The second-nearest supermassive black hole is playing a game of hide and seek with the researchers who expect to discover more about it soon
Two astrophysicists at the Center for Astrophysics | Harvard & Smithsonian have suggested a way to observe what could be the second-closest supermassive black hole to Earth: a behemoth three million times the mass of the sun, hosted by the dwarf galaxy Leo I.
The supermassive black hole, labelled Leo I*, was first proposed by an independent team of astronomers in late 2021. The team noticed stars picking up speed as they approached the centre of the galaxy – evidence for a black hole – but directly imaging emission from the black hole was not possible.
Now, CfA astrophysicists Fabio Pacucci and Avi Loeb suggest a new way to verify the supermassive black hole’s existence.
Fabio Pacucci, lead author of the 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.”
See also: Death of a star reveals midsize black hole lurking in a dwarf galaxy
groundbreaking
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, the astronomers say.
Pacucci explained: “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 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.”
Co-author Avi Loeb added: “Observing Leo I* could be groundbreaking. It would be the second-closest supermassive black hole after the one at the centre 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?”
smaller black hole
Decades of studies show that most massive galaxies host a supermassive black hole at their centre, and the mass of the black hole is a tenth of 1% of the total mass of the spheroid of stars surrounding it.
Loeb continued: “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.”
So, when can we expect an image of the black hole?
“We are not there yet,” Pacucci said.
The team has obtained telescope time on the space-borne Chandra X-ray Observatory and the Very Large Array radio telescope in New Mexico and is currently analysing the new data.
Pacucci concluded: “Leo I* is playing hide-and-seek, but it emits too much radiation to remain undetected for long.”
The research is published in The Astrophysical Journal Letters.
Image: The ultra-faint Milky Way companion galaxy Leo I appears as a faint patch to the right of the bright star, Regulus. © Scott Anttila Anttler.
