Near-infrared line imaging of circumnuclear star formation in nearby active galaxies

We present JHK, Br gamma and H-2 1-0 S(1) imaging of the circumnuclear star forming rings in the active galaxies NGC 1097 and NGC 7771. Their clumpy m orphology varies with wavelength, due to extinction, emission from hot dust and red supergiants, and several stellar generations. The H-2/Br gamma rat ios in the rings indicate the main excitation mechanism of the molecular ga s to be UV radiation from hot young stars. Both galaxies exhibit similar sm all Br gamma equivalent widths. To explain the observations, we prefer a mo del of an instantaneous starburst occurring 6-7 Myr ago, as opposed to a co nstant star forming rate model. A gaseous nuclear bar aligned parallel to t he stellar nuclear bar has been detected in NGC 1097, and we discuss the co nnection between the rings, bars and the fueling of nuclear activity.