The effect of stellar feedback on the formation and evolution of gas and dust tori in AGN

Abstract

Recently, the existence of geometrically thick dust structures in active galactic nuclei (AGN) has been directly proven with the help of interferometric methods in the mid‐infrared. The observations are consistent with a two‐component model made up of a geometrically thin and warm central disc, surrounded by a colder, fluffy torus component. Within the framework of an exploratory study, we investigate one possible physical mechanism, which could produce such a structure, namely the effect of stellar feedback from a young nuclear star cluster on the interstellar medium in centres of AGN. The model is realized by numerical simulations with the help of the hydrodynamics code tramp. We follow the evolution of the interstellar medium by taking discrete mass‐loss and energy ejection due to stellar processes, as well as optically thin radiative cooling into account. In a post‐processing step, we calculate observable quantities like spectral energy distributions (SEDs) and surface brightness distributions with the help of the radiative transfer code mc3d. The interplay between injection of mass, supernova explosions and radiative cooling leads to a two‐component structure made up of a cold geometrically thin, but optically thick and very turbulent disc residing in the vicinity of the angular momentum barrier, surrounded by a filamentary structure. The latter consists of cold long radial filaments flowing towards the disc and a hot tenuous medium in between, which shows both inwards and outwards directed motions. With the help of this modelling, we are able to reproduce the range of observed neutral hydrogen column densities of a sample of Seyfert galaxies as well as the relation between them and the strength of the silicate 10 μm spectral feature. Despite being quite crude, our mean Seyfert galaxy model is even able to describe the SEDs of two intermediate type Seyfert galaxies observed with the Spitzer Space Telescope.

The Impact of Nuclear Star Formation on Gas Inflow to AGN

“…In the later version, the updated stellar wind speeds were all fast, and nearly the entire ISM was blown out. It is reassuring that qualitatively, these simulations yield the same result as those of Schartmann et al (2009) even though the scales are drastically different, because in both cases it is the same processes (i.e., stellar outflows) that govern the evolution of the region.…”

“…It is inevitable that the star formation will have some impact on the gas, and this was argued phenomenologically by Davies et al (2007). Hydrodynamical simulations of the impact of stellar evolution, such as those presented in Schartmann et al (2009) give a clearer picture. To understand this in the context of Seyfert nuclei, these simulations have been scaled to match a typical starburst there, as described by Schartmann et al (these proceedings):…”

Disentangling the Circumnuclear Environs of Centaurus A. III. An Inner Molecular Ring, Nuclear Shocks, and the CO to Warm H2 Interface

“…Three-dimensional radiative hydrodynamical models of the circumnuclear gas in galaxies predict that a clumpy torus-like structure is naturally pro-duced on scales of tens of parsec around an SMBH, due to radiative pressure from the AGN resulting in vertical circulation of gas (or fountains) (Wada 2012), supernova-driven turbulence (Wada et al 2002(Wada et al , 2009Schartmann et al 2009), as well as due to dynamical arguments related to galactic inflows (Hopkins et al 2012). As a result of the energy feedback from SNe and AGN the circumnuclear disks possess a: i) highly inhomogeneous internal density and temperature structure, ii) filamentary structures such as nuclear spiral arms, and iii) a large scale height in the outer regions at galactocentric radii of a few tens of parsec, resembling the shape of a torus and being characterized by large internal turbulent motion.…”

The dusty heart of nearby active galaxies

“…Although the actual properties of the dust clouds are not yet constrained by observations, theoretical predictions and hydrodynamic simulations arguably favor small and compact opticallythick clouds (e.g. Vollmer et al 2004;Schartmann et al 2009). …”