When galaxies collide: the growth of supermassive black holes

Galaxies may look pretty and delicate, with their swirls of stars of many colours – but don’t be fooled. At the heart of every galaxy lies a supermassive black hole, including in our own Milky Way.

Black holes in some nearby galaxies contain ten billion times the mass of our sun in a volume a few times the size of our solar system. That’s a lot of mass in a very small space – not even light travels fast enough to escape a black hole’s gravity.

So how did they get that big? In the journal Science, we tested a commonly-held view that black holes become supermassive by merging with other black holes – and found the answer is not quite that simple.

The answer may lie in a related question: when two galaxies collide to form a new galaxy, what happens to their black holes?

When galaxies collide, they form a new, bigger galaxy. The colliding galaxies’ black holes sink to the centre of this new galaxy and orbit each other, eventually combining to form a new, bigger black hole.

Black holes, as the name suggests, are very hard to observe. But orbiting black holes are the strongest emitters in the universe of an exotic form of energy called gravitational waves. Gravitational waves are a prediction of Einstein’s General Theory of Relativity and are produced by very massive, compact objects changing speed or direction. This, in turn, causes the measured distances between objects to change.

For example, a gravitational wave passing through your computer screen will cause it to first stretch in one direction, then in a perpendicular direction, over and over again.

Fortunately for your laptop, but unfortunately for astronomers, gravitational waves are very weak. Gravitational waves from a pair of black holes in a nearby galaxy causes your screen size to change by one atomic nucleus over ten years.

But fear not – a way to detect these waves exists by using other extreme astronomical objects: pulsars, which are leftovers of massive stellar explosions called supernovae.

While they’re not quite as extreme as black holes, pulsars are massive and compact enough to crush atoms into a sea of nuclei and electrons. They compress up to twice the mass of our sun into a volume the size of a large city.

So how do pulsars help? First, they rotate very quickly – some of them up to 700 times per second – and very predictably. They emit intense lighthouse-like beams of radio waves, which, when they sweep by the Earth, appear as regular “ticks.”