Scientists say they have finally identified a long-sought intermediate-mass black hole for the first time, thanks to their catching it mid-meal.
Black holes are some of the most extreme objects in the universe, which makes them such fascinating objects to study. There have been two types of black holes that we've been able to identify so far: stellar-mass black holes and supermassive black holes.
Stellar-mass black holes are the remains of a star that exhausted its fuel for nuclear fusion and collapsed in on itself. If the star is roughly eight to 15 times the mass of the sun, it erupts in a supernova, leaving the ultra-dense, city-sized remnant of its core behind, known as a neutron star. But if it is between 15 and 100 solar masses, then even the tightly packed core can't withstand the star's collapse and the collapse continues even further, producing a stellar-mass black hole.
Supermassive black holes, meanwhile, are a million solar masses or even greater, and are found in the center of most large galaxies, if not all of them.
But what about the black holes in the middle, between 100 and 1 million solar masses? These are known as intermediate-mass black holes and it has been an open question if such objects even exist – until now.
In a new study in this month's The Astrophysical Journal, a team of University of Arizona (UA) astronomers says that a black hole that was caught shredding apart a star in a so-called tidal disruption event has revealed itself to be a member of this long-elusive class of black hole.
"The fact that we were able to catch this black hole while it was devouring a star offers a remarkable opportunity to observe what otherwise would be invisible," said Ann Zabludoff, a UA professor of astronomy who co-authored the paper. "Not only that, by analyzing the flare we were able to better understand this elusive category of black holes, which may well account for the majority of black holes in the centers of galaxies."
The remains of the dead star caught in the tidal disruption event produced a large amount of X-ray radiation as it was torn apart, known as the J2150 flare, which had been previously observed and recorded. The research team, led by UA astronomer Sixiang Wen, reviewed that data and used it to calculate both the mass and the spin of the black hole that caused it.
"The X-ray emissions from the inner disk formed by the debris of the dead star made it possible for us to infer the mass and spin of this black hole and classify it as an intermediate black hole," Wen said.
While supermassive black holes are the dynamos that power large galaxies like the Milky Way or larger, there are many smaller galaxies that might have these intermediate-mass black holes in their centers.
"We still know very little about the existence of black holes in the centers of galaxies smaller than the Milky Way," said co-author Peter Jonker of Radboud University and SRON Netherlands Institute for Space Research, both in the Netherlands. "Due to observational limitations, it is challenging to discover central black holes much smaller than 1 million solar masses."
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Analysis: intermediate-mass black holes are a big deal for physics
One of the reasons intermediate-mass black holes have been sought after for so long is that they could help answer a lot of outstanding questions in physics.
The most obvious would be the origin and evolution of their bigger cousins, the supermassive black holes that play such a key role in galaxy formation. Knowing how supermassive black holes form, and whether they were once intermediate-mass black holes, would tell us a lot about how the universe developed.
Another question it could help solve is the issue of dark matter, which physicists believe makes up more than 80% of all the matter in the universe but which does not interact with light in any way.
Dark matter may be composed of some exotic particle of matter that we've yet to discover, and one such theoretical particle is the ultralight boson, which should have the effect of slowing down the spin of an intermediate-mass black hole.
"If those particles exist and have masses in a certain range, they will prevent an intermediate-mass black hole from having a fast spin," said study co-author Nicholas Stone, a senior lecturer at Hebrew University in Jerusalem. "Yet J2150’s black hole is spinning fast. So, our spin measurement rules out a broad class of ultralight boson theories, showcasing the value of black holes as extraterrestrial laboratories for particle physics."