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 鈥� 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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鈥淚t鈥檚 one of the lowest luminosity black holes that we see,鈥� says Juod啪balis. 鈥淥ur finding implies strong evidence in favour of black holes in the early universe growing in very fast bursts.鈥�
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鈥檚 central black hole is 10,000 times less massive than the galaxy, for example.
The black hole that Juod啪balis鈥檚 team found appears to have stunted the growth of its galaxy, so star formation hasn鈥檛 yet begun in earnest. 鈥淭his is the most overmassive black hole that has been found,鈥� says at the Harvard-Smithsonian Center for Astrophysics in Massachusetts, who wasn鈥檛 involved in the study.
This could have resulted from this galaxy merging with another, which 鈥渇orce-fed鈥� the black hole, says Juod啪balis. 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鈥檚 growth, but it is more likely that the galaxy will eventually undergo star formation and grow into a more regular galaxy, says Juod啪balis. 鈥淚t鈥檚 still acquiring fresh gas.鈥�
Pacucci isn鈥檛 sure this is an archetypal example of galaxy growth, however. 鈥淚 think this is a very interesting and peculiar source,鈥� he says. 鈥淚 don鈥檛 think this is the normal pathway galaxies go through.鈥�
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.
Juod啪balis and his team鈥檚 black hole could favour the former scenario. 鈥淲e鈥檙e alleviating the need for having a lot of direct-collapse black holes that formed heavy,鈥� he says. 鈥淢aybe black holes did form from the first stars and just grew at a very high rate.鈥�
Finding more overmassive galaxies like this could provide more answers. 鈥淲e are going below the tip of the iceberg in luminosity,鈥� says Pacucci. 鈥淵ou would expect that going lower in luminosity, you would find lower mass objects. This paper is throwing up that this is not the case.鈥�
arXiv
