Near-infrared line imaging of NGC 7771

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We present high spatial resolution near-infrared broad-band JHK images and, for the first time, BrGamma 2.1661 micron and H₂ 1-0 S(1) 2.122 micron emission line images of the circumnuclear star forming ring (major axis diameter 7" = 2 kpc) in the starburst galaxy NGC 7771. These data are used to investigate the morphology and extinction of the starburst ring and to study its star forming properties and history by comparing the observed quantities with an evolutionary population synthesis model. The clumpy morphology of NGC 7771 varies strongly with wavelength, due to the combination of extinction (for which we derive an average value of Aᵥ = 2.8), emission from hot dust and red supergiants, and several stellar generations in the ring. Also, the ellipticity and the position angle of the ring depend on the wavelength. The starburst ring in NGC 7771 exhibits small BrGamma equivalent widths. Assuming a constant star formation model with Mᵤ = 100 Mₛun results in very long lifetimes of the star forming regions (up to 1 Gyr), in disagreement with the clumpy near-infrared morphology and the observed radio spectral index of NGC 7771. This situation is only slightly remedied by assuming a reduced upper mass cutoff (Mᵤ = 30 Mₛun), resulting in ages between 8 and 180 Myr. We prefer an instantaneous star formation model with Mᵤ = 100 Mₛun which can explain the derived BrGamma equivalent widths if a single starburst occurred 6-7 Myr ago. The main excitation mechanism of the molecular gas, based on the observed S(1)/BrGamma ratio, appears to be excitation by UV radiation from hot young stars. We derive M = 1900 Mₛun for the mass of the excited H₂.