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Mystery: How are supermassive black holes made?

IF YOU can get a good view of the Scorpius constellation, look for its tail. Follow its curve into the dark sky nearby and you will find yourself looking right at it: the centre of the Milky Way. There resides the sinkhole of our galaxy, a supermassive black hole known as Sagittarius A*. Staggeringly extreme, it weighs in at 4 million solar masses and yet stretches just 44 million kilometres across. All that matter is squeezed into a space about the size of Mercury’s orbit around the sun.

Bamboozling though black holes are, we think they are a natural consequence of how matter makes a galaxy. It condenses to form stars, and some of those eventually become so big that they collapse under their own gravity to form a stellar-mass black hole.

Such an object can become supermassive over time. Its immense gravity slurps up dust, gas and light from its surrounding galaxy into a disc around itself, eventually pulling this material over its “event horizon”, never to be seen again. The more a black hole eats, the more massive it gets and the stronger its pull becomes. The biggest can capture a tenth of the mass of the sun into their discs each year.

But calculations suggest there is a hard limit to black-hole voracity. “The disc radiates and it can push the matter away, and that will shut off accretion,” says at the Harvard-Smithsonian Center for Astrophysics in Massachusetts.

In reality, however, black holes seem to regularly flout that rule. Sagittarius A* is a baby compared with the heftiest black hole yet found. This freak, which is 3.5 billion light years away at the heart of a quasar called OJ 287, has the mass of 18 billion suns. To get that big, it would have to have eaten about nine sun-sized stars every year for 10 billion years. Even more confounding is the earliest known supermassive black hole, which had grown to 800 million times the sun’s mass when the universe was just 700 million years old.

Perhaps these truly gargantuan monsters form when everyday supermassive black holes merge, giving them a kick-start – although the violence of that process seems equally likely to scatter any nearby material that would help them keep growing, says Kenyon. Or they could be devouring incredibly massive stars.

Or perhaps we have got things precisely the wrong way round: galaxies didn’t give birth to all supermassive black holes, but black holes may have come first, says , also at Harvard. “It all depends on how you define a galaxy,” he says.

If the early universe contained precursor galaxies that were just clouds of warm gas, these might directly collapse into a black hole if they got dense enough, says Loeb. “You can imagine some black holes being made shortly after the big bang and being around for a while, attracting matter and then making a galaxy,” he says. That would give them time to hoover up stars to grow into the supermassive creatures we see today.

This article appeared in print under the headline “Object: Sagittarius A*”