F. P. Israel — Page 6

arXiv1998

The obscured circumnuclear region of NGC 3079

Images in the J, H and K bands and in the the v=1-0 S(1) line of H2 of the central region of the almost edge-on galaxy NGC 3079 reveal contributions from direct and scattered starlight, emission from hot dust and molecular gas, and extinction gradients. The central 100 pc suffers an extinction of 6 mag. Extremely red near-infrared colours require the presence of hot dust at about 1000 K. Less reddened parts of the bulge require either a 20% J-band contribution from young stars in a stellar bar, or a 20-30% contribution from scattered stellar light. The nucleus is surrounded by a dense molecular disk of radius 300 pc. Emission from H2 and hot dust traces a cavity of radius 120 pc. In the central few hundred pc, HI spin temperatures must be less than 275 K and the CO-to_H2 conversion factor is at most 5% of the standard Galactic value. This is consistent with theoretical predictions for environments subjected to dissociative shocks, where reformation of H2 is impeded by high dust grain temperatures. The overall molecular gas content of NGC 3079 is normal for a late-type galaxy.

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arXiv1997

H2 and its relation to CO in the LMC and other magellanic irregulars

H2 column densities towards CO clouds in the LMC and SMC are estimated from their far-infrared surface brightness and HI column density. N(H2)/I(CO) conversion factors are X(LMC) = 1.3 and X(SMC) = 12 (unit: 10**21 mol cm**-2 (K km/s)**-1). LMC and SMC contain total (warm) H2 masses of 1.0 and 0.75 x 10**8 M(sun) respectively. Similar local H2/HI mass ratios are found in the magellanic irregulars NGC 1569 (X = 16), NGC 55, 4214 and 6822 (X = 3--6) and in NGC 4449 as well as the extragalactic HII regions NGC 604, 595 and 5461 in M 33 and M 101 (X = 1--2). The CO--H2 conversion factor X depends on both the ambient radiation field intensity per nucleon s(FIR)/N(H) and metallicity [O]/[H]. The results are interpreted as the consequence of selective photodissociation of CO subjected to high radiation field energy densities and poor (self)shielding in low-metallicity environments, and especially the preferential destruction of diffuse CO in `interclump' molecular gas. Although locally H2 may be the dominant ISM-component, the average global H2/HI mass ratio is 0.2 and the average H2 gas mass fraction is 0.12. Magellanic irregulars thus have warm molecular gas fractions very similar to those of our Galaxy, whereas other global properties (mass, luminosity, metallicity, CO luminosity) are very different.

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