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Astron. Astrophys. 353, 834-846 (2000)

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Near-infrared line imaging of the circumnuclear starburst rings in the active galaxies NGC 1097 and NGC 6574

J.K. Kotilainen 1, J. Reunanen 1, S. Laine 2 and S.D. Ryder 3

1 Tuorla Observatory, University of Turku, Väisäläntie 20, 21500 Piikkiö, Finland
2 Department of Physical Sciences, University of Hertfordshire, College Lane, Hatfield, Herts. AL10 9AB, UK
3 Joint Astronomy Centre, 660 N. A'Ohoku Place, Hilo, HI 96720, USA

Received 31 August 1999 / Accepted 20 October 1999

Abstract

We present high spatial resolution near-infrared broad-band JHK and Br[FORMULA] 2.166 µm and H2 1-0 S(1) 2.121 µm emission line images of the circumnuclear star formation rings in the LINER/Seyfert 1 galaxy NGC 1097 and in the Seyfert 2 galaxy NGC 6574. We investigate the morphology, extinction, and the star formation properties and history of the rings, by comparing the observed properties with an evolutionary population synthesis model.

The clumpy morphology in both galaxies varies strongly with wavelength, due to a combination of extinction, emission from hot dust and red supergiants, and the age of the stellar populations in the rings. The near-infrared and radio morphologies are in general agreement, although there are differences in the detailed morphology. From the comparison of Br[FORMULA] and H[FORMULA] fluxes, we derive average extinctions toward the hot spots AV = 1.3 for NGC 1097 and AV = 2.1 for NGC 6574. The observed H2/Br[FORMULA] ratios indicate that in both rings the main excitation mechanism of the molecular gas is UV radiation from hot young stars, while shocks can contribute only in a few regions.

The starburst rings in both galaxies exhibit small Br[FORMULA] equivalent widths. Assuming a constant star formation rate with Mu = 100 [FORMULA] results in extremely long ages (up to 1 Gyr), in disagreement with the morphology and the radio spectral index of the galaxies. This situation is only slightly remedied by a reduced upper mass cutoff (Mu = 30 [FORMULA]). We prefer a model of an instantaneous burst of star formation with Mu = 100 [FORMULA] occurring [FORMULA]6-7 Myr ago. Gaseous nuclear bars parallel to the stellar nuclear bar were detected in both galaxies, and we derive M [FORMULA]100 [FORMULA] for the mass of the excited nuclear H2 emission. Finally, we briefly discuss the connection between the rings, bars and the fuelling of nuclear activity.

Key words: stars: formation – galaxies: individual: NGC 1097 – galaxies: individual: NGC 6574 – galaxies: Seyfert – galaxies: starburst – infrared: galaxies

Present address: Department of Physics and Astronomy, University of Kentucky, Lexington, KY 40506-0055, USA

Present address: Anglo-Australian Observatory, P.O. Box 296, Epping, NSW 1710, Australia

Send offprint requests to: J.K. Kotilainen (jkotilai@stardust.astro.utu.fi)

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© European Southern Observatory (ESO) 2000

Online publication: January 18, 2000
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