A dwarf galaxy that appears to harbor a supermassive black hole far bigger than expected may have had most of its stars stolen from it by the Milky Way, scientists have proposed.
In a study published in the journal The Astrophysical Journal Letters, a team of researchers put forward the idea as a possible explanation for why the black hole thought to lie at the center of the dwarf galaxy is as large as it is.
The dwarf galaxy in question, known as Leo I, is located around 830,000 light-years away from us. It is thought to be a satellite of the Milky Way, one that orbits our galaxy in an elliptical trajectory. In fact, scientists consider Leo I to be the most distant satellite galaxy of the Milky Way.
A recent study found evidence that Leo I contains a supermassive black hole at its center. Almost all large galaxies are thought to harbor a supermassive black at their heart, but finding one in a dwarf galaxy is unusual.
Intriguingly, Leo I appears to contain a black hole with a mass around 3 million times that of our sun. This is roughly equivalent to the supermassive black hole residing at the center of our own galaxy.
"It resembles in mass and activity the Milky Way's black hole, Sagittarius A," study co-author Avi Loeb told Newsweek.
These characteristics are surprising given that Leo I is over 10,000 times less massive than our own galaxy. In fact, Leo I's black hole is 100-1,000 times more massive than expected for a galaxy of its size.
"This was fascinating because such a massive black hole in a small galaxy challenges our understanding of the relationship between galaxies and their central black holes," Fabio Pacucci, a researcher with the Harvard–Smithsonian Center for Astrophysics, who led the latest study, told Newsweek. "Usually, smaller galaxies host smaller central black holes."
"Typically, we'd expect a black hole in a dwarf galaxy to be a few thousand times the mass of the sun, not millions. If this is confirmed, it would be like finding a whale in a pond. Uncovering a black hole this large in a dwarf galaxy could revolutionize our understanding of how galaxies and black holes co-evolve in time during the evolution of the universe," Pacucci said.
In order to explain this discrepancy, the authors of the latest study have proposed that Leo I may once have been much larger but then had most of its stars stripped away by the Milky Way during close encounters with our own galaxy.
"The satellite galaxy Leo I had at least one close-by passage with the Milky Way galaxy more than one billion years ago. During this passage, gravitational forces from the Milky Way could have led to a significant loss of stars in Leo I, strongly affecting its current size," Pacucci said.
The researchers ran mathematical models and computer simulations which showed that two close passages with the Milky Way could have stripped up to around 90 percent of the stellar mass of Leo I while reproducing its currently observed position and velocity.
"This stellar mass loss is sufficiently large to possibly explain the origin of this overmassive black hole, although some problems remain," Pacucci said.
First, for the stripping to be so effective, Leo I must have passed very close to the Milky Way not once, but two times—at the limit of what the available evidence suggests is possible—according to the researchers.
Second, the abundance of elements heavier than helium observed in Leo I would be expected to be higher if the dwarf galaxy was much more massive billions of years ago.
"However, our study shows that this mechanism is, at least, viable to explain the formation of such an extremely overmassive black hole in a dwarf galaxy," Pacucci said.
Future research will be required to shed light on the mystery of Leo I's oversized black hole. If the presence of the supermassive black hole at the center of Leo I is confirmed, and the team's hypothesis holds up, this would suggest that the dwarf galaxy is halfway through the process of becoming a wandering black hole, according to the authors. These objects are black holes that float around space freely, untethered to any other celestial body.
If further research does not support the team's hypothesis, alternative explanations must be sought for the formation of the supermassive black hole, the scientists said.
"Either way, if confirmed, the supermassive black hole in Leo I would be an exceptional laboratory for studying the intimate connection between black holes and their host galaxies," the authors wrote in the study.
Update 11/22/23, 9:20 a.m. ET: This story was updated with comment from Fabio Pacucci and Avi Loeb.
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