Nowadays, astronomers think that nearly all large galaxies have supermassive black holes at their cores. Our own Milky Way’s central black hole is called Sgr A* (pronounced Sagittarius A-star), and it’s the focus of many fascinating studies. This week (April 24, 2018), astrophysicists announced the results of a new study based not on observations, but on a state-of-the-art cosmological simulation, called Romulus. The simulation showed that galaxies might contain more than one supermassive black hole. In fact, a galaxy with a mass like that of our Milky Way should host several, these scientists said. The extra supermassive black holes might “wander” throughout a galaxy, remaining far from its center. If it’s true, then could our own Milky Way galaxy’s supermassive black hole have an unseen sibling or two?
How did these extra supermassive black holes get into the Milky Way? These scientists – led by Michael Tremmel at Yale – think the sibling black holes, if they exist, indicate mergers between our Milky Way and other galaxies in the early universe. If a smaller galaxy joined ours, it might have deposited its own central supermassive black hole within our galaxy. When the universe was young, this might have happened several times.
In this study, we’re looking at how supermassive black holes move through their galaxies. If we look at massive galaxies the size of the Milky Way, we find that, on average, these galaxies host several supermassive black holes within them, wandering about the galaxy on scales of several thousand light-years from the center of the galaxy.
So, theoretically, multiple supermassive black holes – wandering supermassive black holes – within galaxies are possible. But no one has yet discovered them. Tremmel said that, in the next several decades, gravitational wave telescopes in space should be able to detect the black hole mergers that would cause galaxies to have more than one supermassive black hole. Perhaps this is a normal part of galaxy evolution.
Could we find one of our Milky Way’s sibling black holes now? Probably not. Tremmel said that, since wandering supermassive black holes are predicted to exist far from the centers of galaxies and outside of galactic disks, they’re unlikely to be accreting more gas. Since it’s the accretion of matter that causes all the observable activity near a black hole, any sibling supermassive black holes for our Milky Way or other galaxies would effectively be invisible. Tremmel said:
We are currently working to better quantify how we might be able to infer their presence indirectly.
If they exist, could unseen supermassive black holes in our Milky Way galaxy somehow affect us? No. Tremmel explained:
It is extremely unlikely that any wandering supermassive black hole will come close enough to our sun to have any impact on our solar system. We estimate that a close approach of one of these wanderers that is able to affect our solar system should occur every 100 billion years or so, or nearly 10 times the age of the universe.
An extremely unlikely encounter indeed!
Bottom line: Astronomers ran a sophisticated computer simulation called Romulus to learn that galaxies like our Milky Way might contain multiple supermassive black holes. Thus our Milky Way’s central supermassive black hole might have unseen siblings.
Eleanor Imster has helped write and edit EarthSky since 1995. She was an integral part of the award-winning EarthSky radio series almost since it began until it ended in 2013. Today, as Lead Editor at EarthSky.org, she helps present the science and nature stories and photos you enjoy. She also serves as one of the voices of EarthSky on social media platforms including Facebook, Twitter and G+. She and her husband live in Tennessee and have two grown sons.