Active galactic nuclei (AGN) represent the active growing phases of supermassive black holes. For the first time, we are able to resolve the dusty gas on parsec scales and directly test our standard picture of these objects. While this "unification scheme" relates the parsec-scale IR emission with a geometrically-thick disk, it has been realised recently that the bulk of the dust emission comes from the polar region of the alleged disk where gas is blown out from the vicinity of the black hole. Along with these polar features, the compactness of the dust distribution may depend on the accretion state of the black hole. Neither of these findings have been predicted by current models and lack a physical explanation.
To explain the new observations, I proposed a revision to the AGN unification scheme that involves a dusty wind driven by radiation pressure. Depending on their masses, velocities, and frequency, such dusty winds might play a major role in self-regulating AGN activity and, thus, impact the interplay between host and black hole evolution. However, as of now we do not know if these winds are ubiquitous in AGN and how they would work physically. In the course of this project, the following topics will be addressed:
Work package 1: Winds and disks
How common are dusty winds? How are outflowing and inflowing components linked to the accretion state? How much dust mass is contained in the wind and how does it interact with the host galaxy? And where is the torus? These questions have to be addressed via high angular resolution observations of a well-selected sample of AGN and requires a multi-wavelength approach. We will constrain the contribution and direction of the hot equatorial dust in relation to the cooler dust in the polar region. At the same time, the kinematic state and densities of the different dust-emitting components will be unveiled using molecular lines that trace the typical high densities of this region. This information will be compared to the luminosity, accretion state, and black hole mass, revealing any correlation with the fundamental parameters of an AGN. Taken together, this approach will firmly test the presence of dusty winds, their frequency, and physical origin. As a legacy of the unprecedented combination of the highest angular resolution facilities in the IR and sub-mm, we will make fully reduced and science-ready pc-scale intensity, velocity, and density maps of the immediate environment of the supermassive black holes available to the community.
Work package 2: Physical models of dusty winds
What physical process moves the dust to the polar region? Can radiation pressure solve the puzzle? Do we have to invoke radiative transfer effects? How do we have to adjust the current models to represent the dust distribution correctly? To answer these questions, we will develop a new radiation hydrodynamic (RHD) model that tests if dusty winds can be driven by radiation pressure from the AGN and the dust itself. RHD simulations are a very difficult problem to solve and a hotly debated subject in current literature. Radiation pressure from either the AGN or surrounding clouds can become the dominating force on dusty gas clouds, which is why we will put special emphasis to accurately and efficiently solve the radiative transfer problem locally. Results from these RHD simulations will be used to create new model grids that can be used for modelling IR and sub-mm photometry, spectroscopy, and interferometry. These grids will be made publicly available. Therefore, as one deliverable of this project, the community will have a model for the AGN IR emission where the underlying dust distribution represents the new constraints set by IR interferometry.
Work package 3: The dust emission as a standard ruler
Aside from the physical constraints we will obtain by studying the dusty distribution and kinematics from both the theoretically and observational sides, there is a very practical and synergistic application of this knowledge: We can measure absolute distances to nearby AGN via a combination of IR interferometry and reverberation mapping. The method works by simultaneously determining the time lag between optical and near-IR fluxes (= absolute size of hot dust emission) and measuring the angular size of the near-IR emitting region with IR interferometry. The ratio of both quantities is the absolute distance. The goal of this project is to establish absolute distances to 12 AGN at redshifts z = 0.004 − 0.04 (∼15−200 Mpc) with a combined precision of ∼3%. This "network of AGN" can be used as an independent anchor for the cosmic distance ladder, a set of objects used to calibrate extragalactic distances in order to constrain the cosmological parameters. For that we are currently undertaking a multi-year photometric monitoring campaign of the sample of type 1 AGN and secure simultaneous near-IR interferometry. In addition, advanced modelling techniques will be developed to properly characterise statistical and systematic errors.