The nuclear stellar cluster in the Seyfert 1 galaxy NGC 3227: High angularresolution near-infrared imaging and spectroscopy

Near-infrared (NIR) high angular resolution speckle imaging and imaging spe ctroscopy of the nuclear region (similar to 10" approximate to 840 pc) of t he Seyfert 1 galaxy NGC 3227 are presented. The images reveal an unresolved nuclear source in the K band in addition to a nuclear stellar cluster that is slightly resolved in the J and H bands. The contribution of this stella r cluster to the NIR continuum is increasing from the K to the J bands. The stellar absorption lines are extended compared to the neighboring continuu m suggesting a nuclear stellar cluster size of similar to 70 pc FWHM. Analy sis of the stellar absorption lines suggests that the stars are contributin g about 65% (40%) of the total continuum emission in the H (K) band in a 3. "6 aperture. The dominant stellar type is cool M-type stars. Population syn thesis in conjunction with NIR spectral synthesis indicates that the age of the mapped nuclear stellar cluster is in the range of 25-50 Myr when red s upergiants contribute most to the NIR light. This is supported by published optical data on the Mg I b line and the Ca II triplet. Although a higher a ge of similar to0.5 Gyr where asymptotic giant branch (AGB) stars dominate the NIR light cannot be excluded, the observed parameters are at the limit of those expected for a cluster dominated by AGB stars. However, in either case the resolved stellar cluster contributes only about similar to 15% of the total dynamical mass in the inner 300 pc. This immediately implies at l east one other much older stellar population that contributes to the mass b ut not the NIR luminosity. Pure constant star formation over the last 10 Gy r can be excluded based on the observational fact that in such a scenario t he total observed (spatially unresolved and spectrally resolved) Br gamma f lux would be of stellar origin that is spatially extended. Therefore, at le ast two star formation/starburst events took place in the nucleus of NGC 32 27. Since such sequences in the nuclear star formation history are also obs erved in the nuclei of other galaxies a link between the activity of the st ar formation and the AGN itself seems likely.