Dust formation in AGN winds

TL;DR

This paper models silicate dust formation in magnetohydrodynamic winds from AGN accretion disks, showing that such winds can produce significant dust, especially in luminous quasars, supporting the AGN unification model.

Contribution

It provides detailed chemical calculations demonstrating dust formation in AGN winds and links dust production to black hole mass and accretion rate, advancing understanding of AGN dust environments.

Findings

AGN winds can form significant dust, especially near Eddington accretion.

Dust distribution in winds resembles a torus with high optical depth.

Dust production scales with black hole mass and accretion rate.

Abstract

Infrared observations of active galactic nucleus (AGN) reveal emission from the putative dusty circumnuclear 'torus' invoked by AGN unification, that is heated up by radiation from the central accreting black hole (BH). The strong 9.7 and 18 micron silicate features observed in the AGN spectra both in emission and absorption, further indicate the presence of such dusty environments. We present detailed calculations of the chemistry of silicate dust formation in AGN accretion disk winds. The winds considered herein are magnetohydrodynamic (MHD) winds driven off the entire accretion disk domain that extends from the BH vicinity to the radius of BH influence, of order of 1 to 100 pc depending on the mass of the resident BH. Our results indicate that these winds provide conditions conducive to the formation of significant amounts of dust, especially for objects accreting close to their Eddington limit, making AGN a significant source of dust in the universe, especially for luminous quasars. Our models justify the importance of a r to the power -1 density law in the winds for efficient formation and survival of dust grains. The dust production rate scales linearly with the mass of the central BH and varies as a power law of index between 2 to 2.5 with the dimensionless mass accretion rate. The resultant distribution of the dense dusty gas resembles a toroidal shape, with high column density and optical depths along the equatorial viewing angles, in agreement with the AGN unification picture.