Hunting for the second closest supermassive black hole to Earth

The supermassive black hole closest to Earth is right in the center of our galaxy, the Milky Way. It is called Sagittarius A*, it is about 26,000 light years from us and has a mass equivalent to that of four million suns.

But it is not the only ‘monster’ of this type that we have around. Astronomers, in fact, know that a small satellite galaxy of ours, called Leo I and situated ‘only’ 820,000 light years awayhosts a black hole of similar size, one that we have not yet been able to observe.

‘Seeing’ a black hole, however large it may be, is not an easy task. Suffice it to remember that the one we have closest at hand, that of our own galaxy, could only be photographed for the first time this year. And that apart from ‘ours’ we only have images of another supermassive black hole much further away (M87*, 54 million light years away) and much larger (about 6,000 million solar masses), photographed in 2019. And there it is end the list.

three million suns

But let’s see. The existence of Leo I* was first suggested in 2021, when a team of astronomers noticed that the stars’ orbits were accelerating as they approached the center of the dwarf galaxy, a telltale sign of Leo’s own enormous gravitational pull. a big black hole.

And it was thus, by calculating the acceleration of the stars as they were attracted by the gravitational field of the black hole, that the researchers deduced that Leo I* has approximately three million times the mass of our Sun. In other words, it is barely a 25% smaller than Sagittarius A*. However, despite the fact that gravitational effects indirectly revealed the presence of the black hole, seeing it is an entirely different challenge. Now, a team of researchers from Harvard Smithsonian Center for Astrophysics just proposed a method to achieve it. The work is published in ‘Astrophysical Journal Letters’.

“Black holes,” he explains. Fabio Pacucci, who with Harvard astrophysicist Avi Loeb is the lead author of the paper – are very elusive objects and sometimes enjoy playing hide-and-seek with us. Light rays 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 does not accumulate mass, it does not emit light and becomes impossible to find with our telescopes.”

an inactive black hole

That is precisely the case with Leo I*. The dwarf galaxy that hosts it does not have enough gas to power it, so it remains inactive and, in fact, invisible. Let’s remember that the photographs we have of M87* and Sagittarius A* show, in both cases, bright orange rings with their respective black holes inside. Those rings are made of matter that spins very rapidly around the two black holes before being swallowed up, emitting brilliant radiation that can be picked up by telescopes. But if there is no matter to gobble up, there is no bright ring around the black hole, and it remains entirely invisible.

According to Pacucci and his colleagues, however, there might be a way to get a good look at Leo I*. “In our study -explains the scientist- we suggest that a small amount of mass lost by the stars that wander near the black hole could provide the necessary accretion rate to observe it. Old stars become very big and red, we call them red giant stars. Red giants often have strong winds that carry a fraction of their mass out into the environment. The space around Leo I* appears to contain enough of these ancient stars for it to be observable.”

“Observing Leo I* could be innovative,” he says for his part Avi Loeb-. It would be the second closest supermassive black hole after the one at the center of our galaxy, with a very similar mass but housed in a galaxy a thousand times smaller than the Milky Way. This fact challenges everything we know about how galaxies and their central supermassive black holes coevolve. How did such a big baby end up being born to such a skinny father?

a bewildering size

Decades of studies show that most massive galaxies harbor a supermassive black hole at their center, with the mass of the black hole typically no more than one tenth of the total mass of the surrounding spheroid of stars.

«In the case of Leo I -continues Loeb- we would expect a much smaller black hole. Instead, this small galaxy appears to contain a black hole several million times the mass of the Sun, similar to the one that hosts the Milky Way. This is exciting because science often advances further when the unexpected happens.”

For when then an image of Leo I*? According to Pacucci “we haven’t got there yet.” But the team has gotten airtime from the Chandra X-ray Space Observatory and the radio telescope Very Large Array, in New Mexico, and is currently analyzing the data obtained. “Leo I* is playing hide-and-seek,” says Pacucci, “but he emits too much radiation to go unnoticed for much longer.”


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