G2: A gas cloud on its way around the black hole at the Galactic Centre

 

The recent discovery of a gas and dust cloud approximately three times the mass of Earth that is orbiting in the accretion zone of Sgr A* is investigated with the help of numerical modelling.
The observations tightly constrain the cloud's orbit to be highly eccentric, with an innermost radius of approach of only roughly 2,400 times the Schwarzschild radius of the central massive black hole in Sgr~A*. It was reached in 2014 and over the past years the cloud has begun to disrupt, mainly through tidal shearing arising from the black hole's gravitational force. Models to explain this unique object fall in two categories: compact cloud models in which G2 is assumed to be a pure gas and dust cloud and compact source models in which an additional source of gas and dust hides in the observable gas and dust distribution. We find that both models are able to explain the observations equally well and are consistent with a formation of the cloud in the disc of young stars in the GC.
The image shows the example of the evolution of the cloud modelled as a mass losing source, which might be a T-Tauri star. In the latter case the cloud will reform after peri centre passage, allowing to distinguish between the two scenarios.

 

The life cycle of starbursting circumnuclear discs

 

High-resolution observations from the submm to the optical wavelength regime resolve the central few 100 pc region of nearby galaxies in great detail. They reveal a large diversity of features: thick gas and stellar discs, nuclear starbursts, inflows and outflows, central activity, jet interaction, etc. Concentrating on the role circumnuclear discs play in the life cycles of 


galactic nuclei, we employ 3D adaptive mesh refinement hydrodynamical simulations with the RAMSES code to self-consistently trace the evolution from a quasi-stable gas disc, undergoing gravitational (Toomre) instability, the formation of clumps and stars and the disc’s subsequent, partial dispersal via stellar feedback. Our approach builds upon the observational finding that many nearby Seyfert galaxies have undergone intense nuclear starbursts in their recent past and in many nearby sources star formation is concentrated in a handful of clumps on a few 100 pc distant from the galactic centre. We show that such observations can be understood as the result of gravitational instabilities in dense circumnuclear discs. By comparing these simulations to available integral field unit observations of a sample of nearby galactic nuclei, we find consistent gas and stellar masses, kinematics, star formation and outflow properties. Important ingredients in the simulations are the self-consistent treatment of star formation and the dynamical evolution of the stellar distribution as well as the modelling of a delay time distribution for the supernova feedback. The knowledge of the resulting simulated density structure and kinematics on pc scale is vital for understanding inflow and feedback processes towards galactic scales.

 

 

 

Radiative transfer modelling of AGN tori


Active Galactic Nuclei (AGN) are thought to be enshrouded by geometrically and optically thick toroidal dust distributions, the so-called molecular tori. We derive observable properties for a self-consistent model of such toroidal gas and dust distributions, where the geometrical thickness is achieved and maintained with the help of X-ray heating and radiation pressure due to the central engine (Wada et al., 2012). We simulate spectral energy distributions (SEDs) and images with the help of dust continuum radiative transfer calculations with RADMC-3D (Dullemond et al., 2012). This results - for the first time - in time-resolved SEDs and images for a physical model of the central obscurer. We find that temporal changes are mostly visible at shorter wavelengths, close to the combined peak of the dust opacity -- which follows a loal dust extinction law -- as well as the central source spectrum and are caused by variations in the column densities of the generated outflow. Because of the three-component morphology of the hydrodynamical models -- a thin disc with high-density filaments (the remainder of the initial condition and determined by the inflow from larger scales), a surrounding fluffy component (the obscurer) and a low-density outflow along the rotation axis - we find dramatic differences depending on the observed wavelength: whereas the mid-infrared images are dominated by the elongated appearance of the outflow cone, the long wavelength emission is mainly given by the cold and dense disc component. Overall, we find good agreement with observed characteristics, especially for those models, which show clear outflow cones in combination with a geometrically thick distribution of gas and dust, as well as a geometrically thin, but high column density disc in the equatorial plane. 

 

G2: A gas cloud on its way around the black hole at the Galactic Centre

 

The recent discovery of a gas and dust cloud approximately three times the mass of Earth that is orbiting in the accretion zone of Sgr A* is investigated with the help of numerical modelling.
The observations tightly constrain the cloud's orbit to be highly eccentric, with an innermost radius of approach of only roughly 2,400 times the Schwarzschild radius of the central massive black hole in Sgr~A*. It was reached in 2014 and over the past years the cloud has begun to disrupt, mainly through tidal shearing arising from the black hole's gravitational force. Models to explain this unique object fall in two categories: compact cloud models in which G2 is assumed to be a pure gas and dust cloud and compact source models in which an additional source of gas and dust hides in the observable gas and dust distribution. We find that both models are able to explain the observations equally well and are consistent with a formation of the cloud in the disc of young stars in the GC.
The image shows the example of the evolution of the cloud modelled as a mass losing source, which might be a T-Tauri star. In the latter case the cloud will reform after peri centre passage, allowing to distinguish between the two scenarios.

 

warped diskStar formation occurs due to the collapse of dense parts of molecular clouds. To study star formation in detail it is important to also study the interstellar medium (ISM) and molecular clouds as precursors to the star formation. Once a star has formed it has a strong impact on the evolution of the surrounding medium. The UV radiation ionises the surrounding gas and creates a hot HII region. The heated gas starts to expand into the surrounding cold molecular cloud, creating a shock-wave. This shock-wave compresses the cold gas leading to pillar like structures. In the tip of those structures the formation of new stars is possible. Another important task to understand star formation in more detail is to study this process under extreme conditions. One example is the region close to the milky-way galactic centre black hole. Here tidal forces, which would disrupt a typical molecular cloud due to the deep potential, prevent star formation in the standard way. Still there are observations of a disk of stars on a sub-parsec scale near the black hole. The large scale impact of star formation on the ISM is due to supernovas. This process can heat and stir the ISM, leading to its multiphasic structure. It also enriches the ISM with elements of higher order. The so called local bubble, a region of low density gas inside which our solar system currently resides, is a interesting source to study the effect of supernova feedback. The bubble is believed to have formed by supernovas blowing out a large, peanut shaped region in the ISM. At the edges of this region, clusters of stars can form. The supernova feedback of those stars is directed towards the low density bubble (the so called champagne-effect) and enriches the bubble with heavy elements.

spiral structure

 


Stars are known to form in massive clouds of molecular hydrogen (GMCs), which are highly structured and turbulent. Understanding the origin and evolution of these clouds is a key problem in astrophysics. Here at the LMU and MPE we perform simulations of galaxies and colliding flows to better understand the nature of GMCs and their formation. Simulations of the interstellar medium (ISM) in galaxies show the formation of GMCs by the agglomeration of smaller clouds, as gas passes through the spiral arms (left). These calculations highlight how the properties of the clouds are related to their formation: the spacing of the clouds along the arm is proportional to the strength of the spiral potential, whilst the cloud rotations and internal velocity dispersions are regulated by cloud-cloud interactions and collisions. Without stellar feedback, these clouds last several 10's, or even 100 or more Myrs. We are currently performing calculations which include stellar feedback. In this case, the lifetimes of the GMCs are typically much shorter, the lifetimes and also the morphologies of the clouds governed by the star formation efficiency adopted in the calculations. 

 

 

 

 

 

 

 

 

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