A Rogue Black Hole Roaming Our Galaxy May Have Just Been Confirmed

The discovery of what looks to be a renegade black hole wandering across the Milky Way, which was announced earlier this year, has now been confirmed.

A second group of scientists has reached almost the same conclusion after completing a separate, independent examination, lending credence to the theory that we've discovered a wayward black hole wandering the galaxy.

The new research, led by astronomers Casey Lam and Jessica Lu of the University of California, Berkeley, has reached a somewhat different result. According to the latest research, the object might be a neutron star rather than a black hole, based on its mass range.

In any case, gravitational microlensing might be a new method for finding 'dark', compact objects that are otherwise unseen in our galaxy by measuring how their gravitational fields bend and distort the light of distant stars as they pass in front of them.

"This is the first free-floating black hole or neutron star discovered with gravitational microlensing," Lu explains.

"With microlensing, we're able to probe these lonely, compact objects and weigh them. I think we have opened a new window onto these dark objects, which can't be seen any other way."

The collapsing cores of huge stars that have reached the end of their lifetimes and expelled their outer material are thought to be black holes. Black hole precursor stars with masses greater than 30 times that of the Sun are predicted to have brief lifetimes.

According to our best estimations, there might be as many as 10 million to 1 billion stellar-mass black holes floating around in the galaxy, drifting happily and quietly.

But there's a reason black holes are called that. Unless material falls upon them, which creates X-rays from the region around the black hole, they emit no light that we can see. We have practically no method of recognizing a black hole that is just hanging there and doing nothing.

Almost. A black hole does have a very intense gravitational field that warps any light passing through it. As observers, this may cause us to notice a faraway star that seems brighter and at a different position than usual.

That's exactly what occurred on June 2, 2011. The Optical Gravitational Lensing Experiment (OGLE) and Microlensing Observations in Astrophysics (MOA), two distinct microlensing surveys, both independently documented an event that peaked on July 20.

MOA-2011-BLG-191/OGLE-2011-BLG-0462 (reduced to OB110462) was the designation given to this event, and scientists honed down on it because it was extremely lengthy and extraordinarily brilliant.

"How long the brightening event lasts is a hint of how massive the foreground lens bending the light of the background star is," Lam adds.

"Long events are more likely due to black holes. It's not a guarantee, though, because the duration of the brightening episode not only depends on how massive the foreground lens is, but also on how fast the foreground lens and background star are moving relative to each other."

"However, by also getting measurements of the apparent position of the background star, we can confirm whether the foreground lens really is a black hole."

Illustration showing how Hubble views a microlensing event

Up till 2017, the Hubble Space Telescope observed the area eight times.

A team of scientists led by Kailash Sahu of the Space Telescope Science Institute deduced that the culprit was a microlensing black hole at a distance of 5,153 light-years distant, weighing 7.1 times the mass of the Sun.

More Hubble data, as recently as 2021, has been added to Lu and Lam's research. The object is between 1.6 and 4.4 times the mass of the Sun, according to their findings.

It's possible that the item is a neutron star. That's also the collapsed core of a large star, one with an initial mass of 8 to 30 times that of the Sun.

The resultant item is held together by neutron degeneracy pressure, which occurs when neutrons do not want to share the same space, preventing it from collapsing entirely into a black hole. A mass limit of about 2.4 times the mass of the Sun exists for such an entity.

Surprisingly, no black holes smaller than 5 times the mass of the Sun have been discovered. The lower mass gap is what it's called. If Lam and her colleagues' findings are true, we may be on the verge of detecting a smaller mass gap item, which is quite exciting.

Because their studies yielded different conclusions for the relative movements of the compact object and the lensed star, the two teams came up with different masses for the lensing object.

As a result of a natal kick, a lopsided supernova explosion can send the collapsed core rushing away, Sahu and his team discovered that the compact object is travelling at a comparatively high velocity of 45 kilometers per second.

Lam and her colleagues, on the other hand, achieved a speed of 30 kilometers per second. This finding shows that a supernova explosion may not be required for the creation of a black hole, according to the researchers.

At this time, it's hard to say which estimate is true based on OB110462, but astronomers anticipate to learn a lot from the discovery of more of these objects in the future.

"Whatever it is, the object is the first dark stellar remnant discovered wandering through the galaxy unaccompanied by another star," Lam adds.