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

We present high spatial resolution near-infrared broad-band JHK and Br gamma 2.166 mu m and H-2 1-0S(1)2.121 mu m emission line images of the circumnuclear star Formation rings in the LINER/Seyfert I galaxy NGC 1097 and in the Seyfert 2 galaxy NGC 657 1. 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 gamma and Her fluxes, we derive average extinctions toward the hot spots A(V) = 1.3 for NGC 1097 and A(V) = 2.1 for NGC 6574. The observed H-2/Br gamma 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 gamma equivalent widths. Assuming a constant star formation rate with M-u, = 100 M., results in extremely long ages (up to I 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 (M-u = 30 M.). We prefer a model of an instantaneous burst of star formation with M-u =100 M. occurring similar to 6-7 Myr ago. Gaseous nuclear bars parallel to the stellar nuclear bar were detected in both galaxies, and we derive M similar to 100 M., for the mass of the excited nuclear H-2 emission. Finally, we briefly discuss the connection between the rings, bars and the fuelling of nuclear activity.