Astrophysicists looking for the second-closest supermassive black hole

Verifying the existence of the supermassive black hole.

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Decades of studies show that most massive galaxies host a supermassive black hole at their center, and the mass of the black hole is a tenth of a percent of the total mass of the spheroid of stars surrounding it.

A method to observe what may be the second-closest supermassive black hole to Earth has been proposed by two astrophysicists at the Center for Astrophysics | Harvard & Smithsonian. The supermassive black hole, which is housed by the dwarf galaxy Leo I, has a mass that is three million times that of the Sun.

The supermassive black hole Leo I* was first proposed by an independent team of astronomers in late 2021. The astronomers noticed stars picking up speed as they approached the galaxy’s center, but directly imaging emission from the black hole was impossible.

Now, CfA physicists Fabio Pacucci and Avi Loeb propose a new method to confirm the presence of the supermassive black hole.

Fabio Pacucci, a lead author of the ApJ Letters study, said, “Black holes are very elusive objects, and sometimes they enjoy playing hide-and-seek with us. 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.”

“This is the challenge with Leo I — a dwarf galaxy so devoid of gas available to accrete that it is often described as a “fossil.” So, shall we relinquish any hope of observing it? Perhaps not.”

“In our study, 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. Old stars become very big and red — we call 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.”

Avi Loeb, the study’s co-author, said, “Observing Leo I* could be groundbreaking. 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. 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 Leo I, we would expect a much smaller black hole. 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“So, when can we expect an image of the black hole?”

“We are not there yet.”

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

The study is published today in the Astrophysical Journal Letters.