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Astrophysical Fluid Dynamics

Gas flow structure around high-redshift quasars

Gas flow around quasar

 Credit: Michael Curtis and Debora Sijacki

Quasars are extremely bright objects that reside in the centres of galaxies. They are believed to consist of a supermassive black hole (some as large as a billion times the mass of our own Sun) that is surrounded by gas. These black holes grow in size by swallowing their reservoir of gas and, in doing so, release an enormous amount of energy into their host galaxies. This energy affects the growth of each galaxy, changing where and when stars form and playing an important role in the different appearances of galaxies that we observe in the Universe today.
The image above shows state-of-the-art cosmological simulation of a quasar together with it's large scale environment (Curtis & Sijacki, 2016, MNRAS, 457, 34) performed with a moving mesh code Arepo.

Long thin structures, called filaments (shown in blue), transport gas from the outskirts of the galaxy all the way to the central region where the supermassive black hole resides. These filaments grow a disc of gas that orbits around the black hole (the temperature and velocity structure of this disc is shown in two inset plots). The gas in this disc is relatively cold, which allows it to become dense enough to form stars at a dramatic rate. A fraction of the gas escapes the disc and falls onto the black hole, releasing large quantities of energy. This drives a powerful flow of super-heated gas out of the central galaxy (shown in red in the left inset plot and in the main image), oriented in such a way that it does not destroy the surrounding disc, which is the key novel result of the model. 


Angular momentum transport in magnetised convective regions in stars 

 Angular momentum transport in magnetised convective regions in stars

 Credit: Adam Jermyn

Ordinarily, flows do not experience shear as a result of bulk rotation. However, in the presence of magnetic fields, convective turbulence and radiative thermal diffusion, the flow can develop an intrinsic angular momentum (vorticity) which couples to the bulk rotation. This is particularly relevant near the border between radiative and convective heat transport in the Sun and may help explain the development of differential rotation near that boundary.

The colours on the plot indicate the strength of this intrinsic rotation, while the vertical and horizontal axes represent the bulk rotation rate and the magnetic field strength respectively.