Monster black holes may be behemoths from birth
Exactly how supermassive black holes became so big so quickly has had astronomers stumped. Italian scientists present evidence that they skip a few steps. Belinda Smith reports.
Monster black holes may have received a jump start when gigantic gas clouds collapsed to produce a "seed" from which they grow, astronomers suggest, rather than merging with others.
This explains why some supermassive black holes became so big so quickly – within a billion years following the Big Bang.
Italian astronomers, led by Fabio Pacucci from the Scuola Normale Superiore in Pisa, used data from Hubble, Spitzer and Chandra telescopes combined with computer models to identify hints of these cosmic seeds.
"Our discovery, if confirmed, explains how these monster black holes were formed," Pacucci says.
Normal black holes form when large-enough stars, around 30 times the mass of the Sun, collapse on themselves.
But supermassive black holes, thought to lie in the centre of large galaxies including our Milky Way, can be billions of times the mass of the Sun. And, perplexingly, some of these black holes formed when the Universe was still young – less than a billion years old.
Theories as to how black holes became so big so quickly range from voraciously gobbling gas to merging with other black holes, but these can't fully explain their immense size in the early Universe.
So rather than starting small and growing quickly, could these young monsters started big and grown at a normal rate?
The idea's not a new one – it's been around a decade or so – but finding exactly how supermassive black holes start huge has posed a problem.
Obtaining data from the first few hundred million years of the Universe's existence means peering far, far away.
So using a suite of telescopes, Pacucci and colleagues found two promising black hole seed candidates, called 33160 and 29323, based on X-ray and infrared radiation which matched theoretical calculations.
Estimates suggest they're around 13 billion light-years away, meaning they formed when the Universe was less than a billion years old.
The pair of seeds, the researchers suggest, possibly formed when a massive gas cloud collapsed on itself to produce a black hole progenitor more than 100,000 times the mass of the Sun.
By skipping the star formation, burn-out and collapse steps seen in normal black holes, these seeds gave supermassive black holes a leg-up in the growth stakes.
The data are promising, but "as scientists, we cannot say at this point that our model is 'the one'", Pacucci says, so the team plans to search for more seeds.
Things should become clearer with the October 2018 launch of the James Webb Space Telescope, they write, which "will mark a breakthrough in this field, by detecting light from the most distant stars and accreting black holes, probing the mass range of the first black hole seeds".
The work will be published in the Monthly Notices of the Royal Astronomical Society. You can find the preprint on ArXiv.