The most distant, nearly invisible dormant black hole has been detected and 'weighed' by an international team of astronomers that includes researchers from UCL.
The study, published in Science, identified a dormant black hole at the heart of a galaxy known as MRG-M0138 located over 10 billion light years away. It is the most distant dormant black hole yet detected, 15 times farther away than the previous record.
The black hole's mass is about 6 billion times that of the sun, and is being observed at a time when the universe was only about 3 billion years old, about a quarter of its current age, offering unprecedented details into black holes in the early universe.
To find this, the team used data from NASA's James Webb Space Telescope to track the motion of stars orbiting around the otherwise invisible black hole to measure its mass. Though the technique – known as stellar dynamics – has been used to measure dormant black holes in galaxies much closer to Earth, this is the first time it has been used to weigh one located such a great (cosmological) distance away.
Senior Author, Professor Richard Ellis (UCL Physics & Astronomy) said: "Determining how stars collectively move within the core of this distant galaxy has allowed us to measure the mass of its otherwise undetectable supermassive black hole. By demonstrating the feasibility of such a technique for galaxies in the early universe, we can now undertake a more complete census of how black holes develop over time and infer their role in shaping galaxy evolution."
Measuring an invisible black hole
Though black holes themselves don't emit any light, gaseous material that falls into them can emit a great deal of radiation. These "active galactic nuclei" are sometimes referred to as quasars and are easy to identify as they are some of the most luminous objects in the cosmos.
However, the supermassive black hole in MRG-M0138 is dormant. With no gaseous material falling into it, its presence can only be inferred from the motions of nearby stars.
The team was able to detect its presence and accurately measure its mass by observing the collective motion of the stars moving around it. How fast they move and the differences between the motions of stars close to the black hole and those farther away allowed the researchers to accurately calculate the mass of the black hole at the centre.
It's a technique similar to what has been used to measure the mass of the black hole at the centre of our own galaxy – the 'Milky way – and several other nearby galaxies, but it's the first time it's been used on an object at such a great distance. Previously, the most distant galaxy similarly studied with this technique was only 700 million light years away.
Ordinarily the motions of stars in a galaxy so far away would be impossible to observe, but the team overcame this by using a natural cosmic magnifying glass known as gravitational lensing. The gravitational influence of another galaxy, located directly between MRG-M0138 and Earth, bends the light around it, refocusing the background image and enlarging it 30 times. Using this, the researchers were able to reconstruct the internal details of the distant galaxy to a much higher resolution than would otherwise be possible.
Lead author Dr Andrew Newman of the Carnegie Science in Pasadena California said: "By combining JWST data with gravitational lensing, we could peer inside the black hole's sphere of influence, where its gravity boosts the speeds of stars. This is one of the best techniques we have to weigh a black hole, so we were excited to extend it to a much earlier period in cosmic history."
Only a few dormant black holes this massive have previously been found, but all at far closer distances.
A peek into the early universe
The discovery offers new clues about how black holes and galaxies grew together in the early universe. Local galaxies have revealed a close relationship between their masses and those of their central black holes, but more data is needed from earlier cosmic times for both active and dormant supermassive black holes to fully understand this relationship.
The researchers found that not only is the black hole itself dormant, but the surrounding galaxy is similarly inert and no longer forming new stars. Likely, MRG-M0138 probably once hosted a luminous quasar in its past. The researchers think that when the black hole first formed and rapidly grew, the energy it released burned off or ejected the free-floating gas in the galaxy which is critical for the formation of new stars.
The team expects that additional observations from the JWST and other space telescopes will reveal many more dormant black holes from the early universe. This would offer new insights into their role in stopping star formation, as well as how dormant black holes can reactivate again when large amounts of matter start to flow into them.
