Massive black hole pushes the boundaries of what's possible

Astronomers have uncovered what could be one of the most massive black holes ever detected. It is located in a galaxy so enormous that it bends space itself, twisting the light of a background galaxy into a giant horseshoe-shaped ring. This remarkable sight is known as the Cosmic Horseshoe.

The discovery comes from scientists at the University of Portsmouth and the Universidade Federal do Rio Grande do Sul (UFRGS) in Brazil.

The study suggests that this black hole is close to the theoretical upper limit of what is possible in the universe – about 36 billion times the mass of our Sun.

For comparison, that’s roughly 10,000 times heavier than the black hole at the center of the Milky Way.

How the black hole was measured

“This is amongst the top 10 most massive black holes ever discovered, and quite possibly the most massive,” said Professor Thomas Collett of the University of Portsmouth.

“Most of the other black hole mass measurements are indirect and have quite large uncertainties, so we really don’t know for sure which is biggest. However, we’ve got much more certainty about the mass of this black hole thanks to our new method.”

The team combined two techniques – gravitational lensing and stellar kinematics – to measure the black hole.

Stellar kinematics, the study of how stars move around black holes, is often called the gold standard for mass measurements. But it rarely works for distant galaxies because they appear too small to resolve clearly.

Gravitational lensing, where massive objects bend the path of light, allowed the team to “push much further out into the universe,” Professor Collett explained.

Finding a black hole without light

“We detected the effect of the black hole in two ways – it is altering the path that light takes as it travels past the black hole and it is causing the stars in the inner regions of its host galaxy to move extremely quickly (almost 400 km/s),” said Professor Collett.

“By combining these two measurements we can be completely confident that the black hole is real.”

According to study lead author Carlos Melo-Carneiro, the black hole was dormant and not actively accreting material at the time of observation.

“Its detection relied purely on its immense gravitational pull and the effect it has on its surroundings,” said Melo-Carneiro.

“What is particularly exciting is that this method allows us to detect and measure the mass of these hidden ultramassive black holes across the universe, even when they are completely silent.”

Measurements in remote systems

The Cosmic Horseshoe black hole is located about 5 billion light-years from Earth.

“Typically, for such remote systems, black hole mass measurements are only possible when the black hole is active,” said Melo-Carneiro. “But those accretion-based estimates often come with significant uncertainties.”

“Our approach, combining strong lensing with stellar dynamics, offers a more direct and robust measurement, even for these distant systems.”

Powering a massive galaxy

Scientists believe there is a deep connection between the size of a galaxy and the size of its central black hole.

“We think the size of both is intimately linked, because when galaxies grow they can funnel matter down onto the central black hole,” noted Professor Collett.

“Some of this matter grows the black hole but lots of it shines away in an incredibly bright source called a quasar. These quasars dump huge amounts of energy into their host galaxies, which stops gas clouds condensing into new stars.”

The Milky Way’s own black hole has a mass of about four million Suns. It’s quiet now, but has likely acted as a quasar in the past. When the Milky Way merges with the Andromeda Galaxy in roughly 4.5 billion years, it could flare up as a quasar again.

A fossil galaxy group

The Cosmic Horseshoe’s host galaxy is a “fossil group” – a giant galaxy left behind after smaller galaxies merged into it.

“It is likely that all of the supermassive black holes that were originally in the companion galaxies have also now merged to form the ultramassive black hole that we have detected,” said Professor Collett.

“So we’re seeing the end state of galaxy formation and the end state of black hole formation.”

Accidental discovery with big potential

The black hole discovery was not the team’s original goal. They were studying the distribution of dark matter throughout the galaxy when evidence of the black hole became clear.

Now that the method has been proven, they hope to use data from the European Space Agency’s Euclid space telescope to spot more of these hidden giants and understand how they influence the birth of stars in galaxies.

The full study was published in the journal Monthly Notices of the Royal Astronomical Society.

Image Credit: NASA/ESA

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