Astronomers have discovered what may be the largest black hole ever discovered - one with a mass 36 billion times that of the sun. This huge black hole is located at the center of the Horseshoe Galaxy in the universe, about 5 billion light-years away from Earth, and its mass is almost 10,000 times that of the black hole at the center of our Milky Way Galaxy.
Scientists from the University of Portsmouth and the Federal University of Rio Grande do Sul in Brazil collaborated on the research, published in the Monthly Notices of the Royal Astronomical Society.
Cosmic horseshoe galaxies are notable for their massive mass, which bends light from more distant galaxies. This phenomenon is known as Einstein's ring, and is where the galaxy's name comes from.
Researchers believe that supermassive black holes of this size are typically found in the largest galaxies in the universe, which are known as fossil galaxy groups - the remnants of several smaller galaxies that merged over time. The Cosmic Horseshoe Galaxy is the only surviving bright galaxy in the group, suggesting that it has devoured other galaxies in the past.
To measure the mass of a black hole, astronomers use two techniques. The first is gravitational lensing, which studies how the gravity of a galaxy bends passing light. The second is stellar kinematics, which analyzes the speed and motion patterns of stars near a black hole.
The cosmic horseshoe is one of the best examples of an Einstein ring, a lenticular galaxy formed by the precise alignment of two galaxies.
Usually, tracking the motion of stars is the most reliable way to measure the mass of a black hole, but this method is less effective for very distant galaxies. By combining stellar kinematics and gravitational lensing, the team was able to measure the mass of black holes at greater distances than usual.
Thomas Collette, a professor at the University of Portsmouth and one of the authors of the paper, said: "This is one of the ten most massive black holes discovered so far, and it is probably the most massive black hole."
Collette explained that most methods of measuring black hole masses are indirect and often subject to uncertainty, so scientists rarely know for sure which black hole is the largest. "Thanks to our new method, however, we have greater certainty about the mass of this black hole," he said.
The black hole is currently dormant, meaning it is not actively accreting new matter - making its discovery all the more remarkable. "This discovery was made for a dormant black hole - that is, a black hole that was not actively accreting material at the time of observation," said UFRGS principal investigator Carlos Melo. "Its detection relies entirely on its enormous gravitational pull and its impact on its surroundings."
Mello noted that their method could help scientists discover and measure other hidden supermassive black holes in the universe, even if they are silent.
The research team believes that the black holes in the "cosmic horseshoe" grew to their current extreme size through a series of galaxy mergers. In such a system, the galaxy and its central black hole combine over time to form a single supermassive black hole.
"It's likely that all the supermassive black holes that were originally in companion galaxies have now merged to form the supermassive black hole we detected," Collette said. "We are witnessing the end state of galaxy formation, and the end state of black hole formation."
The discovery reveals a black hole close to the theoretical mass limit and demonstrates a powerful new way to search for similar black holes. The team now plans to combine their method with data from the European Space Agency's Euclid telescope to search for more hidden giant stars and study how such black holes affect the growth and evolution of galaxies.