Black holes are some of the strangest and most fascinating objects in outer space. They’re incredibly dense, with such strong gravitational attraction that even light cannot escape their grasp if it comes near enough.
In space, black holes appear in different sizes and masses. An example presently holds the record in the Abell 85 cluster of galaxies, where an ultra-massive black hole with 40 billion times the mass of our Sun sits in the middle of the central galaxy Holm 15A.
Astronomers at the Max Planck Institute for Extraterrestrial Physics and the University Observatory Munich found this by assessing photometric data from the Wendelstein Observatory as well as new spectral observations with the Very Large Telescope.
Even though the central galaxy of the cluster Abell 85 has an enormous visible mass of around 2 trillion (10^12) solar masses in stars, the center of the galaxy is extremely diffuse and faint. This is the reason a joint group of astronomers at the Max Planck Institute for Extraterrestrial Physics (MPE) and the University Observatory Munich (USM) got interested in the galaxy. This central diffuse region in the galaxy is nearly as enormous as the Large Magellanic Cloud, and this was a suspicious clue for the presence of a black hole with a high mass.
Consists of more than 500 individual galaxies, the Abell 85 cluster of galaxies is at a distance of 700 million lightyears from Earth.
MPE scientist Jens Thomas, who led the study, said, “There are only a few dozen direct mass measurements of supermassive black holes, and never before has it been attempted at such a distance. But we already had some idea of the size of the Black Hole in this particular galaxy, so we tried it.”
The data was gathered at the USM Wendelstein observatory of the Ludwig-Maximilians-University and with the MUSE instrument at the VLT, which enabled scientists to perform a mass estimate based on directly on the stellar motions around the core of the galaxy.
Roberto Saglia, a senior scientist MPE and lecturer at the LMU, said, “This is several times larger than expected from indirect measurements, such as the stellar mass or the velocity dispersion of the galaxy.”
USM doctoral student Kianusch Mehrgan, who performed the data analysis, said, “The light profile of the galaxy shows a center with an extremely low and very diffuse surface brightness, much fainter than in other elliptical galaxies. The light profile in the inner core is also very flat. This means that most of the stars in the center must have been expelled due to interactions in previous mergers.”
Generally, the cores in such massive elliptical galaxies form via so-called ‘core scouring’: In a merger between two galaxies, the gravitational interactions between their merging, central black holes lead to gravitational slingshots that eject stars on predominantly radial orbits from the center of the remnant galaxy. If there is no gas left in the center to form new stars – as in younger galaxies – this leads to a depleted core.
Jens Thomas, who also provided the dynamical models, said, “The newest generation of computer simulations of galaxy mergers gave us predictions that do indeed match the observed properties rather well. These simulations include interactions between stars and a black hole binary, but the crucial ingredient is two elliptical galaxies that already have depleted cores. This means that the shape of the light profile and the trajectories of the stars contain valuable archaeological information about the specific circumstances of core formation in this galaxy – as well as other very massive galaxies.”
Be that as it may, even with this excellent merging history, the scientists could establish another and powerful connection between the black hole mass and the galaxy’s surface brightness: With each merger, the black hole gains mass and the galaxy center loses stars. Astronomers could utilize this relation for black hole mass gauges in more distant galaxies, where direct estimations of the stellar motions sufficiently close to the black hole are unrealistic.