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Rare Free-Floating Black Hole Traveling at 30 km/s Spotted in Our Galaxy

A photograph showing the potential free-floating Black Hole near the center of the Milky Way Galaxy. Image credit: NASA, ESA, Kailash Sahu(STScI), with image processing by STScI’s Joseph DePasquale

A group of astronomers believes to have spotted a rare, invisible black hole, freely floating through our galaxy at a speed of thirty kilometers per second.

Gravitational microlensing may have helped astronomers find the first free-floating black hole in our galaxy. Future observations should continue to uncover many more similar black holes, researchers have revealed.

Black holes should be scattered throughout our Milky Way galaxy if, as astronomers assume, the death of large stars leaves black holes in their wake. Unfortunately, isolated black holes cannot be seen.

A team of researchers led by the University of California, Berkeley, astronomers have reported having discovered, for the first time ever, a strange free-floating black hole. This discovery was only possible by studying the brightening of a distant star and its distorted light due to the object’s gravitational field. This is known as gravitational microlensing.

Researchers led by UC Berkeley professor Jessica Lu and graduate student Casey Lam estimate that the mass of the invisible compact object ranges between 1.6 and 4.4 times that of the sun. As a result, researchers at UC Berkeley believe that a dead star’s remnant must be heavier than 2.2 solar masses to collapse into a black hole.

However, they caution that the object could also be a neutron star. The gravity of neutron stars is balanced by their internal neutron pressure, preventing them from collapsing into black holes.

The object is a stellar “ghost,” roaming through the galaxy without a companion, whether it’s a neutron star or a black hole.

“This is the first free-floating black hole or neutron star discovered with gravitational microlensing,” Lu revealed. “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.”

Knowing how many of these dense objects exist in the Milky Way galaxy will help astronomers understand the evolution of stars – specifically, how they die – and of our galaxy. It may also reveal whether the unseen black holes are primordial black holes, which are thought to have been produced in large quantities at the time of the Big Bang.

The research led by Lam, Lu, and their international team has been accepted for publication in The Astrophysical Journal Letters. In the research, the team found four other microlensing events not caused by black holes. However, two of the others may have been caused by white dwarfs or neutron stars. They also determined that there are likely to be 200 million black holes in the galaxy – an estimate that most theorists predicted.

An analysis of the same microlensing event by researchers from the Space Telescope Science Institute (STScI) in Baltimore determined that the compact object is closer to 7.1 solar masses and is undoubtedly a black hole.

In The Astrophysical Journal, scientists led by Kailash Sahu described their analysis of the data.
To come to these conclusions, both studies used the same astronomical data: measurement of the brightening of the distant star caused by the light being bent by the super-compact object.

Additionally, the researchers measured the astrometric shift in the distant star’s position in the sky caused by the lensing object’s gravitational distortion.

The photometric data are from two microlensing surveys; one carried out by Warsaw University with a 1.3-meter telescope in Chile and the other by Osaka University with a 1.8-meter telescope in New Zealand.

NASA’s Hubble Space Telescope provided the astrometric data. STScI is responsible for the science program and telescope operations.

Both microlensing surveys identified the same object, so it has two names: MOA-2011-BLG-191 and OGLE-2011-BLG-0462, or OB110462.

Every year, surveys like this find about 2,000 stars brightened by microlensing within the Milky Way galaxy. Still, the addition of astrometric data served as the link to determining the mass and distance of the compact object.

UC Berkeley-led researchers estimate it lies between 2,280 and 6,260 light-years (700-1920 parsecs) away, near the bulge surrounding the Milky Way Galaxy’s star-forming center.

Based on estimates from STScI, it lies about 5,153 light-years (1,580 parsecs) from us.

Furthermore, both teams calculated the super-compact lensing object’s velocity. The team from Lu/Lam found a relatively slow speed of less than 30 km/h. On the other hand, scientists from STScI found a velocity of 45 km/s, which they interpreted as the result of an extra kick from the supernova that created the purported black hole.

According to Lu, the team’s low-velocity estimate supports a new hypothesis that black holes do not come from supernovae – the current theory – but instead are formed by failed supernovae that do not make a bright splash in the universe or give the black hole a kick.

You can read more about the discovery in this and this paper published in the preprint server arXiv.

Astronomers have estimated that there are around 40 billion billions Black Holes in our universe.

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