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

The obscured circumnuclear region of the outflow galaxy NGC 3079

F.P. Israel ¹, P.P. van der Werf ¹, T.G. Hawarden ² and C. Aspin <sup>2, 3

1</sup> Sterrewacht Leiden, P.O. Box 9513, 2300 RA Leiden, The Netherlands
² Joint Astronomy Centre, 660 N. A'ohoku Pl., Hilo, HI 96720, USA
³ Nordic Optical Telescope, Apartado 474, E-38700 Santa Cruz de la Palma, Canary Islands, Spain

Received 12 December 1997 / Accepted 17 April 1998

Abstract

Images of the central region of the almost edge-on Sc galaxy NGC 3079 in the J, H and K-bands and in the S(1) line of molecular hydrogen are presented. The inner few kiloparsecs of NGC 3079 exhibit a large range of near-infrared colours caused by varying contributions from direct and scattered stellar light, emission from hot dust and extinction gradients. Our results show that interpretation of the observed light distribution requires high-resolution imaging in order to separate the different effects of these contributions.

The central 1" (87 pc) of NGC 3079 suffers a peak extinction . Its extremely red near-infrared colours require the additional presence of hot dust, radiating at temperatures close to 1000 K. The least reddened eastern parts of the bulge require either a contribution of 20% of light in the J-band from a younger population in a stellar bar or a contribution of 20-30% from scattered starlight; scattered light from a nuclear source would require a less likely emission spectrum for that source.

The nucleus is surrounded by a disk of dense molecular material, extending out to a radius of about 300 pc and with a central cavity. Bright H₂ emission and emission from hot dust mark the hole in the CO distribution and trace the inner edge of the dense molecular disk at a radius of 120 pc. Less dense molecular gas and cooler dust extend out to radii of about 2 kpc. In the inner few hundred parsecs of NGC 3079, spin temperatures appear to be well below 275 K and the CO-to-H₂ conversion factor has at most 5 of the Galactic value. An underabundance of H₂ with respect to CO 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.

Key words: infrared: galaxies galaxies: nuclei galaxies: ISM individual: NGC 3079 galaxies: active

Send offprint requests to: F.P. Israel
Correspondence to: israel@strw.leidenuniv.nl

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Contents

• 1. Introduction
• 2. Observations
  • 2.1. Broad-band images
  • 2.2. Molecular hydrogen vibrational emission line images
• 3. Results
  • 3.1. Broad-band images
  • 3.2. H₂ images
• 4. Analysis and discussion
  • 4.1. Continuum colours
    • 4.1.1. The outer bulge
    • 4.1.2. Disk and western dust lane
    • 4.1.3. Stellar emission and K-band excess in the central kiloparsec
  • 4.2. The central region of NGC 3079
    • 4.2.1. Hot dust
    • 4.2.2. H₂ kinematics
    • 4.2.3. Structure of the central region
  • 4.3. Molecular gas in NGC 3079
    • 4.3.1. Relation of CO emission to H₂ column density
    • 4.3.2. Gaseous content of NGC 3079
• 5. Conclusions
• Acknowledgements
• References

© European Southern Observatory (ESO) 1998

Online publication: July 20, 1998
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