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

TL;DR

This study uses interferometry to show that most mid-IR emission in NGC 3783's AGN comes from a polar dusty wind rather than the traditional torus, challenging existing models.

Contribution

It provides high-resolution evidence that mid-IR emission in AGN is dominated by polar dust, not the torus, suggesting a need to revise current AGN IR emission models.

Findings

Mid-IR emission is elongated toward the polar axis, not the equatorial plane.

Half-light radii at 12.5 μm are approximately 4.2 pc by 1.4 pc.

Majority of mid-IR emission (60-90%) is from polar-elongated dust.

Abstract

(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.