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An important discovery in the past quarter century or so of extragalactic astronomy is the existence of scaling relations which very simply relate different properties of those galaxies.

One of the most intriguing relations connects the total stellar mass of a galaxy to the mass of its black hole. Virtually every evolved galaxy in the sky has a supermassive black hole in its center, and the mass of each of those black holes is about 0.2% the mass of the stars in its galaxy.

At first glance, this relation is pretty simple -- big galaxies have big black holes. But more profoundly this relation suggests that supermassive black holes co-evolve with the galaxies they live in.

This winds up begin a really useful tool: If you know the mass of all the stars in a galaxy, you can reasonably estimate the mass of its black hole! So if we want to build a census of black holes, we can start out with a census of galaxy stellar masses!

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