The earliest supermassive black holes may have been big to start with. If so, it would help explain the recent detection of such beasts within a billion years of the big bang.
Supermassive black holes take a long time to build, so we expect to see only a few in the early universe. The more of them we find, the less likely it is that they all grew the way most modern black holes do, by devouring dust and gas.
“You can have a few black holes that accrete at the maximum possible rate for a very long time, but not all of them,” says Marco Ajello at Clemson University in South Carolina.
In principle, though, stars can gain mass faster than black holes. Joseph Smidt at the Los Alamos National Laboratory in New Mexico and his colleagues say this could explain the presence of supermassive black holes so early on.
If a star of around 100,000 solar masses collapses, it could form a substantial black hole right away. Fed by streams of cold gas, that black hole could grow at a stately pace to reach a billion solar masses within the first billion years of the universe.
Smidt and his colleagues performed the most detailed simulation yet of this “direct-collapse” scenario. It produced the black holes we observe, as well as the ionised gas around them and the star formation rate in their host galaxies.
“Other results showed that you can get the right mass – but black holes are more than mass,” says Smidt. “We’ve shown that we can match several other independently observed features.”
So far, astronomers have not directly spotted supergiant stars in the early universe. But if they exist, the James Webb Space Telescope should be able to see them after its launch next year.
Direct collapse is still just one possibility, however. Other theories, like mergers of smaller black holes, remain viable, and researchers aren’t quite sure yet how many black holes there really are in the early universe.
“We think that the most massive black holes out there in the early universe formed from direct collapse, but the less massive ones could have formed in other ways,” says Priyamvada Natarajan at Yale University.
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