Since the 1970s, scientists have known that within the cores of most massive galaxies in the Universe, there beats the heart of a Supermassive Black Hole (SMBH). The presence of these giant black holes causes these galaxies to be particularly energetic, to the point where their central regions outshine all the stars in their disks combined – aka. Active Galactic Nuclei (AGN). The Milky Way galaxy has its own SMBH, known as Sagitarrius A*, which has a mass of over 4 million Suns.
For decades, scientists have studied these objects in the hopes of learning more about their role in galactic formation and evolution. However, current research has shown that SMBHs may not be restricted to massive galaxies. In fact, a team of astronomers from the University of Texas at Austin’s McDonald Observatory recently discovered a massive black hole at the heart of a dwarf galaxy that orbits the Milky Way (Leo I). This finding could redefine our understanding of how black holes and galaxies evolve together.
The research team was led by Dr. María José Bustamante-Rosell, a physicist at UT Austin, who was joined by UT astronomers Eva Noyola, Karl Gebhardt, and Greg Zeimann, as well as colleagues from the Max Planck Institute for Extraterrestrial Physics (MPE) in Garching, Germany. The study that describes their findings appeared in a recent issue of The Astrophysical Journal.
Artist’s impression of the path of the star S2 as it passes very close to the supermassive black hole at the centre of the Milky Way. Credit: ESO/M. Kornmesser
For their study, Bustamante-Rosell and her colleagues decided to study Leo I because it does not appear to contain much dark matter, compared to other dwarf galaxies that orbit the Milky Way. Like other galaxies, Leo I has a “dark matter profile,” which describes how the density of dark matter changes from the outer edges of the galaxy into its center. To determine changes in density, astronomers measure how fast a galaxy’s stars are moving, with faster speeds indicating that more matter is enclosed in their orbits.
In particular, the team wanted to know whether dark matter density increases toward the galaxy’s center. In addition, they wanted to know if their profile measurement would match previous data, along with sophisticated computer models. To do this, they relied on data obtained by the Visible Integral-field Replicable Unit Spectrograph-W (VIRUS-W) on the McDonald Observatory’s 2.7-meter (8.85-foot) Harlan J. Smith Telescope.
The team then fed their results and models into a supercomputer at UT Austin’s Texas Advanced Computing Center, and received some unprecedented results. Basically, their results implied that Leo I has considerably more mass at its center than in its disk, which could only be described by the presence of a black hole almost as massive as Sagittarius A*. As UT Astronomy Prof. Karl Gebhardt explained in a recent UT News release:
“The models are screaming that you need a black hole at the center; you don’t really need a lot of dark matter. 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 Fornax Dwarf galaxy, one of the earliest known dwarf spheroidal galaxies. Credit: ESO/DSS2
This difference in results to previous observations of Leo I is apparently due to a combination of better data and supercomputer simulations. Another major difference is that the central region of the dwarf galaxy was not explored by previous studies, which were mainly concerned with the velocities of individual stars. What’s more, Bustamante-Rosell and her team found that there was a bias in previous studies towards low velocities that decreased the inferred amount of matter enclosed within their orbits.
The data in this present study is concentrated in the central region, which is unaffected by this bias and therefore increased estimates on the matter enclosed in their orbits considerably. The finding could drastically shake up astronomers’ understanding of galactic evolution, as well as how SMBHs form and grow over time. Said Gebhardt:
“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 result is made all the more significant as astronomers have studied “dwarf spheroidal galaxies” like Leo I for 20 years to understand how dark matter is distributed within galaxies. In addition, SMBHs and all particularly-massive black holes are known to be the result of mergers, so the existence of this new type of black hole will give gravitational wave observatories a new signal to search for.
The study also verifies the effectiveness of the VIRUS-W spectrograph, which is the only instrument in the world capable of conducting this type of dark matter profile study. In the coming years, the Giant Magellan Telescope (GMT) – of which UT Austin is a founding partner – and other next-generation telescopes will have comparable spectrographs. Along with space telescopes like the James Webb (JWST) and Nancy Grace Roman (RST), these observatories will shine new light on the “Dark Universe.”
Further Reading: University of Texas at Austin, The Astrophysical Journal
