Thick turbulent gas disks with magnetocentrifugal winds in active galactic nuclei

Model infrared emission and optical polarization

¹ Observatoire astronomique de Strasbourg, Université de Strasbourg, CNRS, UMR 7550, 11 rue de l’Université, 67000 Strasbourg, France
e-mail: Bernd.Vollmer@astro.unistra.fr
² Max-Planck-Institut für extraterrestrische Physik, Postfach 1312, Gießenbachstr., 85741 Garching, Germany
³ University Observatory Munich, Scheinerstraße 1, 81679 München, Germany
⁴ Centre for Astrophysics and Supercomputing, Swinburne University of Technology, PO Box 218, Hawthorn, Victoria 3122, Australia
⁵ Sterrewacht Leiden, Universiteit Leiden, Niels-Bohr-Weg 2, 2300 CA Leiden, The Netherlands
⁶ Department of Physics and Astronomy, University of Southampton, Southampton SO17 1BJ, UK

Received: 9 May 2017
Accepted: 16 March 2017

Abstract

Infrared high-resolution imaging and interferometry show that the dust distribution is frequently elongated along the polar direction of an AGN. In addition, interferometric mm line observations have revealed a bipolar outflow in a direction nearly perpendicular to the nuclear disk. To explain these findings, we developed a model scenario for the inner ~30 pc of an AGN. The structure of the gas within this region is entirely determined by the gas inflow from larger scales. We assumed a rotating thick gas disk between about one and ten parsec. External gas accretion adds mass and injects energy via gas compression into this gas disk and drives turbulence. We extended the description of a massive turbulent thick gas disk developed in a recent paper by adding a magnetocentrifugal wind. Our disks are assumed to be strongly magnetized via equipartition between the turbulent gas pressure and the energy density of the magnetic field. In a second step, we built 3D density cubes based on the analytical model, illuminated them with a central source, and made radiative transfer calculations. In a third step, we calculated mid-infrared (MIR) visibility amplitudes and compared them to available interferometric observations. We show that magnetocentrifugal winds starting from a thin and thick gas disk are viable in active galaxy centers. The magnetic field associated with this thick gas disk plays a major role in driving a magnetocentrifugal wind at a distance of ~1 pc from the central black hole. Once the wind is launched, it is responsible for the transport of angular momentum and the gas disk can become thin. A magnetocentrifugal wind is also expected above the thin magnetized gas disk. The structure and outflow rate of this wind is determined by the properties of the thick gas disk. The outflow scenario can account for the elongated dust structures, outer edges of the thin maser disks, and molecular outflows observed in local AGN. The models reproduce the observed terminal wind velocities, the scatter of the MIR – intrinsic X-ray correlation, and point source fractions. An application of the model to the Circinus galaxy and NGC 1068 shows that the infrared spectral energy distribution, available MIR interferometric observations, and optical polarization can be reproduced in a satisfactory way, provided that (i) a puff-up at the inner edge of the thin disk is present and (ii) a local screen with an optical depth of τ_V ~ 20 in form of a local gas filament and/or a warp of the thick disk hide a significant fraction of both nuclei. Our thick disk, wind, thin disk model is thus a promising scenario for local Seyfert galaxies.