Accretion discs: from quasars to planets

Discs occur throughout the Universe, from massive spiral galaxies to Saturn's rings. We have all seen an ice-skater spin faster as she pulls her arms closer to her body. The reason this happens is because angular momentum is conserved, and that same process, writ large, causes discs to form throughout the Universe. Most astrophysical objects form via gravitational collapse, and as gravity pulls material inwards it rotates progressively faster and faster, resulting in discs. Around stars and black holes these discs act as a conduit for infalling gas, and are called 'accretion discs'. Accretion on to a black hole is the most efficient means of energy generation we know of (many times more efficient than nuclear fusion), and consequently accretion discs are responsible for some of the most spectacular phenomena in the Universe. Accretion discs regulate the growth of super-massive black holes. We now believe that most, if not all, galaxies have super-massive black holes at their centres. These cosmic giants are many millions of times heavier than the Sun, and they have an enormously strong gravitational pull, and their formation appears to be intimately linked to the formation of the galaxies in which they reside. The 'feeding' of gas, through accretion discs, into super-massive black holes in distant galaxies is thought to be the engine that powers quasars, the brightest objects in the Universe. Locally, we now know that a super-massive black hole also lives at the centre of our galaxy, the Milky Way. This black hole is approximately four million times more massive than the Sun, but it is not currently being fed (thankfully for us!). Instead, it is surrounded by a rotating disc of very young stars, whose motions modern telescopes map with exquisite precision. The existence of these young stars was initially a puzzle, because the tenuous gas clouds which usually host star birth would be shredded by gravity so close to the black hole. We now believe that these stars formed during an earlier feeding episode, when the black hole's accretion disc broke apart under its own gravity. By studying our local super-massive black hole we can learn a great deal about the how black holes feed and form stars, and this in turn will enhance our knowledge of how black holes and galaxies form and grow in the distant Universe. Planets form in accretion discs around young stars. In recent years it has become clear that planets around other stars are common, with over 200 'extra-solar' planets now known. Accretion discs around young stars were the birthplaces of these distant solar systems. Within the discs around these young suns, dust and rocks stick together and eventually grow into planets. However, the solid material that makes up the building blocks for planets represents only a tiny fraction of the disc material; the vast majority of the disc is gaseous. Observations tell us that these discs live only for a few million years (a mere blink of the eye, in astrophysical terms), so understanding how their gas is removed is crucial to understanding how planets form. The processes of planet formation and disc evolution are inextricably linked, and only by understanding both can we hope to gain a full understanding of how planets form, and of how we came to exist. I will conduct large computer simulations of accretion discs, to study how stars form near super-massive black holes and how planets form in discs around young stars. Similar physics applies to both of these problems, despite their widely differing scales, so techniques developed in one context can readily be applied in the other. My research will show us how black holes form stars, and how star formation in turn regulates the growth of black holes at the centres of galaxies. On smaller scales I will simulate the formation of planets around young stars like our Sun, learning how accretion discs form violent 'hot Jupiters' and distant Earths.