Models Constraints from Observations of Active Galaxies

Studying the unresolved stellar content of galaxies generally involves disentangling the various components contributing to the spectral energy distribution (SED), and fitting a combination of simple stellar populations (SSPs) to derive information about age, metallicity, and star formation history. In the near-infrared (NIR, 0.85-2.5 mu m), the thermally pulsing asymptotic giant branch (TP-AGB) phase the last stage of the evolution of intermediate-mass (M less than or similar to 6 M-circle dot) stars is a particularly important component of the SSP models. These stars can dominate the emission of stellar populations with ages similar to 0.2-2 Gyr, being responsible for roughly half of the luminosity in the K band. In addition, when trying to describe the continuum observed in active galactic nuclei, the signatures of the central engine and from the dusty torus cannot be ignored. Over the past several years we have developed a method to disentangle these three components. Our synthesis shows significant differences between Seyfert 1 (Sy 1) and Seyfert 2 (Sy 2) galaxies. The central few hundred parsecs of our galaxy sample contain a substantial fraction of intermediate-age populations with a mean metallicity near solar. Two-dimensional mapping of the near-infrared stellar population of the nuclear region of active galaxies suggests that there is a spatial correlation between the intermediate-age stellar population and a partial ring of low stellar velocity dispersion (sigma(star)). Such an age is consistent with a scenario in which the origin of the low-sigma(star) rings is a past event which triggered an inflow of gas and formed stars which still keep the colder kinematics of the gas from which they have formed. We also discuss the fingerprints of features attributed to TP-AGB stars in the spectra of the nuclear regions of nearby galaxies.