Black Holes

Black Holes

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.

 Figure 1.

Such quasars are very rare in the present day Universe, but were much more active in the past. Because of their brightness, some of the most distant objects we can see are some of the most massive quasars, existing barely 1 billion years after the Big Bang. The presence of these objects raises many questions: how were these objects able to grow to such a large mass so quickly? What do the galaxies that host such objects look like? How does the growth of such a massive black hole affect the surrounding environment?

Using the DiRAC supercomputers at Universities of Cambridge, Leicester and Durham, researchers at the Institute of Astronomy and Kavli Institute for Cosmology, Cambridge simulated the growth of a galaxy containing a quasar similar to those we observe. These results are summarised in a recent Letter to MNRAS by Curtis and Sijacki, accepted on December 10, 2015. Vital to the work was the use of a new computational technique to significantly increase the resolution of simulations around the central black hole, which Curtis and Sijacki developed over the last year (2015, MNRAS, 454, 3445) thanks to unique computational resources provided by DiRAC. This novel technique enabled the capture of the flow of gas around black holes with unprecedented accuracy both in term of mass and spatial resolution.

The work by Curtis and Sijacki provides new insights into what we might hope to observe in the near future around these distant quasars with the James Webb space telescope and the Atacama Large Millimeter/submillimeter Array. Long thin structures, called filaments, transport gas from the outskirts of the galaxy all the way to the central region. These filaments grow a disc of gas that orbits around the black hole. 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, oriented in such a way that it does not destroy the surrounding disc, which is the key novel result of the model.

In the coming years there will be many observations of quasars in the early Universe. The ways in which they compare with the predictions of Curtis and Sijacki will provide clues to the physical processes that we do not fully understand, allowing deeper insight into the role black holes play in shaping our Universe.