The Near-Infrared Stellar Populations of Seyfert Galaxies: a First Approach

In this work we investigate the NIR spectra of 24 Seyfert galaxies (9 Sy 1 and 15 Sy 2) observed with the IRTF SpeX. Our main focus was the stellar population (SP), active galactic nuclei (AGN) featureless continuum (FC) and hot dust contribution properties in the spectral range between 0.8 mu m and 2.4 mu m. We have analysed the absorption features located in the near infrared (NIR). The approach followed here for the SP study made use of the STARLIGHT code, for first instance in this spectral range. Besides, in this work we also consider for the first time hot dust as an additional element base. We found evidence of correlation among the equivalent widths (W-lambda) of W-CaT versus W-Mg11.58 mu m W-CaT versus W-CO 2.3 mu m; W-Na I 2.21 mu m versus W-Mg I 1.58 mu m and W-SiI11.59 mu m versus W-Mg11.58 mu m, equally for both kinds of activity. Part of the W-NaI2.21 mu m and W-CO 2.3 mu m strengths and the correlation between W-NaI2.21 mu m and W-Mg I 1.58 mu m appears to be accounted for by galaxy inclination. Also, part of the scatter in the diagrams involving CaT may result from heavy reddening. For the 7 objects in common with previous optical studies using the same method of analyses, the NI R stellar population synthesis does not reproduce well the optical results. Our synthesis shows significant differences between Sy 1 and Sy 2 galaxies. The hot dust component is required to fit the K-band spectra of similar to 80% of the Sy 1 galaxies, and only of similar to 40% of the Sy 2. Besides, about 50% of the Sy 2 galaxies require a FC component contribution >20%, while this fraction increases to about 80% in the Sy 1. Also, in about 50% of the Sy2, the combined FC and young components contribute with more than >20%, while this occurs in 100% of the Sy1, suggesting recent star formation in the central region of our galaxy sample. The light at 1.223 mu m in central few hundred parsec of our galaxy sample contain a substantial fraction of intermediate-age SPs with a meam metalicity near to solar. Our SP synthesis confirms that the 1.1 mu m CN band can be taken as an unambiguous evidence of intermediate-age SP. Our simultaneous fitting of SP, PC and hot dust components, allows us to increase in 150% the number of AGNs with hot dust detected and the mass estimated. The NIR emerges as an excellent window to study the stellar population of Sy 1 galaxies, as opposed to the usually heavily attenuated optical range. Our approach opens a new way to investigate and quantify the individual contribution of the three most important NIR continuum components observed in AGNs.