Our Galaxy May Have Once Had a Second Central Black Hole

The Milky Way may have had a second black hole at its heart between 10 billion and 10 million years ago—one that acted a bit like a star goalkeeper.

This is the conclusion of a study by researchers from China, who said such a presence could explain the mysterious nature of rapidly moving stars in our galaxy's halo.

"Hypervelocity stars currently observed in the galactic halo were ejected from the galactic center 50-250 million years ago and thus carry rich information about the dynamical environment of the galactic center in the recent past," the researchers said in their paper.

"The supermassive black hole-intermediate black hole binary formed probably after the merger of the Milky Way with the Gaia-Sausage-Enceladus dwarf galaxy."

In the study, astronomer Chunyang Cao of Peking University and colleagues studied massive "hypervelocity stars" whizzing through the Milky Way's halo—the roughly spherical region around the galaxy's iconic disc that contains some of the oldest stars.

These far-flung stars (to use an epithet that, as we'll see, works literally and metaphorically) began their life at the heart of the Milky Way, in the region of the central black hole, Sagittarius A* (Sgr A*).

Life is dangerous for the massive, young stars found in the galactic center. The awesome gravitational forces of Sgr A* have the capacity to tear apart pairs of stars known as stellar binaries—snagging one into a tight orbit and flinging the other away at high speed.

Past studies tracing the routes of these hypervelocity stars in the galactic halo have indicated that they left the galactic core some 50 million to 250 million years ago.

As Cao and his colleagues note, however, there is something strange about the population of hypervelocity stars; firstly, their speeds max out at around 435 miles per second, even though our calculations suggest this should be only their average velocity.

Secondly, none of the stars that remain in orbit around Sgr A* appear to be the left-behind, erstwhile companions of these hypervelocity stars.

According to the researchers, both of these oddities could be explained, however, if the Milky Way once had a second, intermediate-sized, black hole at its heart—one that has long since been subsumed into Sgr A*.

The supermassive black hole could have gained such a partner, the team explained, during one of the occasions that the Milky Way previously collided with, captured and incorporated a dwarf galaxy.

Orbiting together with Sgr A* in their own binary pair, the smaller black hole would have acted rather like a gravitational goalkeeper, kicking away any stars approaching the galactic center.

In this way, the presence of the intermediate-mass black hole would have prevented binary systems from being disrupted very close to Sgr A*, where they would have received the largest gravitational boost and, as a result, the highest velocities.

Once this influence was factored into their models, the team reported, the resulting predicted velocity and radius distributions of hypervelocity stars ending up in the Milky Way's halo matches what is actually seen—with only 2.5 percent of hypervelocity stars having velocities greater than 435 miles per second.

According to the researchers, the most likely source for the donation of an intermediate black hole to the Milky Way's core in the right time frame would be the so-called Sausage Galaxy.

This dwarf galaxy merged with the Milky Way around 10 billion years ago, added at least eight globular clusters and some 50 billion solar masses' worth of stars, gas and dark matter.

It is estimated that the Sausage Galaxy's central black hole was some 300 times less massive than Sgr A*—with the two merging around 10 million years ago.

Even though Sgr A*'s long-lost goalkeeping companion may be long gone, other signs of its presence may still be felt today, the team explained. Specifically, the recoil resulting from the merger of the two black holes 10 million years ago may explain aspects of Sgr A*'s motion seen in the present day.

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Reference

Cao, C., Liu, F. K., Li, S., Chen, X., & Wang, K. (2025). A Recent Supermassive Black Hole Binary in the Galactic Center Unveiled by the Hypervelocity Stars. The Astrophysical Journal Letters, 982(2). https://doi.org/10.3847/2041-8213/adbbf2