Black holes growing faster

Print edition : March 08, 2013

The famous early type spiral galaxy the Sombrero (the Mexican hat) galaxy, called so because of its shape, has the black hole that has grown the most. It is estimated 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. Photo: ESO/P. BarthelAcknowledgments: Mark Neeser (Kapteyn Institute, Groningen) and Richard Hook (ST/ECF, Garching, Ger

Centres of most galaxies harbour black holes that can weigh anywhere from one million to one billion times the mass of the sun. A new study, published in The Astrophysical Journal, shows that these black holes are growing much faster than previously thought. Even the black hole in our own Milky Way, which otherwise appears very quiet, has probably been consuming the equivalent of one sun every 3,000 years.

Until recently, astronomers believed that black holes grow mostly when galaxies crash into each other, causing a large concentration of gas to form around the black hole and get very hot, shining very brightly in what is known as an active galactic nucleus. The 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 such as the Milky Way cannot grow much. However, computer simulation work led by Victor Debattista of the University of Central Lancashire, the United Kingdom, has shown that black holes in spiral galaxies grow by large amounts without the need for collisions.

Some spiral galaxies have active galactic nuclei, which may outnumber those in merging galaxies. Closer home, astronomers 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 researchers used a property of black holes that their masses can be quite accurately predicted from the speed of stars in the galaxies they live in. By using computer simulations to compare the masses of black holes in spiral galaxies with those of elliptical galaxies, the team successfully disproved the previous theory that black holes are unable to grow while the galaxy itself grows. Elliptical galaxies are those that have suffered collisions and have stopped forming stars. The team 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.