Host galaxy-active galactic nucleus alignments in the SDSS-DR7

C. DEL P. LAGOS, N. D. PADILLA, M. A. STRAUSS, S. A. CORA, L. HAO

MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY

WILEY-BLACKWELL PUBLISHING, INC

Año: 2011

0035-8711

We determine the intrinsic shapes and orientations of 27, 450 type I and IIactive galactic nucleus (AGN) galaxies in the spectroscopic sample of theSloan Digital Sky Survey Data Release 7, by studying the distribution ofprojected axis ratios of AGN hosts. Our aim is to study possible alignmentsbetween the AGN and host galaxy systems (e.g. the accretion disc and thegalaxy angular momentum) and the effect of dust obscuration geometry onthe AGN type. We define control samples of non-AGN galaxies that mimic themorphology, colour, luminosity and concentration distributions of the AGNpopulation, taking into account the effects of dust extinction and reddening.By assuming that AGN galaxies have the same underlying three-dimensionalshape distribution as their corresponding control samples, we find that thespiral and elliptical type I AGN populations are strongly skewed towardface-on galaxies, while ellipticals and spirals type II AGN are skewed towardedge-on orientations. These findings rule out random orientations for AGNhosts at high confidence for type I spirals (2  230) and type II ellipticals(2  15), while the signal for type I ellipticals and type II spirals is weaker(2  3 and 2  6, respectively). We obtain a much stronger tendencyfor the type II spirals to be edge-on when just high [OIII] equivalent width(EW) AGN are considered, meaning that > 20% of low [OIII] EW edge-ontype II AGN may be missing from the optical sample. Galactic dust absorptionof the broad-line region alone cannot explain the observed inclination angledistributions of type I and II Seyfert types, implying that obscuration by asmall-scale circumnuclear torus is necessary. These results favour a scenarioin which the angular momentum of the material which feeds the black holeretains a memory of its original gas source at least to some small, non-negligible degree.