Galactic Core Black Holes Grow Faster Than Expected

Black holes are growing faster than previously thought possible, which includes the black hole in our own Milky Way Galaxy, according to an astronomical study published this week in the Astrophysical Journal.

The standing theory was that the massive black holes at the core of spiral galaxies are usually quiet and often don't grow at all, and only get significantly bigger--or rather, heavier, since the singularity has, again in theory, zero size and infinite density--when another galaxy crashs into it. It can than be observed that 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.

The 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.

The central black hole in our galaxy, which otherwise appears very quiet, has probably been consuming the equivalent of one Sun every 3000 years. And we are actually going to see it feed right now, this year. Astronomers have recently discovered a gas cloud near the center of the Milky Way that later this year, we will see ripped apart by the black hole. Over the next 10 years, the black hole is predicted to be seen having swallowed up to as much as 15 times the mass of the Earth from this cloud.

The team disproved the previous theory by using computer simulations to compare the masses of black holes in spiral galaxies with those of elliptical galaxies--interestingly, black-hole masses can be accurately predicted from the speed of stars in the galaxies they live in.

The comparison of spiral and elliptical galaxies showed that there is no mismatch between how big their black holes are, implying that black holes have been growing along with the galaxy and refuting the theory that spiral galaxies are different.

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 our Sun.

The new study 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.

Study:
Victor P. Debattista, Stelios Kazantzidis, Frank C. van den Bosch, DISK ASSEMBLY AND THE M BH-σ e RELATION OF SUPERMASSIVE BLACK HOLES, Astrophysical Journal, 2013, DOI: 10.1088/0004-637X/765/1/23