While combing through some of the oldest galaxies in the universe, researchers have discovered a galaxy that appears to contain an actively feeding central black hole. Based on the amount of radiation it emits, researchers estimate that it represents roughly half the mass of the entire galaxy, which is astonishingly high compared to modern galaxies.
The fact that such a large object could only exist half a billion years after the Big Bang places strict constraints on how it formed, strongly suggesting that supermassive black holes formed without going through an intermediate step involving a star.
The first galaxies we know of in the universe were identified using the James Webb Space Telescope, which took advantage of a foreground galaxy cluster that zoomed in on distant galaxies through gravitational lensing. Using the lens provided by a specific array, Webb identified 11 galaxies that were imaged because they existed less than a billion years after the Big Bang.
An international team of astronomers decided to examine these galaxies to confirm the existence of supermassive black holes located at the center of modern galaxies. When these galaxies feed, they emit copious amounts of X-rays, so the researchers turned to the Chandra To collect enough data, Chandra spent up to two weeks photographing one location.
There was a clear match with a galaxy named UHZ1, which was magnified nearly four times by gravitational lensing. The X-rays from this location stood out above the background by four standard deviations. (There may be information about X-rays associated with the other 10 galaxies, but researchers say they will publish that separately.) UHZ1 is at a redshift of z=10, which means we’re looking at it as it existed about 500 million years after the Big Bang.
The amount of energy coming from this X-ray source corresponds to active galactic nuclei, which are objects that derive their energy from a supermassive black hole located at the center of the galaxy. Based on the wavelengths detected, researchers believe the object is contained in a shell of dust and gas within its host galaxy.
How fast is a black hole eating?
To understand these results, you need to understand the Eddington limit, which determines how quickly a black hole merges material from its surroundings. It was named after Arthur Eddington, who performed the first calculations on it, the limit was determined by the fact that matter must lose energy to fall into the black hole, otherwise it would simply remain in orbit around it. This energy will be lost as radiation, which will be absorbed by nearby matter and pushed away from the black hole.
As a result, even if there is plenty of material available for a black hole to feed on, its diet ends up being limited: it feeds on too much, and the radiation chokes off its food supply. So, given the mass of the black hole, the Eddington limit can be calculated as the maximum amount of matter it can hold in a given time.
There are ways to exceed the Eddington limit if material is directed toward the black hole. But these require very specific compositions of gas feeding the material directly below the gravity well, so Eddington superfeeding is thought to be a temporary aberration.
The first way the Eddington limit comes into play is that it helps researchers estimate the size of the black hole. Given the amount of energy it’s emitting and assuming it’s feeding at the Eddington limit, you can put a lower bound on the mass of the black hole (if it was feeding below the Eddington limit, it would be heavier). Based on this calculation, the black hole in UHZ1 must be at least 107 times the mass of the sun.
Based on estimates of the mass of stars in UHZ1, this suggests that the central black hole represents half the mass of the galaxy. Or to put it another way, the black hole is almost as massive as everything else in the galaxy combined.
In the current universe, central supermassive black holes account for only about 0.1% of the mass of their galaxies. So, this suggests that we captured UHZ1 at a very early stage in its development, which is not surprising, given its age.
How to build a supermassive black hole
This work also has important implications for the formation of supermassive black holes. There were two ideas about how something of this size could develop. One idea is that the first stars were very large, and that they formed unusually large black holes. These would have grown rapidly through mergers, feeding on the dense environment of gas present in the first galaxies.
The opposite view is that this growth would have occurred too slowly. Instead, people argue that supermassive black holes have always been very large, and they formed very early in the universe’s history through the direct collapse of extremely dense clouds of gas.
The researchers performed the calculations, assuming that the black hole formed about 200 million years after the Big Bang. They found that a black hole formed by the direct collapse of a gas cloud would need to feed at the Eddington limit throughout its entire history to reach the mass of that found in UHZ1. In contrast, a black hole created by a supernova of one of the first stars would have to feed at twice the Eddington limit over its entire history.
This analysis doesn’t take mergers into account, but the researchers point out that a smaller black hole would have a relatively small gravitational pull, so it wouldn’t be able to capture as many of its neighbors to merge. Although Eddington superfeeding is possible, it is unlikely to last for the hundreds of millions of years needed to build a black hole of this size.
It is important to treat this result with some caution, since it is the first supermassive black hole that we have been able to subject to this type of analysis. But as Webb is used to learn about more of these early galaxies, we will likely be able to develop a complete set of early black holes to analyze. This may ultimately give us a clearer picture of its formation and development.
Physical Astronomy, 2023. DOI: 10.1038/s41550-023-02111-9 (About digital IDs).
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