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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
Received 31 August 1999 / Accepted 20 October 1999
We present high spatial resolution near-infrared broad-band JHK and Br 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 and H 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 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 equivalent widths. Assuming a constant star formation rate with Mu = 100 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 ). We prefer a model of an instantaneous burst of star formation with Mu = 100 occurring 6-7 Myr ago. Gaseous nuclear bars parallel to the stellar nuclear bar were detected in both galaxies, and we derive M 100 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 (email@example.com)
Online publication: January 18, 2000