Dark matter – how does it work? Scientists still aren’t completely sure (they only know it exists due to its gravitational effects). Now, Harvard researchers have observed elliptical galaxies that reveal a connection between galaxies, their black holes and the dark matter that appears to feed them.
“There seems to be a mysterious link between the amount of dark matter a galaxy holds and the size of its central black hole, even though the two operate on vastly different scales,” says lead author Akos Bogdan of the Harvard-Smithsonian Center for Astrophysics (CfA).
The larger a galaxy, the larger it’s central black hole – but why? Even the largest black holes are several million times less massive than the galaxies that contain them. In studying football-shaped elliptical galaxies, two correlations are evident: Black holes correlate with the total star mass of the galaxy, but also with the size of the dark matter “halo” that surrounds it.
Now, it’s becoming clear which correlation is the driving force.
In studying over 3,000 elliptical galaxies, Bogdan and his colleagues used star motions to derive the masses of the black holes at the galaxies’ centers. By X-raying the hot gas surrounding the galaxies, they were able to determine the size of their dark matter halos (galaxies with stronger halos can hold more gas).
What they found was that without fail, larger dark matter halos lead to larger black holes.
Elliptical galaxies form when two spiral galaxies collide. When that happens, their halos also merge. Because dark matter outweighs normal matter at a ration of six to one, the resulting combined halo is orders of magnitude more massive than the galaxy it contains, and thus guides and determines its growth.
“In effect, the act of merging creates a gravitational blueprint that the galaxy, the stars and the black hole will follow in order to build themselves,” explains Bogdan.