The nuclear dust lane of Circinus: collimation without a torus

TL;DR

This study shows that in the Circinus galaxy, nuclear dust lanes on 10 pc scales, not a traditional torus, are responsible for collimating ionized gas and obscuring the nucleus, challenging the standard AGN Unified Model.

Contribution

It demonstrates that nuclear dust lanes, rather than a torus, can cause collimation and obscuration in AGN, supported by high-resolution infrared and optical imaging.

Findings

Dust lanes provide one-third of the obscuration of the nucleus.

Collimation of ionized gas is caused by host galaxy dust, not a torus.

Most pc-scale dust is located in the polar direction.

Abstract

In some AGN, nuclear dust lanes connected to kpc-scale dust structures provide all the extinction required to obscure the nucleus, challenging the role of the dusty torus proposed by the Unified Model. In this letter we show the pc-scale dust and ionized gas maps of Circinus constructed using sub-arcsec-accuracy registration of infrared VLT AO images with optical \textit{Hubble Space Telescope} images. We find that the collimation of the ionized gas does not require a torus but is caused by the distribution of dust lanes of the host galaxy on $\sim$10 pc scales. This finding questions the presumed torus morphology and its role at parsec scales, as one of its main attributes is to collimate the nuclear radiation, and is in line with interferometric observations which show that most of the pc-scale dust is in the polar direction. We estimate that the nuclear dust lane in Circinus provides $1/3$ of the extinction required to obscure the nucleus. This constitutes a conservative lower limit to the obscuration at the central parsecs, where the dust filaments might get optically thicker if they are the channels that transport material from $\sim$100 pc scales to the centre.