Leo I is one of a group of small galaxies that orbit the Milky Way – but even among many, it is far from common. It appears that at the center of this small galaxy is a supermassive black hole comparable in mass to the Milky Way’s own supermassive black hole, Sagittarius A*.

Sagittarius A* is estimated to be 4.15 million times the mass of our Sun. Certainly a huge object, but tiny compared to the mass of the Milky Way, which is roughly one trillion solar masses. Leo I is a dwarf spheroidal galaxy much smaller than the Milky Way, and yet its supermassive black hole, as reported in The Astrophysical Journal, appears to weigh roughly 3 million times our Sun.

“There is no explanation for this kind of black hole in dwarf spheroidal galaxies,” lead author Dr María José Bustamante said in a statement.

The team did not set out to measure a supermassive black hole, and not one that could potentially be incredibly heavy with respect to its host galaxy mass. Leo I has intrigued astronomers because it seems to possess little dark matter, the hypothetical invisible substance that keeps galaxies in good shape. The team conducted detailed observations of how the stars in this little galaxy move and tried to model the dark matter distribution – but the model told a very different story.

“The models are screaming that you need a black hole at the center; you don't really need a lot of dark matter,” co-author Karl Gebhardt, from the University of Texas at Austin explained. “You have a very small galaxy that is falling into the Milky Way, and its black hole is about as massive as the Milky Way's. The mass ratio is absolutely huge. The Milky Way is dominant; the Leo I black hole is almost comparable.”

While more observation will be needed to better understand this galaxy, the team sees this finding as an explanation for the growth of supermassive black holes in massive galaxies. Galaxies grow and evolve through mergers, and so do their supermassive black holes. The presence of oversized supermassive black holes in small galaxies could explain the enormous size of supermassive black holes in galaxies larger and heavier than the Milky Way.

“If the mass of Leo I's black hole is high, that may explain how black holes grow in massive galaxies," Gebhardt said. "That's because over time, as small galaxies like Leo I fall into larger galaxies, the smaller galaxy's black hole merges with that of the larger galaxy, increasing its mass.”

The team hopes to conduct observations of many other small galaxies around the Milky Way, studying their dark matter profile and supermassive black holes.