Dust in the central parsecs of unobscured AGN: more challenges to the torus

TL;DR

This study reveals that dust filaments near galactic centers, with varying optical depths and locations, challenge traditional AGN unification models by acting as the obscuring torus.

Contribution

It provides new observational evidence of dust filaments in both type 1 and type 2 AGN, highlighting their role in AGN classification and challenging existing torus models.

Findings

Dust filaments are common in the central regions of galaxies.

Optical thickness of filaments varies, affecting AGN visibility.

Gas kinematics suggest ongoing mass inflows.

Abstract

A parsec-scale dusty torus is thought to be the cause of Active Galactic Nuclei (AGN) dichotomy in the 1/2 types, narrow/broad emission lines. In a previous work, on the basis of parsec-scale resolution infrared / optical dust maps it was found that dust filaments, few parsecs wide, several hundred parsecs long, were ubiquitous features crossing the centre of type 2 AGN, their optical thickness being sufficient to fully obscure the optical nucleus. This work presents the complementary view for type 1 and intermediate-type AGN. The same type of narrow, collimated, dust filaments are equally found at the centre of these AGN. The difference now resides in their location with respect to the nucleus, next to it but not crossing it, as it is the case in type 2, and their reduced optical thickness towards the centre, $A_V \lesssim 1.5 \rm{mag}$, insufficient to obscure at UV nucleus wavelengths. It is concluded that large scale, hundred pc to kpc long, dust filaments and lanes, reminiscent of those seen in the Milky Way, are a common ingredient to the central parsec of galaxies. Their optical thickness changes along their structure, in type 2 reaching optical depths high enough to obscure the nucleus in full. Their location with respect to the nucleus and increasing gradient in optical depth towards the centre could naturally lead to the canonical type 1/2 AGN classification, making these filaments to play the role of the torus. Dust filaments and lanes show equivalent morphologies in molecular gas. Available gas kinematic indicates mass inflows at rates $ < ~ 1 M\odot~ yr^{-1}$.