The earliest black holes seen by JWST appear to be unusually massive

Observations by the James Webb Space Telescope suggest supermassive black holes from the early universe are more massive in relation to their galaxies compared with those near us.

The universe’s earliest supermassive black holes appear to be much more massive than black holes today, according to an analysis of observations by the James Webb Space Telescope (JWST). The findings also provide the strongest evidence yet for long theorised but unobserved black holes that form without stars, known as direct collapse black holes.

The James Webb Space Telescope has captured images of extremely distant galaxies and supermassive black holes that existed when the universe was less than 1 billion years old
NASA, ESA, CSA, S. Finklestein

Astronomers know that supermassive black holes are intimately tied to their host galaxy’s properties and growth. In galaxies near us, they contribute around one- thousandth of the galaxies’ total mass, on average.

It is unclear, however, if this is the case for the universe’s first supermassive black holes and whether they formed in the same way, because they are very faint and difficult to spot from Earth.

Fabio Pacucci at Harvard University and his colleagues analysed more than 20 supermassive black holes found by JWST since it started its observations in 2022. They found that the relationship between black holes and their host galaxies’ mass is very different from that in our local universe.

Older galaxies, which are further away from us, appear in different hues due to the stretching of light waves, called the redshift. At high redshifts, which correspond to ages between 700 million and a billion years after the big bang, black holes are 10 to 100 times more massive than black holes in our local universe, for a given galaxy size.

Apart from quasars, which are exceptionally bright black holes that outshine their host galaxy’s starlight, black holes at this age are so faint that we were unable to measure them until recently, but JWST’s sensitive instruments have revealed their unusual nature.

“It seems that, statistically, the population of galaxies in the local universe is different from the population of galaxies at higher redshifts,” says Pacucci.

It isn’t clear exactly why this is the case, but astronomers have previously shown that black holes can grow at different rates depending on rates of star formation and supernova explosions. In the early universe, when these rates were different to today, black holes may have grown more quickly and become more massive compared with their galaxies. The team’s calculations suggest that supernovae may have played less of a role in high-redshift galaxies.

The massive black holes also hint at more exotic origins than the black holes we see in the recent universe, which are all formed from collapsed stars. In the early universe, there were large amounts of hydrogen and helium gas unpolluted by heavier elements from stars. If enough of this gas pools together, astronomers predict that it can collapse under gravity to form a direct collapse black hole (DCBH).

The black holes’ mass relative to that of their galaxies so close to the universe’s beginning strongly suggests they must have formed this way, as there probably wasn’t enough time for them to form from stars. But we can’t be sure we have found a DCBH until we see a black hole just 100 million years after the big bang with no pollution, says Pacucci.

“We know that black holes and galaxies grow together, but the way in which they do that is obviously a bit more complicated than we had thought,” says Andy Lawrence at the University of Edinburgh, UK. “It’s not quite clear what it means yet or how you interpret it — it’s just a very striking result.”


arXivDOI: arXiv:2308.12331

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