The mystery of the fastest growing black hole

A team of astronomers at ANU have found a black hole 12 billion lightyears away, and it’s not your ordinary black hole.

The fastest that any human has ever run is recorded as 44km/h, the top speed Usain Bolt reached in a 100-metre race in 2009. Even though the records are broken from time to time, we can’t go much faster than that, because there is more or less a set speed for human beings.

Something similar has been found to apply to black holes, those places in space that really suck: their gravity is so strong that nothing can get out. In astronomy there is a known rule that explains how fast and how large black holes can grow, sometimes referred to as “the speed limit”. They can’t grow faster than one percent of their mass in a million years.

The black hole that the research team found adds to an existing mystery about the origin of black holes: how do some of them get so big so quickly?

When they applied the speed limit rule and counted backwards in time, they ran into a puzzle: this black hole must have formed from an extremely heavy star. So heavy, that they have no idea how a star like that would come into being.

“It glows the brightest of all the supermassive black holes in the Universe,” says Dr. Christian Wolf from the ANU Research School of Astronomy and Astrophysics. The brightness means that it’s sucking in the most stuff and growing at the fastest rate of all black holes out there.

It was found 12 billion lightyears away in the early days of the Universe, only 1.2 billion years after the Big Bang. It already has a mass of 20 billion suns and it sucks in the equivalent of a star every two days.

Black holes can’t be seen, because even light can’t get out once it gets in. Yet black holes are bright, because the outer parts have a beautiful shine resulting from objects piling up and going in. The ingoing mass competes for space creating friction and heat radiation, which presents as hot glowing gas clouds that astronomers can spot with special telescopes.

But if no one can see it, can we be sure that it’s a black hole? This question has been asked before about the black hole in the middle of our own galaxy, the Milky Way.

“There are plenty of people who thought about alternatives: must it be a black hole, can it be something else?” Wolf says.

However, the black hole in the Milky Way had all the characteristics of a black hole, and it was an object that didn’t let any light come out, so eventually it was declared as one. Because of this, the black hole the research team found is also thought to be a black hole, even though we can’t see its inner structure. The secret of spotting black holes is indeed looking at what goes in, not what is inside.

Black holes are born when a star with a large mass dies and compresses into a small but very dense object. Large black holes called supermassive black holes have been found in the center of every galaxy, which has made researchers wonder about their purpose in the formation of galaxies.

Astronomers are trying to find more large black holes to uncover the secret of how they are growing so rapidly. Could this be a violation of the rules of physics?

Not necessarily, Wolf thinks. “It just means there is something we haven’t discovered yet.”