It did not shine like a quasar, it did not devour gas with visible violence or send out signals that were easy for scientists to recognize. It was almost off, hidden in the center of a reddish, gravity-warped galaxy. Even so, an international team of astronomers has managed to measure it.
The discovery, published in Science, places the most distant dormant black hole detected so far in the heart of MRG-M0138, a galaxy observed when the universe was only about 3 billion years old. Its mass is around 6,000 million suns.
The figure is impressive in itself, but the real revolution is in how it has been detected. Black holes do not emit light, so they usually only reveal themselves when the matter falling towards them heats up and releases enormous radiation. That activity turns some into cosmic lighthouses.
Just the opposite happened here. The central monster of MRG-M0138 was not feeding appreciably. There was no bright disk to mark its presence, nor an easy-to-detect active nucleus. To find it, researchers had to look at the behavior of surrounding stars.
As detectives they reached a concrete conclusion. If the stars at the center of a galaxy are moving faster than expected, something very massive is pulling on them. By measuring those velocities, they can calculate the mass of the invisible object governing the central region.
"By studying how stars move collectively in the core of this distant galaxy, we have been able to measure the mass of its otherwise undetectable supermassive black hole," says Richard Ellis, Professor of Physics and Astronomy at University College London (UCL) and one of the authors of the discovery.
This technique, known as stellar dynamics, has already been used to study nearby black holes, including the one at the center of the Milky Way. What is extraordinary is having applied it to a galaxy located more than 10 billion light years away, a distance that until recently seemed out of reach for this type of measurements.
The jump was made possible by an unusual combination: the James Webb Space Telescope and a natural magnifying glass made by the universe itself. Between Earth and MRG-M0138 there is a cluster of galaxies whose gravity bends and amplifies the light of the distant galaxy, multiplying its image.
That effect, called gravitational lensing, works like a cosmic telescope. In this case, he enlarged the image about 30 times, enough for Webb to study internal details that would otherwise have been mixed into a blurry, too-small, red blob.
Using the NIRSpec instrument on James Webb, the researchers analyzed how the stars moved in different areas of the magnified image. They were not seeing the black hole, but its imprint on the cosmos: the way its gravity accelerated the stars within its sphere of influence.
The result changes the playing field. Until now, the most distant galaxy studied with a comparable technique was about 700 million light years away. MRG-M0138 is about 15 times farther away, allowing us to look at a much earlier stage of cosmic history.
It also raises an uncomfortable question for physics today: How could a black hole grow so large when the universe was still so young? Six billion solar masses at 3 billion years of the cosmos suggest an accelerated infancy, perhaps marked by an earlier phase of very luminous quasar.
The monster that turned off its own galaxy
The galaxy itself seems to tell that story. MRG-M0138 not only contains a sleeping black hole; It is also virtually off in terms of star formation. It barely makes new stars. That fits with an increasingly solid idea: some black holes not only grow inside galaxies, they can also help alter them at this drastic level.
When a supermassive black hole feeds rapidly, it releases energy capable of heating, dispersing or expelling the gas that galaxies need to continue forming stars. It is as if the central engine had had a ferocious stage and, after consuming or sweeping away its fuel, had left an aging cosmic city.
This case also offers another oddity: a dormant supermassive black hole in the early universe. Quasars are striking and relatively easy to find; Sleeping black holes, on the other hand, hide better and can give a more complete picture of the real population.
If Webb and future observations find more such bodies, astronomers will be able to compare active and extinguished black holes at different times. That would help reconstruct when they grew, when they stopped feeding, and to what extent they participated in the end of star formation in their galaxies.
