Quasar feeding and star formation in the central parts of galaxies.

Galaxies are formed when gas, cooling in massive dark matter halos, contracts gravitationally into stars. A small fraction of the gas apparently reaches supermassive black holes that reside in the centres of galaxies. The resulting energy release is enourmous and affects evolution of the galaxies themselves. The details of these processes are still poorly understood. However, exciting progress is being made on all fronts. Numerical simulations allow us to stage experiments that expand our analytical understanding of the complicated physics. Observations of distant quasars and AGN (actively accreting black holes) constrains the gross picture of quasar feeding. Observations of our Galactic Centre, with its supermassive black hole, revealed that accretion and star formation might be taking place simultaneously in the inner parsecs of galaxies. The prospective PhD student will use a range of tools -- numerical, analytical or data interpretation, depending on the student's interests and abilities -- to advance our knowledge in the linked areas of supermassive black holes, star formation and galaxy evolution.

SnapshotSnapshot of a simulation by the 1st year PhD student Alex Hobbs. The simulation shows the result of two massive gaseous clouds colliding in the central parsec of our Galaxy. The inner circular disc is gravitationally unstable and forms stars. The outer eccentric filament also forms stars. These results are relevant to the origin of two discs of young stars recently discovered in the centre of our Galaxy.

More detailed description of the projects:

1. Quasar/AGN feeding and star formation

Quasars and AGN are supermassive black holes powered by gravitational energy release by the gas that falls or "accretes" onto these. At the moment, the only certain thing about this process is that it happens in Nature but we do not understand how. There are several challenges here. On small (sub-parsec) scales, gas probably forms a disc since there is too much angular momentum. Theories predict that these discs should be very massive and cool, and form stars much more readily that transfer matter inside the supermassive black holes. However, models need to be developed further to include additional effects, such as outflows and radiation from massive stars.

On the larger scales, the challenge is to understand how the gas is delivered into the central regions of galaxies: via larger scale quasi-static discs or rather via chaotic deposition of individual molecular clouds. Are mergers of galaxies important? And finally, how does this work when supermassive black hole turns on its powerful radiation and gas outflows?

To answer these questions, a numerically-minded student would use existent numerical codes and the in-house super-computing facilities to simulate the important processes ab initio. He/she would then analyse the results in order to understand understand implications of these simulations to the real AGN and galaxies. The sudent would also gain hands on experience coding and modifying some of the best available astrophysical codes. An analytically capable student could instead build models to search the parameter space and compare to the results of numerical simulations obtained by other people in the group. Finally, the eventual goal is to relate this work to observations of AGN and our Galactic Centre, in particular.
The project might include collaborative visits to MPA-Garching, Germany, for code development.

2. Star formation in extreme environment

The theoretical prediction of star formation within massive accretion discs (see the project above) has been recently dramatically confirmed with discovery of several tens of massive young stars near Sgr A*, the supermassive black hole in the centre of our Galaxy. Apparently, these stars "stole" about 10,000 solar masses of gas that Sgr A* managed to lure into the inner parsec. An extremely interesting and important point to address is what do these observations imply for star formation as a process. Specifically, star formation has been studied for half a century in our Galaxy and other galaxies, and one normally observes it to take place in a relatively similar environments, e.g., a certain ambient gas density, temperature, external radiation background, etc. In contrast, the supermassive black hole produces a very strong tidal field that shreds "normal gas clouds" in pieces, and holds on to matter tightly. This changes the conditions for star formation to very extreme, never observed before.

This project could involve students of a wide range of interests and abilities. Simulations with the existing code and the results analysis would be main tools for the numerically able. (Semi)analytical modelling and probably light programming for those thinking in terms of equations. Finally, literature reading and contrasting star formation near
Sgr A* with that elsewhere in the galaxy, and interpretation of observational results, including the latest ones, such as stellar orbits and stellar types, would be the tool of for those wishing to be in between theory and observations.