Extreme general relativity to one side, what does this mean for black hole studies?
“Where did the black hole get its spin? It could have some spin when it was born, but most of it, particularly at these fast rates, must be accumulated as the black hole grows,” said Harrison. “It can accumulate because the accretion disk -- as the matter swirls onto the black hole it can add spin. And also the process by which black holes merge – when two galaxies merge, their black holes also merge – this can add, or subtract, spin from the black hole.
“If we can measure the spins of a large number of black holes, we can begin to say things very concrete about how they grew.”
This research isn’t restricted to NGC 1365, however.
“(NGC 1365’s) properties are pretty "normal", so we expect to find similar line broadening in other (maybe most) supermassive black holes,” said Risaliti.
“Actually, it is even better than this: we already have measurement of line broadening for many (~20-30) supermassive black holes. Until now however we could not be sure about the uniqueness of the interpretation. So our result for one black hole also validates previous studies on many others.”
Knowing how black holes grow and evolve is critical to the understanding of galactic development. AGNs (driven by the engine of a ravenous black hole) can dramatically impact star formation in a galaxy. The more we know about black hole spin rates, the more we may be able to understand about the life cycle of an entire galaxy.
We may even be able to use these data to do some galactic archaeology and ask whether a given galaxy was actually the result of a galactic merger.
“What excites me is the fact that we are able to do this for the very massive black holes at the centers of galaxies but we can also make the same measurement for black holes in our galaxy ... black holes that resulted from the explosion of a star ... The fact we can extend this from billions of solar masses to 10 solar masses is pretty cool,” Harrison concluded.