Leicester Highlight: Tearing up the Disk: How Black Holes Accrete

Almost all galaxies have supermassive black holes in their centres. These holes grow because gas from the galaxy falls into them, a process which also makes the holes spin. In general the gas spirals slowly in to the hole in a disk, initially lying in a plane which is at an angle to its spin equator. But near to the hole, powerful forces drag it into this plane. This means that the gas flows through a warped disk, whose plane changes as it moves in. Until recently it was thought that this process occurred fairly smoothly, with a smooth warp gently straightening the disk plane. However work by Ogilvie in 1999 suggested that in some cases the warp could be much more abrupt, because the forces holding the disk together actually weaken in a warp. The disk would then break into a pair of disks, the outer ring in the original misaligned plane, and the inner one aligned to the black hole spin.

Figure 1. Simulation of disk tearing. Gas spirals towards a supermassive black hole in a plane which is highly inclined to the black hole spin. Close to the hole the gas is forced to orbit in the equatorial plane, and this breaks the disk. The inner disk tumbles in space, and eventually is rotating partially in the opposite direction to the outer disk. Where the two disks touch their gas collides. This removes the rotation that supports it against the black hole gravity, so it falls to a new orbit much closer to the hole. But there it also encounters gas moving in the opposed direction, so the process repeats. This produces rapid gas infall on the black hole, making it grow rapidly, and producing an extremely luminous source of radiation at the center of the host galaxy.

Only detailed work with a powerful supercomputer can check this possibility. Our work with DiRAC2 shows for the first time that it actually occurs in many realistic cases. The spin forces near the black hole break the central regions of tilted disks around spinning black holes into a set of distinct planes with only tenuous flows connecting them. These component disks then precess – their planes tumble in space – independently of each other. In a most cases the continued precession of these disks eventually sets up partially counterrotating gas flows, so that gas in one disk is moving almost oppositely to gas in a nearby disk. As these opposed flows interact, this drives rapid infall towards the black hole. The process can repeat itself as disks form and interact even closer to the black hole. This produces a cascade of tumbling and precessing disks closer and closer to the black hole. This is important because rapid gas infall makes the black hole grow rapidly. This in turn makes the infalling gas extremely hot and bright, accounting for the most luminous objects in the entire Universe.