Leo I is one of the Milky Way’s dwarf satellite galaxies. Recently, astronomers at The University of Texas at Austin’s McDonald Observatory have discovered an unusually massive black hole at the center of these satellite galaxies. The black hole is as massive as the black hole in our galaxy.
Leo I is peculiar: Unlike other dwarf galaxies, it does not contain much dark matter. Scientists measured Leo I’s dark matter profile, i.e., how the density of dark matter changes from the galaxy’s outer edges into its center.
They gauged its gravitational pull on the stars. The faster the stars move, the more matter there is enclosed in their orbits. Scientists, specifically, wanted to determine:
1. Whether dark matter density increases toward the galaxy’s center.
2. Whether their profile measurement would match previous ones made using older telescope data combined with computer models.
Scientists used the observations from a unique instrument called VIRUS-W on McDonald Observatory’s 2.7-meter Harlan J. Smith Telescope.
UT astronomer Karl Gebhardt said, “The models are screaming that you need a black hole at the center; you don’t need a lot of dark matter. You have a tiny galaxy falling into the Milky Way, and its black hole is about as massive as the Milky Way’s. The mass ratio is huge. The Milky Way is dominant; the Leo I black hole is almost comparable.”
Scientists noted, “The result was different from the past studies of Leo I due to a combination of better data and the supercomputer simulations. The central, dense region of the galaxy was mostly unexplored in previous studies, which concentrated on the velocities of individual stars.”
“The current study showed that for those few velocities that were taken in the past, there was a bias toward low velocities. This, in turn, decreased the inferred amount of matter enclosed within their orbits.”
The data used in the study- focused on the central region and is unaffected by this bias. The amount of inferred matter enclosed within the stars’ orbits skyrocketed.
Gebhardt 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. 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.”
- M. J. Bustamante-Rosell et al. Dynamical Analysis of the Dark Matter and Central Black Hole Mass in the Dwarf Spheroidal Leo I. DOI: 10.3847/1538-4357/ac0c79