The physics of the ISM under extreme conditions in Seyfert galaxies

Alessandro Ballone, Andreas Burkert, Philipp Plewa, Marc Schartmann

This page is intended to give an overview over the science we have done within the framework of our ISM-SPP project. The sub-projects are listed in inverse chronological order. Click on the images for a larger version of them!

Dynamics of gas and dust clouds in Active Galactic Nuclei

We analyse the motion of single optically thick clouds in the potential of a central mass under the influence of an anisotropic radiation field, a model applicable to the inner region of active galactic nuclei. Resulting orbits are analytically soluble for constant cloud column densities. All stable orbits are closed, although they have non-trivial shapes (see figure). Furthermore, there exists a stability criterion in the form of a critical inclination, which depends on the luminosity of the central source and the column density of the cloud.

Plewa, Schartmann & Burkert, 2013, MNRAS, 431, 127

Observing G2's pericenter passage in the Galactic Center

With the help of new, very sensitive SINFONI data, we are able to observe for the first time blue-shifted emission in the position-velocity diagram of the gas cloud G2. As the cloud is extremely stretched out (more than 15,000 Schwarzschild radii in the meantime), part of it seems to have passed pericenter (~2000 Schwarzschild radii) already, a process which will continue for roughly another year. The new data also enables us to update the observed kinematical and physical properties of the gas cloud, confirming the near-radial orbit of the object. Very puzzling for current models for the cloud is the constant Brackett-gamma luminosity of the head (over the past 9 years).

Gillessen, Genzel, Fritz, Eisenhauer, Pfuhl, Ott, Schartmann, Ballone, Burkert, 2013, astro-ph: 1306.1374

G2 - a compact mass-loosing source?

G2 might be a diffuse cloud or the result of an outflow from an invisible star embedded in it. With hydrodynamical simulations of the evolution of different spherically symmetric winds of a stellar object embedded in G2, we find that the thermal pressure of the hot and dense atmosphere confines the wind, while its ram pressure shapes it via stripping along the orbit. Tidal forces squeeze the wind near pericenter, reducing it to a thin and elongated filament. Most of the Brackett-gamma luminosity comes from the dense shocked wind shells, which have a highly filamentary structure with low volume filling factor. For our assumed atmosphere, the observations can be best matched by a mass outflow rate of 8.8e-8 Msun/yr and a wind velocity of 50 km/s, being compatible with a young TTauri star wind.

Ballone, Schartmann, Burkert, Gillessen, Genzel, Fritz, Eisenhauer, Pfuhl, Ott, 2013, astro-ph: 1305.7238

Observations of the Gas Cloud G2 in the Galactic Center

New observations of the recently discovered gas cloud G2 (Gillessen et al., 2012) currently falling toward the massive black hole in the Galactic Center are presented. The new data confirm that G2 is on a highly elliptical orbit with a predicted pericenter passage mid-2013. The updated orbit has an even larger eccentricity of 0.966 and smaller pericenter distance (2200 Schwarzschild radii). The velocity gradient of G2 has developed further to 600 km/s FWHM in summer 2012, emphasizing the currently ongoing tidal disruption of the cloud. We also detect the tail of similar total flux and on the same orbit as G2 along the trajectory. The Brackett gamma flux of G2 has not changed by more than 10% between 2008 and 2012.

Gillessen, Genzel, Fritz, Eisenhauer, Pfuhl, Ott, Cuadra, Schartmann, Burkert, 2013, ApJ, 763, 78

Simulations of the origin and fate of the GC cloud G2

Our hydrodynamical investigations of the origin and fate of the recently discovered gas cloud G2 with the PLUTO code in combination with currently available observations favor two scenarios: a Compact Cloud which started around the year 1995 and a Spherical Shell of gas, with an apocenter distance within the disk(s) of young stars and a radius of a few times the size of the Compact Cloud. The latter can account for both G2's signal as well as the fainter extended tail-like structure G2t seen at larger distances from the black hole and smaller velocities. From these first idealized simulations, we expect a roughly constant feeding of the supermassive black hole through a nozzle-like structure over a long period, starting shortly after the closest approach in 2013.51 for the Compact Cloud, which might lead to a boost of the luminosity of Sgr A* in several wavelength bands. We also find that the near-future evolution of the cloud will be a sensitive probe of the conditions of the gas distribution in the milli-parsec environment of the massive black hole in the Galactic center (image credit: ESO/MPE/Marc Schartmann).

Schartmann, Burkert, Alig, Gillessen, Genzel, Eisenhauer, Fritz, 2012, ApJ, 755, 155

Physics of the Galactic Center Cloud G2

We investigate the origin, structure, and evolution of the small gas cloud G2, which is on an orbit almost straight into the Galactic central supermassive black hole (SMBH). G2 is a sensitive probe of the hot accretion zone of Sgr A*. Its mass is equal to the critical mass below which cold clumps would be destroyed quickly by evaporation. Its mass is also constrained by the fact that at apocenter its sound crossing timescale was equal to its infall timescale. If the cloud had formed at apocenter in the "clockwise" stellar disk as expected from its orbit, it would be torn into a very elongated spaghetti-like filament by 2011, which is not observed, but an in-situ formation model in 1995 is consistent with the data. Another possibility is a model in which G2 is the head of a larger, shell-like structure that formed at apocenter. Our numerical simulations show that this scenario explains not only G2's observed kinematical and geometrical properties but also the Brackett-gamma observations of a low surface brightness gas tail that trails the cloud.

Burkert, Schartmann, Alig, Gillessen, Genzel, Fritz, Eisenhauer, 2012, ApJ, 750, 58

A gas cloud on its way towards the SMBH at the GC

We report the presence of a dense gas cloud approximately three times the mass of Earth that is falling into the accretion zone of Sgr A*. Our observations tightly constrain the cloud's orbit to be highly eccentric, with an innermost radius of approach of only roughly 3,100 times the event horizon that will be reached in 2013. Over the past three years the cloud has begun to disrupt, probably mainly through tidal shearing arising from the black hole's gravitational force. The cloud's dynamic evolution and radiation in the next few years will probe the properties of the accretion flow and the feeding processes of the supermassive black hole. The kilo-electronvolt X-ray emission of Sgr A* may brighten significantly when the cloud reaches pericentre. There may also be a giant radiation flare several years from now if the cloud breaks up and its fragments feed gas into the central accretion zone.

Gillessen, Genzel, Fritz, Quataert, Alig, Burkert, Cuadra, Eisenhauer, Pfuhl, Dodds-Eden, Gammie, Ott, 2012, Nature, 481, 51

Logo of the German Research Foundation This priority program is funded by the German Research Foundation DFG.