Estudo de populações estelares em galáxias ativas a partir de cubos de dados no infravermelho próximo

Abstract

The correct characterization of the spectral energy distribution (SED) components in Active Galactic Nuclei (AGN) is the key for their understanding and their role in the evolution of the host galaxies. In the near-infrared (NIR) spectral range, the SED of AGNs, besides the contribution of stellar populations (SPs), presents also components due to the accretion disk (represented by a power law function) and dust emission (represented by a Planck’s function). In this work we have used the Starlight code to perform spectral synthesis using integral field spectroscopy for the inner 1:005 (few hundreds of parsecs) of a sample of six nearby Seyferts galaxies, witth 0:004 z 0:017. The data were obtained with the NIFS (Near-infrared Integral Field Spectrograph) instrument installed on the Gemini North telescope and comprise observations in the J and K bands with an angular resolution of a few tens of parsecs. So far, the methodology used here has been previously used to study the SED of four galaxies and thus, the present work represents an increase of 150% in the sample of studied galaxies. We obtained two-dimensional maps for stellar kinematics by fitting the CO absorption bandheads at 2.3mm present in the spectra of five galaxies of the sample. The exception is Mrk 3, for which the CO bands were not detected by our observations. The stellar velocity fields for all galaxies show a disk rotation pattern, and the velocity dispersion maps (σ) present structures of low values (50–80kms⁻¹) merged in regions of greater values (σ > 100kms⁻¹). Two-dimensional maps were constructed for the contributions to the K-band flux and mass of SPs in four age bins: young (age 100 Myr), young-intermediate (100 < age 700 Myr), intermediate-old (700 Myr < age 2 Gyr) e old (2 < age 13 Gyr). The intermediate age populations are associated to low-s structures, suggesting that these structures are originated by stars recently formed, that are not still viralized with the bulge stars. The two-dimensional maps and radial profiles for the flux contributions of each SP show that the intermediate-old and old populations present the highest contributions in the central region of the galaxy, decreasing with distance to the nucleus, while the young and young-intermediate populations present the opposite behavior, with values increasing with distance to the nucleus. This result supports the “inside-out” star formation scenario, in which stars are first formed at the nucleus and then successively at larger distances from it. Therefore, older SPs are expected to being located at the nucleus and younger populations at larger distances from it. A contribution of blackbody emission was detected at the nucleus of five objects (Mrk 3, Mrk 573, Mrk 607, NGC2110, NGC3227), while the power law component was necessary to fit the nuclear spectrum of four galaxies (Mrk 3, Mrk 573, NGC2110, NGC3227). We estimated the hot dust masses for the object with detected blackbody components, resulting in values of the order of 10⁻²M⨀ , consistent with the range of values available in the literature. It should be noticed that the estimated mass represents only the hot dust component, that presents continuum emission in the NIR, and is possibly associated with the inner region of the dusty torus, predicted by the AGN unified model. Finally, we found a trend of more luminous AGN to present older stellar populations as compared to less luminous objects, contradicting results obtained at optical wavelengths.