightly and to increase the gas-phase CO abundance.