Astronomers from the Max Planck Institute for Extraterrestrial Physics and at the University Observatory Munich have found the largest known black hole in our galactic neighborhood using direct mass measurements. Located 700 million light-years from Earth in the Abell 85 cluster of galaxies in the constellation of Cetus, it has a mass 40 billion times that of the Sun.
Ever since the existence of the first black hole was confirmed in 1971, they have gone from being a mathematical curiosity to a major player in the field of astrophysics. Theoretically, the smallest black hole is 22 micrograms, but at the other end of the scale a Max Planck team led by Ralf Bender's research group has found one that outweighs both the Large and Small Magellanic Cloud star clusters combined.
Discovered from photometric data and spectra recorded by the Very Large Telescope, the supermassive black hole is located in the center of the central galaxy Holm 15A of the Abell 85 cluster, which is composed of 500 individual galaxies. Holm 15A is no lightweight itself, boasting an equivalent to two trillion solar masses.
However, the central region of Holm 15A is very diffuse and faint as it spreads about 15,000 light-years across, which made the team suspect that a supermassive black hole might be present. It also provided a rare chance to directly measure a black hole's mass by tracking the movements of stars and gases in its vicinity – and at 700 million light-years distant, it's the farthest attempted, though the team refers to it as "local."
What the team found was that the center of Holm 15A has a very flat light curve, indicating that the stars in that region were expelled by some event – most likely "core scouring," which occurs when two galaxies with black holes at their centers merge. As the central black holes come together, the stars in the vicinity are ejected, leaving insufficient gas behind to form new stars.
"The newest generation of computer simulations of galaxy mergers gave us predictions that do indeed match the observed properties rather well," says Jens Thomas, who provided the dynamical models. "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."
The team hopes that by studying the structure of Holm 15A, it may be possible to estimate the mass of black holes at the core of far more distant galaxies where direct measurements are not possible.
Source: Max Planck Society
