An Explanation for the Overmassive Black Hole in the Dwarf Galaxy Leo I

Leo I is a distant dwarf galaxy that appears to host a supermassive black hole 100 times more massive than expected for a galaxy of its size. Could tidal stripping explain this galaxy’s black hole–stellar mass mismatch?

An Anomalously Large Black Hole

dwarf galaxy Leo I

The dwarf galaxy Leo I. [Sloan Digital Sky Survey; CC BY 4.0]

The dwarf galaxy Leo I is the most distant satellite of the Milky Way. At 830,000 light-years away, Leo I is so distant that its status as a satellite of our galaxy has been debated, with early studies suggesting it was bound to our neighboring galaxy, Andromeda, instead. Now, Leo I is thought to trace a highly elliptical orbit around the Milky Way that brings it to within about 150,000 light-years of our galaxy’s center.

Leo I is remarkable not just because it’s the Milky Way’s most distant satellite, but also because this 80-million-solar-mass galaxy appears to harbor a 3-million-solar-mass black hole — a black hole nearly as massive as the Milky Way’s central supermassive black hole but in a galaxy more than 10,000 times less massive. To explain this discrepancy, researchers have proposed that Leo I was once a much larger galaxy that has had its stars stolen by passing too close to the Milky Way.

Simulating Stellar Stripping

Fabio Pacucci (Center for Astrophysics | Harvard & Smithsonian; Black Hole Initiative) and collaborators used analytical and dynamical models to explore how Leo I’s past journeys close to the Milky Way might have robbed the smaller galaxy of its stars. Using a simple analytical model, the team estimated that Leo I could have lost 32–57% of its stars after one close passage, though losses up to 78% are possible based on precise observations by the star-mapping Gaia spacecraft.

plot showing the simulated stellar mass as a function of time

Stellar mass (top panel) and distance between Leo I and the Milky Way (bottom panel) over time. [Pacucci et al. 2023]

More detailed N-body simulations backed up this result. The team found that a billion-solar-mass galaxy (an estimate of Leo I’s original mass based on the size of its supermassive black hole) shrinks to roughly Leo I’s current mass over the course of 8 billion years and two close encounters with the Milky Way. Intriguingly, this scenario predicts the formation of two tidal tails — expansive stellar streams created as a galaxy loses its stars — but suggests that these streams would fall along our line of sight and thus be challenging to detect.

Possible, but Probable?

Pacucci’s team notes that this scenario, though possible, may be unlikely for two reasons. First, it requires Leo I to pass very close to the Milky Way on two occasions, at the limit of what is permitted by Gaia observations. Second, Leo I’s metal content — the abundance of elements heavier than helium — matches what we’d expect for a galaxy of its size; a galaxy that used to be more massive should be more metal rich.

plot of model results showing the current location of Leo I's stars

Modeled locations of Leo I’s stars today (green circles) and black hole over the past 8 billion years (purple line). You can see an animation of this figure here. [Adapted from Pacucci et al. 2023]

While these issues may confound the proposed tidal-stripping scenario, they aren’t deal-breakers: though unlikely, multiple close passages aren’t prohibited, and closer analysis of Leo I’s star formation history might explain its meager metal content.

In-depth statistical modeling delving into the likelihood of this scenario awaits future work, and observations may yet dredge up evidence of Leo I’s stellar streams. If future work supports the team’s hypothesis, another possibility waits in the wings: that Leo I will someday relinquish all of its stars to the Milky Way and become a wandering black hole.


“Extreme Tidal Stripping May Explain the Overmassive Black Hole in Leo I: A Proof of Concept,” Fabio Pacucci et al 2023 ApJL 956 L37. doi:10.3847/2041-8213/acff5e