For decades, the scientists charged with the task of detecting black holes faced a bit of a Goldilocks problem.
These experts had discovered luminous supermassive black holes with masses as large as billions of Suns in the center of most galaxies. In our own galaxy, they’d found one with a mass four million times that of our sun. They had also established strong evidence of the existence of smaller, more common black holes, called stellar black holes, that range from five to 100 times the mass of our sun. In the Milky Way alone, it’s now estimated that there are around 10,000 stellar black holes lurking in the cosmos.
But sizes in between these extremes have long escaped observation by astronomers. Until now.
Thanks to a rare telltale sign of radioactivity and decades’ worth of archival data from a global network telescopes, scientists have captured the best-ever evidence of the elusive mid-size black hole—thought to be the missing link in our understanding of black hole evolution. The discovery, published in Nature Astronomy, could help researchers better fill in gaps on the birth and behavior of black holes.
“It is definitely quite exciting,” says Dacheng Lin, a physicist at the University of New Hampshire and lead author of the paper. “Before, a few candidates were identified, but they were either too far away or found too late. This is the best intermediate-mass black hole candidate observed so far.”
These mid-size black holes have previously eluded detection because they are neither as luminous as the heavyweights at the center of galaxies nor as common as the smaller stellar black holes. They also tend to be located in star clusters at sites devoid of gas, leaving them with little fuel to consume and thus little radiation to emit. The confluence of these circumstances made them virtually impossible for scientists to spot.
However, scientists can get help in detecting this elusive type of black hole with the help of an unknowing accomplice. That help comes in the form of a nearby star, which, in passing too close, gets sucked into the black hole and subsequently devoured. This process shreds the star, but it activates the black hole to emit an enormous flare of radiation—a signature of sorts observable from Earth. The only problem with relying on these events, called tidal disruption events (TDEs), is that they only happen roughly once every 1,000 years.
“It’s like trying to find a needle in a haystack,” says Lin. “You shouldn’t expect to see it in your lifetime.”
Instead of waiting around for these rare celestial events to happen, Lin and collaborators sifted through decades of archival data from one of the largest telescopes in the world, the European Space Agency’s XMM-Newton, to see whether they could identify one that happened in the past. “Fortunately, we found one and we’re lucky because it has a lot of good data,” says Lin.
Their luck came in the form of X-ray data collected from a galaxy nearly 740 million light-years away, taken in 2006 and 2009 as part of a massive XMM-Newton galaxy survey. In these data, which were further confirmed with data from NASA’s Chandra X-Ray Observatory and Swift X-Ray Telescope, the researchers found evidence of a black hole with an enormous flare of radiation from a TDE on the outskirts of the distant galaxy.
By measuring the radiation at different times—and at each time point, analyzing the distribution of photons over energy—Lin and the team of collaborators began mapping the density of their discovery. They found that the black hole had a mass of around 50,000 times that of the Sun: the size of an intermediate-mass black hole. But that wasn’t the only noteworthy finding.
“There were two points about the discovery that I was really excited about: not just that the data fit, but that the location was off the center of the galaxy,” says Ronald Remillard, a physicist at the Massachusetts Institute of Technology and co-author of the paper. “We are so used to thinking, for external galaxies, that there will be a very massive black hole in the center. That’s the part that I hadn’t really visualized and thought through: that they can be way off the center and we can learn from them. It’s the first really good case that it has the potential to be a window into intermediate black holes.”
Both Lin and Remillard hope this approach—detecting intermediate black holes via TDEs and using a global network of retrospective data—opens the door for further discoveries of the existence of intermediate-mass black holes.
“People were getting frustrated, wondering why there weren’t some out there,” says Lin. “Now, there’s this expectation that there should be quite some more out there.”
These mid-size black holes could be the key to better understanding the hierarchy of black hole evolution and how they influence other galactic phenomena. For example, one popular theory is that these mid-size black holes are the “seeds” of supermassive black holes, and that by comparing their X-ray emissions with the large amounts of data we already have on supermassive black holes, scientists can fill in missing intermediary information in current models of black hole lifespans. This, in turn, may even be able to provide new insights into the formation of entire galaxies, says Lin.
“These are really very fundamental questions of studying black holes,” says Lin. “And we now have very high-quality data to help answer them.”