“Like A Mini Galaxy-Size Big Bang” –Evolution of Ultramassive Black Holes


Galaxies are not necessarily safe from celestial, ultramassive behemoths lurking at their centers. The higher a black hole’s mass, the greater its power. “It would be like a mini, galaxy-sized Big Bang,” says Julie Hlavacek-Larrondo, professor in the Department of Physics at Université de Montréal, who holds the Canada Research Chair in Observational Astrophysics of Black Holes.

“We have discovered black holes that are far larger and way more massive than anticipated,” said Mar Mezcua, postdoctoral fellow at the Institute of Space Sciences in Spain, who studied 72 galaxies located at the center of the universe’s brightest and most massive galaxy clusters.. “Are they so big because they had a head start or because certain ideal conditions allowed them to grow more rapidly over billions of years? For the moment, there is no way for us to know.”

“We do know that black holes are extraordinary phenomena,” says Hlavacek-Larrondo. “So it’s no surprise that the most extreme specimens defy the rules that we have established up until now.”

Thanks to data collected by NASA’s Chandra X-ray telescope on galaxies up to 3.5 billion light years away from Earth, an international team of astrophysicists that included Hlavacek-Larrondo and Mezcua reported this past February 2018 that they detected what are likely to be the most massive black holes ever discovered in the universe. The team’s calculations showed that these ultramassive black holes are growing faster than the stars in their respective galaxies.

Now, new research in the journal Monthly Notices of the Royal Astronomical Society released today has found evidence for a large number of double supermassive black holes, likely precursors of gigantic black hole merging events. This confirms the current understanding of cosmological evolution – that galaxies and their associated black holes merge over time, forming bigger and bigger galaxies and black holes.

Astronomers from the University of Hertfordshire, together with an international team of scientists, have looked at radio maps of powerful jet sources and found signs that would usually be present when looking at black holes that are closely orbiting each other.

Before black holes merge they form a binary black hole, where the two black holes orbit around each other. Gravitational wave telescopes have been able to evidence the merging of smaller black holes since 2015, by measuring the strong bursts of gravitational waves that are emitted when binary black holes merge, but current technology cannot be used to demonstrate the presence of supermassive binary black holes.

Supermassive black holes emit powerful jets. When supermassive binary black holes orbit it causes the jet emanating from the nucleus of a galaxy to periodically change its direction. Astronomers from the University of Hertfordshire studied the direction that these jets are emitted in, and variances in these directions; they compared the direction of the jets with the one of the radio lobes (that store all the particles that ever went through the jet channels) to demonstrate that this method can be used to indicate the presence of supermassive binary black holes.

Dr Martin Krause, lead author and senior lecturer in Astronomy at the University of Hertfordshire, said: “We have studied the jets in different conditions for a long time with computer simulations. In this first systematic comparison to high-resolution radio maps of the most powerful radio sources, we were astonished to find signatures that were compatible with jet precession in three quarters of the sources.”

The fact that the most powerful jets are associated with binary black holes could have important consequences for the formation of stars in galaxies; stars form from cold gas, jets heat this gas and thus suppress the formation of stars. A jet that always heads in the same direction only heats a limited amount of gas in its vicinity. However, jets from binary black holes change direction continuously. Therefore, they can heat much more gas, suppressing the formation of stars much more efficiently, and thus contributing towards keeping the number of stars in galaxies within the observed limits.

Galaxies are not necessarily safe from the celestial, ultramassive behemoths lurking at their centers. The higher a black hole’s mass, the greater its power. “It would be like a mini, galaxy-sized Big Bang,” said Hlavacek-Larrondo.

“But there’s no need to worry about our own galaxy,” she continued. “Sagittarius A, the Milky Way’s supermassive black hole, is a bit boring. It’s not very active, much like a dormant volcano. It sucks up little matter and probably wouldn’t be able to produce destructive high-energy jets.”

Professor Hlavacek-Larrondo focuses her work on black holes in distant galaxy clusters to show that such objects have been significantly impacting their galactic neighborhoods and the entire universe for billions of years.

“They are the most powerful objects in the universe, and they are anything but quiet,” she said. “Galaxies are the building blocks of our universe, and to understand their formation and evolution, we must first understand these black holes.”

Image credit top of page: Aurore Simonnet, LIGO/Caltech/MIT/Sonoma State

The Daily Galaxy via University of Montreal and Royal Astronomical Society

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