Pasadena, CA— A team of astronomers led by Carnegie's Andrew Newman has made the first direct mass measurement of a dormant black hole lurking at the center of a galaxy from the early universe.
Although the black hole—a behemoth 6 billion times the mass of our Sun—is no longer lighting up its surroundings, the researchers were able to determine its mass by using JWST to detect the motion of stars near the galaxy's center that are being affected by the black hole's gravity.
Their findings are published in Science.
In comparison, actively feeding black holes are easy to spot. Astronomers have been finding them for decades by looking for quasars—some of the brightest objects in the cosmos, which are powered by gas falling into a black hole at the center of a galaxy.
The black hole the team measured sits at the center of MRG-M0138, a massive galaxy whose light has traveled to JWST from a time when the universe was only about 3 billion years old. The galaxy is no longer forming stars and its central black hole is also quiet.
Prior to this result, astronomers had only successfully used this technique for determining black hole masses in the local universe. In 2020, the Nobel Prize was awarded for detecting the black hole at the center of the Milky Way by tracing the orbits of individual stars.
The collective motions of stars in galaxy centers have been used to weigh black holes to a distance of about 700 million light years. But without JWST's sophisticated suite of instruments and the help of a phenomenon called gravitational lensing, it was not possible to undertake this type of measurement for more distant galaxies.
"We were able to detect this black hole at a distance of 10 billion light years by combining JWST's sharp vision with a natural magnifying glass," Newman explained.
MRG-M0138 is located behind a massive cluster of galaxies, which magnifies and stretches its appearance. As a result, the distant galaxy appears about 30 times larger than it normally would.
"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," Newman explained. "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 handful of dormant black holes this massive have been found before, all in the nearby universe.
The discovery offers new clues about how black holes and galaxies grew together in the early universe. Nearby galaxies show close connections between the masses of their central black holes and the properties of the galaxies around them. But it has been difficult to test whether these relationships already existed billions of years ago. The researchers' findings suggest that the densest galaxies were sites of rapid black hole growth early in the history of the cosmos.
Although now dormant, MRG-M0138 was probably a powerful quasar in its past. Energy released by a rapidly growing black hole can burn off or eject the gas that fuels the birth of stars, which could have put the brakes on star formation in the galaxy.
Future observations will push this work even further. The team is now analyzing JWST data on other similar galaxies. The Euclid satellite and the Nancy Grace Roman Space Telescope will reveal far more examples of gravitational lensing than are currently known. And the Giant Magellan Telescope, now under construction at Carnegie Science's Las Campanas Observatory in Chile with Carnegie as a founding partner, will have the power to study the motions of stars in distant galaxies in much greater detail than JWST.
The researchers expect that applying their methods to more galaxies will help astronomers understand how the most massive black holes formed, grew, and shaped the evolution of galaxies.
