Host galaxy and orientation differences between different types of AGN

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Anamaria Gkini, Manolis Plionis, Maria Chira, Elias Koulouridis

The main purpose of this study is to investigate aspects regarding the validity of the AGN unification paradigm (UP). In particular we focus on the AGN host galaxies, which according to the UP should show no systematic differences depending on the AGN classification. For the purpose of this study we use (a) the spectroscopic SDSS (Sloan Digital Sky Survey) DR14 catalogue, in order to select and classify AGNs using emission line diagnostics, up to a redshift of z=0.2, and (b) the Galaxy Zoo Project catalogue, which classifies SDSS galaxies in two broad Hubble types, spirals and ellipticals. We find that the fraction of type-1 Seyfert nuclei (Sy1) hosted in elliptical galaxies is significantly larger than the corresponding fraction of any other AGN type, while there is a gradient of increasing Spiral-hosts from Sy1 to Liner, type-2 Seyferts (Sy2) and Composite nuclei. These findings cannot be interpreted within the standard Unification Paradigm, but possibly by a co-evolution scheme for supermassive black holes (SMBH) and galactic bulges. Furthermore, for the case of spiral host galaxies we find the Sy1 population to be strongly skewed towards face-on configurations, while the corresponding Sy2 population range in all host-galaxy orientation configurations, having a similar, but not identical, orientation distribution with star-forming galaxies (SF). These results also cannot be interpreted by the standard Unification Paradigm, but point towards a significant contribution of the galactic disk to the obscuration of the nuclear region. This is also consistent with the observed preference of Sy1 nuclei to be hosted by ellipticals, ie., the dusty disk of spiral hosts contributes to the obscuration of the broad line region (BLR) and thus relatively more ellipticals are expected to appear hosting Sy1 nuclei.

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