Near-Infrared Line Imaging of Circumnuclear Star Formation in NearbyActive Galaxies

We present JHK, Brγ and H2 1–0 S(1) imaging of the circumnuclear star forming rings in the active galaxies NGC 1097 and NGC 7771. Their clumpy morphology varies with wavelength, due to extinction, emission from hot dust and red supergiants, and several stellar generations. The H2/Brγ ratios in the rings indicate the main excitation mechanism of the molecular gas to be UV radiation from hot young stars. Both galaxies exhibit similar small Brγ equivalent widths. To explain the observations, we prefer a model of an instantaneous starburst occurring 6–7 Myr ago, as opposed to a constant star forming rate model. A gaseous nuclear bar aligned parallel to the stellar nuclear bar has been detected in NGC 1097, and we discuss the connection between the rings, bars and the fueling of nuclear activity.