Thursday June 20th, 2013

Dust in the polar region as a major contributor to the IR emission of AGN

Sebastian F. Hoenig (1), Makoto Kishimoto (2), Konrad R. W. Tristram (2), M. Almudena Prieto (3), Poshak Gandhi (4,5), Daniel Asmus (2), Robert Antonucci (1), Leonard Burtscher (6), Wolfgang J. Duschl (7,8), Gerd Weigelt (2) ((1) UCSB, (2) MPIfR, (3) IAC, (4) ISAS, JAXA, (5) Durham University, (6) MPE, (7) University of Kiel, (8) Steward Observatory)
(Submitted on 18 Jun 2013)
(abridged) It is generally assumed that the distribution of dust on parsec scales forms a geometrically- and optically-thick entity in the equatorial plane around the accretion disk and broad-line region – dubbed “dust torus” – that emits the bulk of the sub-arcsecond-scale IR emission and gives rise to orientation-dependent obscuration. Here we report detailed interferometry observations of the unobscured (type 1) AGN in NGC 3783 that allow us to constrain the size, elongation, and direction of the mid-IR emission with high accuracy. The mid-IR emission is characterized by a strong elongation toward position angle PA -52 deg, closely aligned with the polar axis (PA -45 deg). We determine half-light radii along the major and minor axes at 12.5 {\mu}m of (4.23 +/- 0.63) pc x (1.42 +/- 0.21) pc, which corresponds to intrinsically-scaled sizes of (69.4 +/- 10.8) rin x (23.3 +/- 3.5) rin for the inner dust radius of rin = 0.061 pc as inferred from near-IR reverberation mapping. This implies an axis ratio of 3:1, with about 60-90% of the 8-13 {\mu}m emission associated with the polar-elongated component. These observations are difficult to reconcile with the standard interpretation that most of the
parsec-scale mid-IR emission in AGN originates from the torus and challenges the justification of using simple torus models to model the broad-band IR emission. It is quite likely that the hot-dust emission in NGC 3783 as recently resolved by near-IR interferometry is misaligned with the mid-IR emitting source, which also finds a correspondence in the two distinct 3-5 {\mu}m and 20 {\mu}m bumps seen in the high-angular resolution spectral energy distribution (SED). We conclude that these observations support a scenario where the majority of the mid-IR emission in Seyfert AGN originates from a dusty wind in the polar region of the AGN.
Comments: 15 pages, 9 figures, 3 tables; published in ApJ, 771, 87 (2013), online on June 20, 2013; for artist’s impressions see this http URL
Subjects: Cosmology and Extragalactic Astrophysics (astro-ph.CO) Cite as: arXiv:1306.4312 [astro-ph.CO]
(or arXiv:1306.4312v1 [astro-ph.CO] for this version)

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