Astrophysicists are searching for the second closest supermassive black hole

The Milky Way’s ultrafaint companion galaxy Leo I appears as a faint spot to the right of bright star Regulus. Credit: Scott Anttila Anttler

Two astrophysicists at the Harvard-Smithsonian Center for Astrophysics have proposed a way to observe the second-closest supermassive black hole to Earth: a giant 3 million times the mass of the Sun, hosted by the dwarf galaxy Leo I.

Dubbed Leo I*, the supermassive black hole was first proposed by an independent team of astronomers in late 2021. The team noticed that stars accelerated as they approached the galaxy’s center – evidence of a black hole – but directly imaging the emissions from the black hole was not possible.

Now CfA astrophysicists Fabio Pacucci and Avi Loeb propose a new way to verify the existence of the supermassive black hole; their work is detailed in a study published today in the Astrophysical Journal Letters.

“Black holes are very elusive objects, and sometimes they enjoy playing hide and seek with us,” says Fabio Pacucci, lead author of the ApJ letters to learn. “Rays of light can’t escape their event horizons, but the environment around them can be extremely bright – if enough material falls into their gravity well. But when a black hole doesn’t accumulate mass, instead it doesn’t emit light and becomes impossible to find with our telescopes.”

That’s the challenge with Leo I – a dwarf galaxy that has so little gas available for accretion that it’s often referred to as “fossil”. So should we give up hope of watching it? Maybe not, the astronomers say.

“In our study, we suggested that a small amount of mass lost from stars migrating around the black hole could provide the rate of accretion needed to observe it,” explains Pacucci. “Old stars become very large and red – we call them red giant stars. Red giants typically have strong winds that carry a fraction of their mass around. Space around Leo I* appears to contain enough of these old stars to make it observable.”

“The observation of Leo I* could be groundbreaking,” says Avi Loeb, co-author of the study. “It would be the second closest supermassive black hole after that at the center of our galaxy, with a very similar mass but hosted by a galaxy 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 a plus-size baby come to be born to a skinny parent?

Decades of studies show that most massive galaxies harbor a supermassive black hole at their center, and the black hole’s mass is one-tenth of a percent of the total mass of the surrounding stellar sphere.

“In the case of Leo I,” Loeb continues, “we would expect a much smaller black hole. Instead, Leo I appears to contain a black hole a few million solar masses, similar to that in the Milky Way. This is exciting because science usually makes the most progress when the unexpected happens.”

So when can we expect a picture of the black hole?

“We’re not there yet,” says Pacucci.

The team has obtained telescope time at the Chandra Spaceborne X-ray Observatory and the Very Large Array Radio Telescope in New Mexico and is currently analyzing the new data.

Pacucci says, “Leo I* is playing hide and seek, but it puts out too much radiation to go undetected for long.”

More information:
Accretion of Winds from RGB Stars May Reveal Supermassive Black Hole in Leo I, The Letters of the Astrophysical Journal (2022). DOI: 10.3847/2041-8213/ac9b21

Provided by the Harvard-Smithsonian Center for Astrophysics

Citation: Astrophysicists hunt for second-closest supermassive black hole (2022, November 28), retrieved November 28, 2022 from .html

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