A black hole in the early universe has almost half the mass of its host galaxy despite no longer sucking in matter, raising questions about how black holes grow.
While at the University of Cambridge and his colleagues were looking through data from the James Webb Space Telescope (JWST), they found something unusual. It was a faint galaxy, seen from our perspective as it was about 800 million years after the big bang, with a central black hole almost half the mass of the galaxy itself – 400 million times the mass of our sun and thousands of times more massive than expected.
Active black holes usually emit radiation, but this one was very faint, meaning the black hole must be dormant, having quickly grown to its large size earlier in its life.
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“It’s one of the lowest luminosity black holes that we see,” says Juodžbalis. “Our finding implies strong evidence in favour of black holes in the early universe growing in very fast bursts.”
Most galaxies, including our own, contain a supermassive black hole at their centre, but these are typically much smaller than the host galaxy. The Milky Way’s central black hole is 10,000 times less massive than the galaxy, for example.
The black hole that Juodžbalis’s team found appears to have stunted the growth of its galaxy, so star formation hasn’t yet begun in earnest. “This is the most overmassive black hole that has been found,” says at the Harvard-Smithsonian Center for Astrophysics in Massachusetts, who wasn’t involved in the study.
This could have resulted from this galaxy merging with another, which “force-fed” the black hole, says Juodžbalis. Such periods of black hole growth might last for just 4 million to 10 million years, so the chance of observing a black hole in this phase is slim.
Another merger in future could restart the black hole’s growth, but it is more likely that the galaxy will eventually undergo star formation and grow into a more regular galaxy, says Juodžbalis. “It’s still acquiring fresh gas.”
Pacucci isn’t sure this is an archetypal example of galaxy growth, however. “I think this is a very interesting and peculiar source,” he says. “I don’t think this is the normal pathway galaxies go through.”
The unusual size of the black hole might explain how supermassive black holes grew in the early universe, an open question in astronomy. Some astronomers think the first stars exploded as supernovae and created small black holes 10 to 100 times the mass of our sun that grew rapidly. Others suspect that clouds of gas collapsed directly into black holes 10,000 to 100,000 times the mass of our sun.
Juodžbalis and his team’s black hole could favour the former scenario. “We’re alleviating the need for having a lot of direct-collapse black holes that formed heavy,” he says. “Maybe black holes did form from the first stars and just grew at a very high rate.”
Finding more overmassive galaxies like this could provide more answers. “We are going below the tip of the iceberg in luminosity,” says Pacucci. “You would expect that going lower in luminosity, you would find lower mass objects. This paper is throwing up that this is not the case.”
arXiv
