How to find black holes?

The first step in trying to understand the connection between black holes and galaxies is finding them. But black holes are, well, black. In fact, you might say their blackness is their most defining feature.

So, how do you find them? It turns out that when they’re feeding on infalling gas and dust, a massive black hole can turn into the brightest object known in the whole universe – a quasar!

As the gas and dust falls towards the black hole, it settles into a disk around it, and as it moves in, friction in the disk heats up all the matter in it to such temperatures that it stats shining. In this way, black holes can be very bright, or quite dim, depending in part on how much matter they are munching on.

There are many ways to find feeding black holes and for the Galaxy Zoo paper on black hole growth, we used the emission lines that AGN (active galactic nuclei, or feeding black holes) cause when the light coming from the accretion disk shines on some other gas floating around in the host galaxy and makes that light in turn emit light with a very particular signature that we can detect by carefully analysing the spectra.