Could there have been two massive black holes in our galaxy’s center at one time? New research suggests that this scenario could explain oddities in the population of hypervelocity stars in the Milky Way’s halo.
From the Center to the Halo
A near-infrared image from the Very Large Telescope of stars at the center of the Milky Way. [ESO/S. Gillessen et al.; CC BY 4.0]
Surveys of the Milky Way halo have indeed found massive hypervelocity stars that hastily departed the galactic center some 50–250 million years ago, cementing the idea that interactions between stellar binaries and Sgr A* can send stars careening through the galaxy.
There are two odd things about the observed population of hypervelocity stars, though: 1) their velocities top out around 700 km/s, though theory suggests roughly half of these stars should move faster than this cutoff; and 2) none of the stars remaining in orbit around Sgr A* appear to be the left-behind binary companions of these hypervelocity stars.
A Long Time Ago in a Galaxy Very, Very Close By
A second black hole in the Milky Way’s center, present long ago, may explain these oddities. In a research article published this week, Chunyang Cao (Peking University) and collaborators outlined how the existing population of hypervelocity stars could be produced with the help of an intermediate-mass black hole that entered our galaxy billions of years ago and has since merged with Sgr A*.
Cao’s team outlined how Sgr A* could gain an intermediate-mass black hole companion when our galaxy captures and incorporates a dwarf galaxy — something that is thought to have happened multiple times in the Milky Way’s history. Orbiting with Sgr A* in a binary system, the intermediate-mass black hole acts as a goalkeeper, gravitationally kicking away stars that approach the galactic center. The presence of the intermediate-mass black hole prevents most binary systems from being disrupted very close to Sgr A*, where they would score the largest gravitational boost and achieve the highest velocities.
Predicted (green and blue shaded areas) and observed (gray and black lines) velocity and galactocentric radius distributions of hypervelocity stars in the Milky Way. The two observation lines show the results from the Multiple Mirror Telescope (MMT) hypervelocity star survey and the MMT survey plus proper-motion corrections from the Gaia spacecraft. The pink and purple lines show the predictions of models that do not incorporate an intermediate-mass black hole. Click to enlarge. [Cao et al. 2025]
A Model Population
Cao’s team modeled the population of hypervelocity stars that would result from this scenario and found that the velocity and radius distributions of their model population closely matched what is observed. Crucially, the model predicts that just 2.5% of hypervelocity stars should have velocities greater than 700 km/s and can accurately reproduce the population of stars that remain in the galactic center. The model also predicts that 10% of ejected stars should be intact binary systems rather than lone binary members, which is consistent with observations.
The authors posited that the most likely source of an intermediate-mass black hole in the right time frame for this scenario to play out would be the merger with the Gaia–Sausage–Enceladus dwarf galaxy about 10 billion years ago. Modeling suggests that this dwarf galaxy’s black hole was roughly 300 times less massive than the Milky Way’s central black hole, and the two black holes likely merged 10 million years ago.
Though the intermediate-mass black hole may be long gone, its influence may still be felt: the merger of the two black holes would have caused Sgr A* to recoil, potentially accounting for the motion of Sgr A* seen today.
Citation
“A Recent Supermassive Black Hole Binary in the Galactic Center Unveiled by Hypervelocity Stars,” Chunyang Cao et al 2025 ApJL 982 L37. doi:10.3847/2041-8213/adbbf2