Researchers have discovered a black hole in a Milky Way satellite galaxy

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Leo I, a dwarf galaxy in the Milky Way's orbit, has a gigantic black hole in its core, found by astronomers at The University of Texas at Austin's McDonald Observatory. The Discovery might revolutionize our knowledge of how galaxies—the building blocks of the universe—evolve. The research was just published in The Astrophysical Journal.

To better understand Leo I, the team chose to focus on it. Even though it's a dwarf galaxy, Leo I doesn't have a large amount of dark matter.

Researchers looked at Leo I's dark matter profile, or how the density of dark matter varies as you go from the galaxy's outskirts to its core. They observed its gravitational influence on the stars: the quicker the stars move, the more stuff is trapped in their orbits. The researchers were particularly interested in determining whether the quantity of dark matter rises as one moves closer to the galactic core. They also wanted to see whether their profile measurement would match prior ones obtained using older telescope data mixed with computer models, as they had done before.

The team, which includes UT astronomers Eva Noyola, Karl Gebhardt, Greg Ziemann, and colleagues from Germany's Max Planck Institute for Extraterrestrial Physics, is led by recent UT Austin Ph.D. graduate Mara José Bustamante (MPE). They made their observations using the VIRUS-W instrument on McDonald Observatory's 2.7-meter Harlan J. Smith Telescope.

UT Austin's Texas Advanced Computing Center's supercomputer produced a shocking outcome when the researchers fed their updated data onto it.

Einstein's theory "screams" that you need a black hole at the core; you don't need much dark matter. "You have a little galaxy colliding with the Milky Way, and its black hole is about the same size as the Milky Way's. The mass-to-volume ratio is massive. In comparison to the Milky Way galaxy, the Leo I black hole is almost as powerful." The result is unexpected.

The results were different from previous Leo I experiments, according to the researchers, because of improved data and supercomputer simulations. The galaxy's core was largely ignored in prior studies, which focused on individual stars' velocities. For the few velocities measured in the past, there was a tendency to favor low velocities. As a result, the estimated mass of the objects in their orbits shrank.

Despite this bias, new data is focused on the core area and is unaffected by it. The quantity of inferred stuff included in the orbits of the stars increased dramatically.

The Discovery of a black hole in dwarf spheroidal galaxies "may change the way astronomers think about galaxy development," says Bustamante.

For the last 20 years, scientists have employed "dwarf spheroidal galaxies" like Leo I, known as "dwarf spheroidal galaxies," to study the distribution of dark matter in galaxies. For gravitational wave observatories, this form of black hole merging provides them with a new signal to hunt for.

Leo I's black hole may explain how giant galaxies develop black holes, Gebhardt added. This is because when smaller galaxies like Leo I combine with more giant galaxies, the smaller galaxy's black hole grows in mass.

VIRUS-W is the only instrument in the world that can do this sort of dark matter profile investigation. It was built by a team at MPE in Germany. Numerous dwarf galaxies in the southern hemisphere may be ideal targets, but no telescope in the southern hemisphere can see them. However, the Giant Magellan Telescope (GMT), which is now under construction in Chile, was built in part for this purpose.


M. J. Bustamante-Rosell1, Eva Noyola2, Karl Gebhardt2, Maximilian H. Fabricius3, Ximena Mazzalay3, Jens Thomas3, and Greg Zeimann2, Dynamical Analysis of the Dark Matter and Central Black Hole Mass in the Dwarf Spheroidal Leo I, The Astrophysical Journal (2021). DOI: 10.3847/1538-4357/ac0c79

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