Dwarf Galaxy Leo I Hosts Strangely Massive Black Hole, Astronomers Say

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A black hole in the center of a dwarf spheroidal galaxy called Leo I is almost as massive as the central black hole of our own Milky Way Galaxy.

The dwarf spheroidal galaxy Leo I is 30 times smaller than the Milky Way Galaxy. Image credit: ESA / Gaia / DPAC / SDSS.

Leo I is a dwarf spheroidal galaxy located about 820,000 light-years away in the constellation of Leo.

Also known as DDO 74, LEDA 29488 and UGC 5470, the galaxy is a member of the Local Group.

Leo I is thought to be one of the most distant satellites of our Milky Way Galaxy.

University of Texas at Austin astronomer María José Bustamante-Rosell and her colleagues decided to study this galaxy because of its peculiarity.

“Unlike most dwarf galaxies orbiting the Milky Way, Leo I does not contain much dark matter,” they said.

“We measured Leo I’s dark matter profile — that is, how the density of dark matter changes from the outer edges of the galaxy all the way into its center.”

“They did this by measuring its gravitational pull on the stars: the faster the stars are moving, the more matter there is enclosed in their orbits.”

“In particular, we wanted to know whether dark matter density increases toward the galaxy’s center.”

“We also wanted to know whether their profile measurement would match previous ones made using older telescope data combined with computer models.”

For their observations, the astronomers used the VIRUS-W instrument on the 2.7-m Harlan J. Smith Telescope at McDonald Observatory.

When they fed their improved data and sophisticated models into a supercomputer, they got a startling result.

“The models are screaming that you need a black hole at the center; you don’t really need a lot of dark matter,” said Dr. Karl Gebhardt, an astronomer at the University of Texas at Austin.

“You have a very small galaxy that is falling into the Milky Way, and its black hole is about as massive as the Milky Way’s.”

“The mass ratio is absolutely huge. The Milky Way is dominant; the Leo I black hole is almost comparable. The result is unprecedented.”

“The finding could shake up astronomers’ understanding of galaxy evolution, as there is no explanation for this kind of black hole in dwarf spheroidal galaxies,” Bustamante-Rosell said.

“The result is all the more important as astronomers have used galaxies such as Leo I, called dwarf spheroidal galaxies, for 20 years to understand how dark matter is distributed within galaxies,” Dr. Gebhardt said.

“This new type of black hole merger also gives gravitational wave observatories a new signal to search for.”

“If the mass of Leo I’s black hole is high, that may explain how black holes grow in massive galaxies. That’s because over time, as small galaxies like Leo I fall into larger galaxies, the smaller galaxy’s black hole merges with that of the larger galaxy, increasing its mass.”

The team’s work was published in the Astrophysical Journal.


M.J. Bustamante-Rosell et al. 2021. Dynamical Analysis of the Dark Matter and Central Black Hole Mass in the Dwarf Spheroidal Leo I. ApJ 921, 107; doi: 10.3847/1538-4357/ac0c79


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