Dark matter guides growth of supermassive black holes

A winner has been declared in the debate about stars, dark matter and black holes, and it can help us understand how elliptical galaxies grow.

Michael Franco
Freelancer Michael Franco writes about the serious and silly sides of science and technology for CNET and other pixel and paper pubs. He's kept his fingers on the keyboard while owning a B&B in Amish country, managing an eco-resort in the Caribbean, sweating in Singapore, and rehydrating (with beer, of course) in Prague. E-mail Michael.
Michael Franco
2 min read

This illustration shows two merging spiral galaxies, which will lead to the creation of an elliptical galaxy -- the type the researchers focused on in their study. NASA/CXC/M.Weiss

There's been a debate rippling in the world of astrophysics like a cloud of dust and gas swirling through a nebula: What exactly is the relationship between the supermassive black holes at the center of elliptical galaxies and the halo of dark matter that cocoons them?

New research from the Harvard-Smithsonian Center for Astrophysics might now put that debate to rest.

In this controversy, previous theories said that the size of an elliptical galaxy's supermassive black hole was linked directly to the total mass of the stars it contains. More recent research, though, has shown a strong relationship between the black hole and the dark matter that surrounds the galaxy.

To find where the truth lies, the research team studied 3,000 elliptical galaxies -- football-shaped collections of stars, planets, gas and dust that form when two galaxies merge. The researchers used the motions of stars to weigh each galaxy's central black hole. Then they used X-ray measurements of the hot gas surrounding the galaxies to weigh the dark matter halo -- the larger the halo, the more hot gas the galaxy can retain.

What they found is that a stronger relationship does indeed exist between the black holes and the halos than between the black holes and stars.

"This connection is likely to be related to how elliptical galaxies grow," the Harvard-Smithsonian Center said this week about the research. "An elliptical galaxy is formed when smaller galaxies merge, their stars and dark matter mingling and mixing together. Because the dark matter outweighs everything else, it molds the newly formed elliptical galaxy and guides the growth of the central black hole."

Akos Bogdan, lead author of the research that has been accepted for publication in the Astrophysical Journal, said that the dark matter forms a kind of blueprint that the galaxies can follow as they merge, and it determines the size of the new central black hole.

Although scientists still aren't sure about exactly what dark matter is, they can theorize about its existence and action by monitoring its gravitational effects on other objects in the universe.

"In our universe, dark matter outweighs normal matter -- the everyday stuff we see all around us -- by a factor of 6 to 1," the report said. "We know dark matter exists only from its gravitational effects. It holds together galaxies and galaxy clusters. Every galaxy is surrounded by a halo of dark matter that weighs as much as a trillion suns and extends for hundreds of thousands of light-years."

This research takes us a tiny step closer to understanding how this mysterious force that we know little about is a key player in shaping the very fabric of the universe.