Sub-parsec-scale dynamics of a dusty gas disc exposed to anisotropic AGN radiation with frequency-dependent radiative transfer

Abstract

We explore the gas dynamics near the dust sublimation radius of active galactic nucleus (AGN). For the purpose, we perform axisymmetric radiation hydrodynamic simulations of a dusty gas disk of radius ≈ 1 pc around a supermassive black hole of mass 10 7 M taking into account (1) anisotropic radiation of accretion disk, (2) X-ray heating by corona, (3) radiative transfer of infrared (IR) photons reemitted by dust, (4) frequency dependency of direct and IR radiations, and (5) separate temperatures for gas and dust. As a result, we find that for Eddington ratio ≈ 0.77, a nearly neutral, dense (≈ 10 6 -8 cm −3 ), geometrically-thin (h/r < 0.06) disk forms with a high velocity (≈ 200 ∼ 3000 km s −1 ) dusty outflow launched from the disk surface. The disk temperature is determined by the balance between X-ray heating and various cooling, and the disk is almost supported by thermal pressure. Contrary to Krolik (2007), the radiation pressure by IR photons is not effective to thicken the disk, but rather compresses it. Thus, it seems difficult for a radiation-supported, geometrically-thick, obscuring torus to form near the dust sublimation radius as far as the Eddington ratio is high (∼ 1). The mass outflow rate is 0.05 -0.1 M /yr and the column density of the outflow is N H 10 21 cm −2 . To explain observed type-II AGN fraction, it is required that outflow gas is extended to larger radii (r 10 pc) or that a denser dusty wind is launched from smaller radii (r ∼ 10 4 R g ).

Redefining the Torus: A Unifying View of AGNs in the Infrared and Submillimeter

“…Pier & Krolik 1992;Fabian et al 2009;) and RHD simulations confirm it for both single dusty clumps as well as massive dusty molecular disks (e.g. Dorodnitsyn et al 2011;Schartmann et al 2011;Wada 2012;Namekata et al 2014;Namekata, & Umemura 2016;Chan & Krolik 2017;Williamson et al 2019). Such radiation pressure driving is most effective for regions with τ V ∼ 1.…”

Parsec-scale Dusty Winds in Active Galactic Nuclei: Evidence for Radiation Pressure Driving*

“…Hydrodynamic models (e.g. Wada 2015; Namekata & Umemura 2016;Williamson et al 2019) came to similar conclusions although the geometry of the disk and wind varies from model to model depending on the included model parameters and resolution of the simulation. Williamson et al (2019) make the further point that the wind was highly sensitive to the anisotropy of the central radiation and to the Eddington ratio.…”

Rapid luminosity decline and subsequent reformation of the innermost dust distribution in the changing-look AGN Mrk 590

“…Consider the BLR gas condition just inside the dust sublimation radius in the bright phase of Mrk 590, and the new dust formation of carbon (C) grains after the AGN luminosity drop. When the AGN accretion disk luminosity (which is proportional to the ionisation parameter) decreases, the BLR gas begins to cool rapidly from the initial gas temperature of ∼ 10, 000 K to 2, 000 K via metal line cooling over a period of 20 days, due to the high density (n ∼ 10 10 − 10 11 cm −3 ; e.g., Namekata & Umemura 2016;Ichikawa & Tazaki 2017;Sarangi et al 2018). The radiatively cooled BLR gas can then initiate dust grain formation, as the partial gas pressure of C becomes larger than the vapour pressure of the bulk condensate at the gas temperature of 2, 000 K, i.e.…”

“…After AGN re-brightening, a portion of the dust grains that formed during the faint phase will be ejected from the dust torus as a radiatively-driven dusty outflow (Namekata & Umemura 2016;Hönig & Kishimoto 2017;Williamson et al 2019;Tazaki & Ichikawa 2019). The dusty outflow launched from the inner part of the dust torus is suggested to be responsible for the parsec-scale MIR (8 − 13 µm) dust emission originating from the polar dusty region recently discovered in local AGNs (including Mrk 590) by IR high-resolution imaging observations (e.g., Hönig et al 2013;López-Gonzaga et al 2016;Asmus et al 2016).…”