|
| The ultra-faint Milky Way companion galaxy Leo I appears as a faint patch to the right of the bright star, Regulus. Credit: Scott Anttila Anttler |
Two astrophysicists at the Harvard-Smithsonian Center for Astrophysics have
suggested a way to observe 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.
The supermassive black hole, labeled 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 center 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; their work is described 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
study. "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 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.
"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 Avi Loeb, the co-author of
the study. "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?"
Decades of studies show that most massive galaxies host a supermassive black
hole at their center, and the mass of the black hole is a tenth of a percent
of the total mass of the spheroid of stars surrounding it.
"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
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 says.
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 analyzing the new data.
Pacucci says, "Leo I* is playing hide-and-seek, but it emits too much
radiation to remain undetected for long."
Reference:
Accretion from Winds of RGB Stars May Reveal the Supermassive Black Hole in
Leo I, The Astrophysical Journal Letters (2022).
DOI: 10.3847/2041-8213/ac9b21
Tags:
Space & Astrophysics