The hunt is on in neighboring galaxy for 2nd closest monster black hole to Earth

pale blue bright light against starry background
An image of the faint Milky Way companion galaxy, Leo I, which may hide a monstrous supermassive black hole. (Image credit: Scott Anttila Anttler)

Astronomers may finally have a way to hunt for a monstrous supermassive black hole they suspect lurks in the dwarf galaxy next door.

The behemoth would be the second closest supermassive black hole to Earth, after Sagittarius A* (Sgr A*) at the heart of the Milky Way, in the companion galaxy Leo I. This neighboring supermassive black hole, named Leo I*, was first proposed to exist in 2021, when astronomers noticed stars accelerating as they approached the heart of the dwarf galaxy. While this is good evidence in favor of a supermassive black hole, astronomers frustratingly couldn't get a direct image of emissions from Leo I* to prove it exists. Now, two researchers have proposed a solution.

"Black holes are very elusive objects, and sometimes they enjoy playing hide-and-seek with us," Fabio Pacucci, an astrophysicist at the Harvard & Smithsonian Center for Astrophysics and lead author of the research, said in a statement. "Rays of light cannot escape their event horizons, but the environment around them can be extremely bright — if enough material falls into their gravitational well. But if a black hole is not accreting mass, instead, it emits no light and becomes impossible to find with our telescopes."

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This is the case with Leo I*: Its dwarf galaxy doesn't have enough gas to feed the supermassive black hole, leaving it inactive and in effect invisible. However, Pacucci and a colleague propose that the black hole could simply be enjoying an alternative diet — and perhaps eating enough for astronomers to confirm its existence.

"We suggested that a small amount of mass lost from stars wandering around the black hole could provide the accretion rate needed to observe it," Pacucci said. "Old stars become very big and red — we call them red giant stars. Red giants typically have strong winds that carry a fraction of their mass to the environment. The space around Leo I* seems to contain enough of these ancient stars to make it observable."

If the technique works, the observation of Leo I* could be groundbreaking, according to Avi Loeb, an astrophysicist also at the Harvard & Smithsonian Center for Astrophysics. 

In particular, a detection would resolve another astronomical mystery: whether dwarf galaxies possess supermassive black holes of these tremendous masses at all. (Scientists estimate that Leo I* might be on the order of 3 million times more massive than the sun; the Milky Way's black hole, Sgr A*, is only a bit larger, at 4 million times the mass of the sun.)

"It would be the second-closest supermassive black hole after the one at the center of our galaxy, with a very similar mass but hosted by a galaxy that is a thousand times less massive than the Milky Way," Loeb said in the statement. "This fact challenges everything we know about how galaxies and their central supermassive black holes co-evolve. How did such an oversized baby end up being born from a slim parent?"

In the case of the Milky Way and the supermassive black holes at the heart of most large galaxies, that central object contains about a 10th of the total mass of the sphere of stars that surround it. The existence of Leo I* in a dwarf galaxy would radically depart from this ratio. 

"In the case of Leo I, we would expect a much smaller black hole," Loeb said. "Instead, Leo I appears to contain a black hole a few million times the mass of the sun, similar to that hosted by the Milky Way. This is exciting because science usually advances the most when the unexpected happens."

Pacucci said that astronomers are still a long way from imaging Leo I*, but that he and his team have obtained time on the space-based Chandra X-ray Observatory and the Very Large Array (VLA) radio telescope in New Mexico in the hope of uncovering this theorized cosmic monster.

"Leo I* is playing hide-and-seek, but it emits too much radiation to remain undetected for long," Pacucci said.

The team's research is described in a paper published Monday (Nov. 28) in the Astrophysical Journal Letters.

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Robert Lea
Contributing Writer

Robert Lea is a science journalist in the U.K. whose articles have been published in Physics World, New Scientist, Astronomy Magazine, All About Space, Newsweek and ZME Science. He also writes about science communication for Elsevier and the European Journal of Physics. Rob holds a bachelor of science degree in physics and astronomy from the U.K.’s Open University. Follow him on Twitter @sciencef1rst.