Astrophysicists say they've identified the mainstay of the "diet" that feeds supermassive black holes and determines their size, and it's not the stars in the galaxy the black hole sits in the center of -- it's dark matter.
Previous theories had held that that the size of any elliptical galaxy's supermassive black hole was linked directly to the total mass of the stars the galaxy contains, but more recent research has suggested a strong link between the size of a black hole and the "halo" of dark matter surrounding the galaxy.
A new study has now yielded evidence the invisible gravitational hand of dark matter can influence the growth of a black hole, researchers report in the Astrophysical Journal.
"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.
Dark matter, although invisible, holds galaxies and galaxy clusters together through gravitational effects, and every galaxy has its own halo of surrounding dark matter with the mass of a trillion suns, extending hundred of thousands of light-years out from a galaxy.
Bogdon and his fellow researcher Andy Goulding of Princeton University wanted to find out what the link might be between supermassive black holes and dark matter halos, so they studied more than 3,000 elliptical galaxies.
Using star motions within the galaxies helped them estimate the weight of their central black holes, while X-ray measurements of hot gases surrounding the galaxies allowed them to determined the weight of the surrounding dark matter, since the amount of dark matter determines how much gas a galaxy can hold on to.
A distinct relationship was found between the two masses -- that of the black hole and that of the surrounding dark energy -- a relationship much stronger than between the black hole and the galaxy's stars, Bogdon and Goulding say.
Elliptical galaxies are the outcome of mergers of many galaxies coming together over billions of years, meaning the final result is is not only composed of all the stars of the many merging galaxies but also has a reservoir of all the dark matter those galaxies contained.
Dark matter therefore is responsible for the dominant gravitational impact, affecting galactic evolution and guiding black hole growth, the researches say.
"In effect, the act of merging creates a [dark matter] gravitational blueprint that the galaxy, the stars and the black hole will follow in order to build themselves," Bogdan says.
