The Milky Way's First Solitary Black Hole
Tommaso Freschi
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Black holes, the most extreme and mysterious objects in the universe, are always found in binary systems, i.e. being orbited by other celestial objects, or sometimes even a very large number of objects. An example is the supermassive black hole (SMBH) in the middle of our Milky Way, which is orbited directly by 4+ stars and indirectly by EVERY SINGLE STAR in our galaxy.
Now, usually black holes aren’t supermassive, but are of stellar mass, i.e. around 3-10 times bigger than the Sun. So, theoretically, they shouldn’t be orbited by much at all, as the supernova that formed them would’ve ejected most of the material around the star away, and their relatively low mass would’ve made it unlikely to start binary systems. That is theoretically, because most observed black holes, as I’ve already said, are in binary systems.
Photo of the SMBH in the center of the galaxy M87 taken by the EHT
Despite this, in late January of this year, a large team of researchers led by PhD Kailash C. Sahu, using the Hubble Space Telescope (HST), found a solitary black hole wandering our galaxy. This is a big deal, as no lone black hole has ever been found before. But why is it such a big deal? Well to understand that, I first need to explain why it’s easier to detect black holes with objects orbiting them.
The main idea is black holes absorb all light without reflecting any, but, at the same time, their gravitational pull is so strong that they bend light in very strong ways, literally making photons change their paths because of how powerful their gravity is. This, mixed with the fact that black holes, in binary systems at least, produce regular gravitational waves (waves that travel using literal spacetime as their medium and are formed when strong gravitational bodies move), makes them very easy to detect if another body is regularly traveling around them, since the bending of light can be detected often and at regular intervals, and the gravitational waves can also be detected.
Representation of a BH in a binary system with a star done by an artist
Hence, without another point of reference like a star or a neutron star (bodies formed after stars go supernova, but aren’t massive enough to form black holes—kind of like the little, less extreme sibling of black holes) orbiting the black hole, it is extremely hard to detect them. So how did this team detect this lonely black hole?
They used a method that was already used to detect black holes in binary systems, called astrometric microlensing. To put it simply, stars usually appear to be in a specific place in space, but if a black hole passes near where a star usually would be, it does what black holes do—it bends spacetime around itself, changing the path of photons and making it look like the star is in a different place in the sky.
Using this method, the team observed a certain star every six months for six entire years, noting down every single change in its position: the changes in position that they, over six years, detected, confirmed not only that this particular black hole exists, but also confirmed the hypothesis that solitary black holes as a whole exist.
Discoveries like these are important because not only are they advancements in science as a whole, but, if more solitary black holes were to be discovered, they would confirm the hypothesis that most black holes should be solitary, which would show astronomers and astrophysicists that they’re on the right track.
References
Choi, C. (2022, February 7). Rogue black hole spotted on its own for the first time. Space.com. Retrieved February 17, 2022, from https://www.space.com/rogue-black-hole-isolated-discovery
Gater, W. I. (2022, February 8). Solitary stellar-mass black hole found wandering through the milky way. Physics World. Retrieved February 17, 2022, from https://physicsworld.com/a/solitary-stellar-mass-black-hole-found-wandering-through-the-milky-way/
Sahu, K. C., Anderson, J., Casertano, S., Bond, H. E., Udalski, A., Dominik, M., Calamida, A., Bellini, A., Brown, T. M., Rejkuba, M., Bajaj, V., Kains, N., Ferguson, H. C., Fryer, C. L., Yock, P., Mroz, P., Kozlowski, S., Pietrukowicz, P., Poleski, R., … Steele, I. A. (2022, January 31). An isolated stellar-mass black hole detected through astrometric microlensing. arXiv.org. Retrieved February 17, 2022, from https://arxiv.org/abs/2201.13296
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