Host galaxy-active galactic nucleus alignments in the Sloan Digital Sky Survey Data Release 7

Abstract

We determine the intrinsic shapes and orientations of 27 450 types I and II active galactic nucleus (AGN) galaxies in the spectroscopic sample of the Sloan Digital Sky Survey Data Release 7 by studying the distribution of projected axial ratios of AGN hosts. Our aim is to study possible alignments between the AGN and host galaxy systems (e.g. the accretion disc and the galaxy angular momentum) and the effect of dust obscuration geometry on the AGN type. We define control samples of non‐AGN galaxies that mimic the morphology, colour, luminosity and concentration distributions of the AGN population, taking into account the effects of dust extinction and reddening. Assuming that AGN galaxies have the same underlying 3D shape distribution as their corresponding control samples, we find that the spiral and elliptical type I AGN populations are strongly skewed towards face‐on galaxies, while elliptical and spiral type II AGN populations are skewed towards edge‐on orientations. These findings rule out random orientations for AGN hosts at high confidence for type I spirals (δχ2≈ 230) and type II ellipticals (δχ2≈ 15), while the signal for type I ellipticals and type II spirals is weaker (δχ2≈ 3 and ≈6, respectively). We obtain a much stronger tendency for the type II spirals to be edge‐on when just high [O iii] equivalent width (EW) AGN are considered, suggesting that >20 per cent of low [O iii] EW edge‐on type II AGN may be missing from the optical sample. Galactic dust absorption of the broad‐line region alone cannot explain the observed inclination angle and projected axial ratio distributions of types I and II Seyfert galaxies, implying that obscuration by a small‐scale circumnuclear torus is necessary. These results favour a scenario in which the angular momentum of the material which feeds the black hole retains a memory of its original gas source at least to some small, non‐negligible degree.

Spatially Offset Active Galactic Nuclei. I. Selection and Spectroscopic Properties

“…In this scenario, selection of Type II AGNs may result in a bias toward edge-on galaxies, and therefore a bias toward edge-on dual SMBH systems, assuming they are orbiting in the galaxy plane following a merger. Indeed, there is some evidence that Type II AGN host galaxies show a statistically significant deviation from random orientations, and may deviate toward edge-on systems (Lagos et al 2011).…”

Deep Silicate Absorption Features in Compton-Thick Active Galactic Nuclei Predominantly Arise Due to Dust in the Host Galaxy

“…This is supported by Gandhi et al (2009) who showed in high spatial resolution ground-based photometry of nuclear regions in nearby AGNs that the AGN-produced mid-IR continuum appears almost isotropic and unaffected by Si-absorption. Furthermore, the inclination angle of the host galaxy may have a significant obscuring effect on the AGN emission (e.g., Malkan et al 1998;Matt et al 2000;AlonsoHerrero et al 2011;Lagos et al 2011). In particular, those AGNs presenting strong Si-absorption features are often found to be hosted in highly inclined and/or merging galaxies (Deo et al 2007(Deo et al , 2009).…”

Unveiling the nucleus of NGC 7172

“…high column density towards the nucleus), this may be a non-negligible fraction. Therefore, this work and others (i.e., Blanco et al 1990;Davies et al 2007) has some implications for the results of optical studies such as the results of Shen et al (2010) and Lagos et al (2011). Further investigation of this type of galaxies should be made using optical and NIR data.…”

“…We also note that NGC 7172 was classified as an NHBLR galaxy (Lumsden et al 2001) probably due to the lack of collimated dust grains along the line of sight, supporting our thesis that the torus is almost face-on and the extinction comes from the host galaxy. Shen et al (2010) and Lagos et al (2011) show that there are more Seyfert 2 galaxies in edge-on host galaxies than Seyfert 1's and studying NGC 7172 in the optical yields a Seyfert 2 galaxy. However, a BLR may still be hidden behind the dustlane.…”

“…The UM says that the Seyfert classification is independent of the orientation of the host galaxy. Lately, Seyfert galaxy samples have been investigated with regard to the orientation of their host galaxy by various authors (Shen et al 2010;Ramolla et al 2011;Lagos et al 2011). The results show that Seyfert 1 AGN hosts are predominantly face-on galaxies, whereas Seyfert 2 galaxies tend to be edge-on.…”