Abstract
Near-infrared (NIR) high angular resolution speckle imaging and imaging spectroscopy of the nuclear region (~10'' ≈ 840 pc) of the 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 stellar cluster to the NIR continuum is increasing from the K to the J bands. The stellar absorption lines are extended compared to the neighboring continuum suggesting a nuclear stellar cluster size of ~70 pc FWHM. Analysis of the stellar absorption lines suggests that the stars are contributing about 65% (40%) of the total continuum emission in the H (K) band in a 36 aperture. The dominant stellar type is cool M-type stars. Population synthesis 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 supergiants 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 age of ~0.5 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 ~15% of the total dynamical mass in the inner 300 pc. This immediately implies at least one other much older stellar population that contributes to the mass but not the NIR luminosity. Pure constant star formation over the last 10 Gyr can be excluded based on the observational fact that in such a scenario the total observed (spatially unresolved and spectrally resolved) Brγ flux would be of stellar origin that is spatially extended. Therefore, at least two star formation/starburst events took place in the nucleus of NGC 3227. Since such sequences in the nuclear star formation history are also observed in the nuclei of other galaxies a link between the activity of the star formation and the AGN itself seems likely.
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