Black holes are now understood to exist in different sizes and masses. The record size for a black hole is now held by a specimen in the Abell 85 cluster of galaxies, where an ultra-massive black hole boasting 40 billion times the mass of our Sun sits in the middle of the central galaxy Holm 15A.

Black hole authorities at the Max Planck Institute and the University Observatory Munich discovered this extreme object by examining photometric data from the Wendelstein Observatory as well as new spectral observations with the Very Large Telescope (VLT).

Never before has this been attempted at such a distance

Jens Thomas of the Max Planck Institute

Even though the central galaxy of the cluster Abell 85 actually has the incredible visible mass of approximately 2 trillion solar masses in stars, the galaxy’s centre is surprisingly diffuse and faint.

This is why a joint group of astronomers at the Max Planck Institute and the University Observatory Munich became interested in the area.

This central diffuse region in the galaxy is almost as large as the Large Magellanic Cloud, and this offered a tantalising clue for the presence of a black hole with an unprecedented mass.

Consisting of more than 500 individual galaxies, the Abell 85 cluster sits at a distance of 700 million lightyears from Earth, double the distance for previous direct black hole mass measurements.

Jens Thomas of the Max Planck Institute, 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 new data obtained at the USM Wendelstein observatory of the Ludwig-Maximilians-University and with the MUSE instrument at the allowed the team to perform a mass estimate based directly on the stellar motions around the galaxy’s core.

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With a mass of 40 billion solar masses, this is the most massive black hole known today in the local universe.

Dr Roberto Saglia, senior scientist at the Max Planck Institute, said: ”This is several times larger than expected from indirect measurements, such as the stellar mass or the velocity dispersion of the galaxy.”

The light profile of the galaxy shows a centre with an extremely low and very diffuse surface brightness, far fainter than otherwise similar elliptical galaxies.

Kianusch Mehrgan University Observatory Munich doctoral student, who performed the data analysis, added: “The light profile in the inner core is also very flat.

“This means that most of the stars in the centre must have been expelled due to interactions in previous mergers.”

In the commonly accepted view, the cores in such massive elliptical galaxies form are created in so-called “core scouring”.

During the merger between two galaxies, the gravitational interactions between the combining central black holes lead to gravitational slingshots capable of eject stars on predominantly radial orbits from the centre of the remnant galaxy.

If there is no gas left in the centre to form new stars—as in younger galaxies—this leads to a depleted core.

Dr Jens Thomas, who also provided the dynamical models, added: ”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.”

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