Two astrophysicists at the Harvard-Smithsonian Center for Astrophysics have proposed a method for observing what could be the second-closest supermassive black hole to Earth: a behemoth 3 million times the mass of the Sun hosted by the dwarf galaxy Leo I.
Late in 2021, an independent group of astronomers proposed the supermassive black hole designated Leo I*. The team observed stars gaining speed as they approached the center of the galaxy, which is evidence of a black hole, but it was not possible to directly image the black hole’s emission. Black holes are extremely elusive objects that occasionally like to play hide and seek.
“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,”
said lead author Fabio Pacucci.
Red Giants Around Leo I*
This is the difficulty with Leo I, a dwarf galaxy so devoid of gas that it is frequently referred to as a “fossil.” So, should we give up on trying to detect it? The astronomers say maybe not.
In their research, they propose that the accretion rate required to observe the black hole could be provided by a small amount of mass lost from stars circling 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,”
Second-closest Supermassive Black Hole
Observing Leo I* may prove groundbreaking if it could be done.
“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?”
said Avi Loeb, the other co-author of the study.
Are We There Yet?
Decades of research indicate that the majority of massive galaxies harbor a supermassive black hole at their center, and that the black hole’s mass is a tenth of a percent of the total mass of the spheroid of stars surrounding it.
“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, according to Pacucci. Leo I* is trying to hide, but it produces too much radiation to do so for very long.
The team has acquired time on the Chandra X-ray Observatory in space and the Very Large Array radio telescope in New Mexico and is currently analyzing the new data.
Reference: Fabio Pacucci, Avi Loeb. Accretion from Winds of RGB Stars May Reveal the Supermassive Black Hole in Leo I. The Astrophysical Journal Letters, 940 L33
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