Astron. Astrophys. 336, 433-444 (1998)

5. Conclusions

1. The central 20" (kpc) of NGC 3079 exhibits a large range of near-infrared colours, representing a varying combination of intrinsic stellar colours, scattered stellar light, emission by hot dust and extinction increasing towards the nucleus. As a consequence, proper interpretation of the observed light in terms of nuclear structure and composition cannot be achieved by the use of photometry in multi-arcsec apertures, but requires imaging at the highest possible resolution.

2. The nucleus suffers from significant extinction, even at near-infrared wavelengths. The peak extinction at a resolution of 1" is . The mean extinction of the inner disk is .

3. The eastern part of the NGC 3079 bulge has () colours too blue to be explained by stars in a typical quiescent bulge, and provide evidence either for a 20% contribution of directly emitted light from young stars in the bulge, or a 20-30% contribution by scattered light from stars in the bulge or in the stellar disk. Scattering of power-law () emission from a nuclear source is less likely as it would require a rather unusual intrinsic spectrum.

4. The JHK colours show the presence of two or three dark lanes obscuring stellar light west of the nucleus. These dust lanes cause significant extinction of both bulge and disk.

5. The colours of the central 3" (pc) are extremely red, peaking at and . They can be explained by the presence of hot dust in the central region, radiating at temperatures close to K. The K-band luminosity of this hot dust is at most of the far-infrared luminosity from the central region, and of the mechanical luminosity that appears to be available from nuclear winds in the central region.

6. Molecular hydrogen S(1) emission originates in a compact source centred on the nucleus and elongated along the major axis, surrounded by a region of lower surface brightness. East of the nucleus, some H₂ emission appears associated with the inner outflow seen at radio and optical wavelengths. It may represent material swept away from the molecular disk out of the plane by the impacting winds. A western counterpart is lacking, and the sharp cutoff of H₂ emission testifies to the significant near-infrared extinction caused by the galaxy disk intervening in the line of sight.

7. The distribution of hot dust emission is practically identical to that of the bright molecular hydrogen emission and just covers the central cavity observed in the CO distribution. This morphology, and the kinematic information obtained from the H₂ images, is supporting evidence for the nuclear wind model proposed by Duric & Seaquist (1988) and for the conclusions reached by Veilleux et al. (1994) and Hawarden et al. (1995). The H₂ and hot dust emission appears to originate in dense material shocked by fast nuclear winds impacting at a radius of about 120 pc on the inner edge of a central molecular disk.

8. The high-density inner molecular disk extends out to a radius of about 290 pc. Cooler dust and molecular gas extend out to a radius of at least 1 kpc.

9. The combination of extinction, absorption optical depth and CO emission places upper limits on both the spin temperature and the CO-to-H₂ conversion factor X in the central region of NGC 3079. For gas-to-dust ratios comparable to those in the solar neighbourhood, the spin temperature is well below 250 K, while is less than . Notwithstanding the concentration of CO emission in the center of NGC 3079, the central regions contain only a small fraction of all molecular hydrogen in the galaxy. The molecular hydrogen content of NGC 3079 is similar to that of other late-type galaxies, but the centrally concentrated CO appears unusually overabundant with respect to H₂, possibly related to the nuclear activity.

© European Southern Observatory (ESO) 1998

Online publication: July 20, 1998
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