Astrophysicists suggest a method for observing the second closest supermassive black hole to Earth

The nearest supermassive black hole (SMBH) to us is in the centre of our Milky Way galaxy. The second closest SMBH is in the neighbouring dwarf galaxy, Leo I.

Leo I is about 820,000 lightyears from Earth, or a little over 30 times further than the centre of our own galaxy.

Despite the fact that SMBHs are between 100,000 and ten billion times the mass of our Sun these behemoths aren’t easy to see. The one in the centre of our own galaxy was only imaged for the first time this year. The first image of a SMBH came when Messier 87* was photographed in 2019.

The supermassive black hole at the centre of Leo I, labelled “Leo I*”, was first suggested to exist in 2021. Independent astronomers noticed stars’ orbits speeding up as they approached the centre of the dwarf galaxy – a tell-tale sign of a massive gravitational pull. The most likely explanation is a SMBH.

By calculating the acceleration of the stars as they are pulled into the SMBH’s gravitational field, the researchers deduced that the black hole is roughly three million times the mass of our Sun. For comparison, this is only slightly smaller than the SMBH in the centre of our galaxy, Sagittarius A*, which is four million times more massive than our Sun.

But seeing the black hole, not just its gravitational effects, is a whole different thing and has thus far been impossible.

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A new method devised by scientists at the Center for Astrophysics at the Harvard Smithsonian aims to overcome this problem is described in the Astrophysical Journal Letters.

“Black holes are very elusive objects, and sometimes they enjoy playing hide-and-seek with us,” says lead author Dr Fabio Pacucci. “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 why Leo I* is so hard to see. Its host dwarf galaxy is lacking in gas and other matter, so there is virtually nothing for the black hole to accrete. The galaxy is described as a “fossil.” But the astronomers suggest that all hope is not lost in seeing the SMBH.

“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,” Pacucci explains. “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.”

“Observing Leo I* could be groundbreaking,” says co-author Professor Avi Loeb. “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 smaller 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?”

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It has been well established over recent decades that most massive galaxies host a supermassive black hole at their centre. But, usually, the mass of the black hole is 0.1% the total mass of the stars that encircle it.

“In the case of Leo I, we would expect a much smaller black hole,” Loeb adds. “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.”

But the scientists say we’re not quite ready to get an image of Leo I*. The team is currently analysing new data from the Chandra X-ray Observatory space telescope and the Very Large Array radio telescope in New Mexico.

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

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