13 February 2013
Lurking at the centres of most galaxies are black holes that can weigh anywhere from one million to one billion times as much as the Sun.
New research, published today in the Astrophysical Journal, shows that these black holes are growing at much larger rates than had previously been thought possible. Even the black hole in our own Milky Way Galaxy, which otherwise appears very quiet, has probably been consuming the equivalent of one Sun every 3000 years.
Until recently astronomers had thought that black holes grow mostly when galaxies crash into each other, at which time a large concentration of gas forms around the black hole and gets very hot, shining very brightly in what is known as an active galactic nucleus.
This gas gets so bright that active galactic nuclei can be seen all the way back to shortly after the Universe first formed. This theory held that black holes in the centres of ordinary spiral galaxies like our own Milky Way cannot grow much.
“These simulations show that it is no longer possible to argue that black holes in spiral galaxies do not grow efficiently”
However, a new study led by University of Central Lancashire (UCLan) astronomer Dr Victor Debattista used computer simulations to show that black holes in spiral galaxies must grow by large amounts without the need for collisions.
Recent observations by the Hubble Space Telescope have indicated that black holes can grow even in quiet spiral galaxies. Some spiral galaxies have active galactic nuclei and these may outnumber those in merging galaxies.
Closer to home, astronomers have recently discovered a gas cloud near the centre of the Milky Way that later this year will be ripped apart by the central black hole. Over the next 10 years, the black hole is predicted to swallow up to as much as 15 times the mass of the Earth from this cloud.
The team of researchers, which included Dr Stelios Kazantzidis of Ohio State University and Professor Frank C. van den Bosch of Yale University, used a property of black holes that was first discovered by the Hubble Space Telescope, that their masses can be quite accurately predicted from the speed of stars in the galaxies they live in.
The team successfully disproved the previous theory that black holes are unable to grow while the galaxy itself grows by using computer simulations to compare the masses of black holes in spiral galaxies with those of elliptical galaxies.
“Our simulations will allow us to refine our understanding of how black holes grew in different types of galaxies”
The team’s comparison of spiral and elliptical galaxies found that there is no mismatch between how big their black holes are. For this to have happened black holes had to have been growing along with the galaxy.
The black hole that has grown the most can be found in the Sombrero galaxy.
The researchers estimate that this black hole has been swallowing the equivalent of one Sun every 20 years and is now over 500 million times as heavy as the Sun.
The new study, published today in the Astrophysical Journal, provides the theoretical basis for understanding the emerging picture that galaxy collisions are a relatively small contribution to the growth of black holes, contrary to previous assumptions.
Commenting on the significance of the new research Dr Debattista said: “These simulations show that it is no longer possible to argue that black holes in spiral galaxies do not grow efficiently. Our simulations will allow us to refine our understanding of how black holes grew in different types of galaxies.”