How are supermassive black holes deforming our galaxy?

One of the biggest mysteries about galaxies is how the supermassive black holes at their centers affect the structure of the galaxy itself. Now scientists think they have an answer.

A team of German and American astronomers have discovered that there's a direct relationship between the size of the supermassive black hole and the number of globular star clusters in a galaxy. These clusters, which are densely packed collections of ancient stars, were never considered as possible link candidates before because they're usually located far away from galactic center. That would have seemingly made any potential connection between the two hugely unlikely.

Andreas Burkert of the University of Munich and Scott Tremaine at Princeton's Institute for Advanced Study headed up the research, which they say they undertook originally as little more than a lark. Having picked one of the most random possible correlations to test, they studied thirteen galaxies: nine giant elliptical galaxies, one spiral, and three hybrids between the two other types. To their shock, they found out their bit of fun was actually a pretty prescient hunch, as all thirteen galaxies showed a strong correlation between the size of the black hole and the number of globular clusters. In fact, it's the strongest association with black hole mass yet discovered.

Of course, it's not really enough just to know that the bigger the black hole, the more globular clusters a galaxy will have; the bigger question is why something like that would have happened. University of Texas astronomer John Kormendy, who was not involved with the project but has evaluated the data did not understate the potential importance of figuring that out:

"I think this correlation is telling us something fundamental. That it's such a good correlation suggests that the formation of globular clusters and the growth of black holes were connected."

There are a number of possible theories being floated around, both by the team of Burkert and Tremaine and by those uninvolved in the study. Kormendy suggests that, since both black holes and the clusters are so ancient, that the link originates from the special conditions that existed shortly after the Big Bang. Princeton's Jeremiah Ostriker suggests that the link is an indication that there used to be even more globular clusters in those galaxies, but as they passed through dark matter they lost their galactic orbits and fell into the central black hole, increasing its mass.

For their part, Burkert and Tremaine point to galactic collisions as the best available explanation. When gas-rich galaxies collide with each other, most of the gas ends up in the supermassive black holes, which increase their masses. Some of that excess galaxy, however, hangs around the further edges of the galaxies and causes globular clusters to form.

There is at least one galaxy that does not follow this relationship: our own Milky Way. That's most likely because it's such a loose spiral and therefore quite unlike any of the galaxies originally studied. On the other hand, our nearest galactic neighbor, the Andromeda galaxy, does follow the correlation.

One of the most stunning aspects of the relationship is how unaffected it is by other conditions. For instance, two giant galaxies with roughly equal brightness, M87 and Fornax A, have massively different profiles when it comes to globular clusters: M87 has about 15,000 while Fornax A has 1,200. The size of their black holes? M87 has a black hole around six billion times bigger than the sun, while Fornax A is only 150 million solar masses.

[arXiv]