Black Holes Need Refreshing Cold Gas to Keep Growing

A pair of disc galaxies in the late stages of a merger. Credit: NASA

The Universe is filled with supermassive black holes. Almost every galaxy in the cosmos has one, and they are the most well-studied black holes by astronomers. But one thing we still don’t understand is just how they grew so massive so quickly. To answer that, astronomers have to identify lots of black holes in the early Universe, and since they are typically found in merging galaxies, that means astronomers have to identify early galaxies accurately. By hand. But thanks to the power of machine learning, that’s changing.

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Next Generation Gravitational Wave Observatories Could Detect 100-600 Solar Mass Black Hole Mergers

Simulation of merging supermassive black holes. Credit: NASA's Goddard Space Flight Center/Scott Noble

Humans are born wonderers. We’re always wondering about the next valley over, the next horizon, what we’ll understand next about this vast Universe that we’re all wrapped up in.

In 2015, we finally detected our first long-awaited and long-theorized gravitational wave from the distant merger of two stellar mass black holes. But now we want to know more, and only better detectors can feed our appetite.

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Ultra-Massive Black Holes: How Does the Universe Produce Objects So Massive?

Illustration of the supermassive black hole at the center of the Milky Way. Credit: NRAO/AUI/NSF
Illustration of the supermassive black hole at the center of the Milky Way. It's huge, with over 4 times the mass of the Sun. But ultramassive black holes are even more massive and can contain billions of solar masses. Image Credit: Credit: NRAO/AUI/NSF

Black holes are the most massive objects that we know of in the Universe. Not stellar mass black holes, not supermassive black holes (SMBHs,) but ultra-massive black holes (UMBHs.) UMBHs sit in the center of galaxies like SMBHs, but they have more than five billion solar masses, an astonishingly large amount of mass. The largest black hole we know of is Phoenix A, a UMBH with up to 100 billion solar masses.

How can something grow so massive?

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When Black Holes Collide They Also Produce Neutrinos

Still image from a numerical simulation of an unequal mass binary black hole merger, with parameters consistent with GW190412. [Image credit: N. Fischer, H. Pfeiffer, A. Buonanno (Max Planck Institute for Gravitational Physics), Simulating eXtreme Spacetimes project]

Ever since astronomers first detected ultra high energy neutrinos coming from random directions in space, they have not been able to figure out what generates them. But a new hypothesis suggests an unlikely source: the mergers of black holes.

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Shortly Before They Collided, two Black Holes Tangled Spacetime up Into Knots

A binary black hole system, viewed from above. Image Credit: Bohn et al. (see http://arxiv.org/abs/1410.7775)

In February 2016, scientists at the Laser Interferometer Gravitational-Wave Observatory (LIGO) announced the first-ever detection of gravitational waves (GWs). Originally predicted by Einstein’s Theory of General Relativity, these waves are ripples in spacetime that occur whenever massive objects (like black holes and neutron stars) merge. Since then, countless GW events have been detected by observatories across the globe – to the point where they have become an almost daily occurrence. This has allowed astronomers to gain insight into some of the most extreme objects in the Universe.

In a recent study, an international team of researchers led by Cardiff University observed a binary black hole system originally detected in 2020 by the Advanced LIGO, Virgo, and Kamioki Gravitational Wave Observatory (KAGRA). In the process, the team noticed a peculiar twisting motion (aka. a precession) in the orbits of the two colliding black holes that was 10 billion times faster than what was noted with other precessing objects. This is the first time a precession has been observed with binary black holes, which confirms yet another phenomenon predicted by General Relativity (GR).

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Astronomers Discover two Supermassive Black Holes Orbiting Each Other, Doomed to Collide in the Future.

Until recently, one of the closest orbiting each other pairs of supermassive blackholes was found in NGC 7727. That pair is about 89 million light-years away from Earth. Those black holes are only 1,600 light-years apart from each other. Another pair in OJ 287, about 3.5 billion light-years from Earth, are only separated by about 0.3 light years. Now scientists have discovered a pair orbiting each other at a distance of 200 AU to 2,000 AU apart, about 0.003 to 0.03 light years.

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Merging Black Holes and Neutron Stars. All the Gravitational Wave Events Seen So Far in One Picture

The mergers of compact objects discovered so far by LIGO and Virgo (in O1, O2 and O3a). The diagram shows black holes (blue), neutron stars (orange) and compact objects of unknown nature (grey), which were detected by their gravitational-wave emission. Each merger of a binary system corresponds to three compact objects shown: the two merging objects and the result of the merger. A selection of black holes (violet) and neutron stars (yellow) discovered by electromagnetic observations is shown for comparison. Image Credit: LIGO Virgo Collaboration / Frank Elavsky, Aaron Geller / Northwestern

The Theory of Relativity predicted the existence of black holes and neutron stars. Einstein gets the credit for the theory because of his paper published in 1915, even though other scientists’ work helped it along. But regardless of the minds behind it, the theory predicted black holes, neutron stars, and the gravitational waves from their mergers.

It took about one hundred years, but scientists finally observed these mergers and their gravitational waves in 2015. Since then, the LIGO/Virgo collaboration has detected many of them. The collaboration has released a new catalogue of discoveries, along with a new infographic. The new infographic displays the black holes, neutron stars, mergers, and the other uncertain compact objects behind some of them.

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Behold! The Black Hole Collision Calculator!

This image shows two massive black holes in the OJ 287 galaxy. The smaller black hole orbits the larger one, which is also surrounded by a disk of gas. When the smaller black hole crashes through the disk, it produces a flare brighter than 1 trillion stars. Credit: NASA/JPL-Caltech

Black holes have been the subject of intense interest ever since scientists began speculating about their existence. Originally proposed in the early 20th century as a consequence of Einstein’s Theory of General Relativity, black holes became a mainstream subject a few decades later. By 1971, the first physical evidence of black holes was found and by 2016, the existence of gravitational waves was confirmed for the first time.

This discovery touched off a new era in astrophysics, letting people know collision between massive objects (black holes and/or neutron stars) creates ripples in spacetime that can be detected light-years away. To give people a sense of how profound these events are, Álvaro Díez created the Black Hole Collision Calculator (BHCC) – a tool that lets you see what the outcome of a collision between a black hole and any astronomical object would be!

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Why Can Black Hole Binaries Have Dramatically Different Masses? Multiple Generations of Mergers

Simulated merger of two black holes. Credit: NASA's Goddard Space Flight Center

On the 12th of April, 2019, the LIGO and Virgo gravitational wave observatories detected the merger of two black holes. Named GW190412, one of the black holes was eight solar masses, while the other was 30 solar masses. On the 14th of August that year, an even more extreme merger was observed, when a 2.5 solar mass object merged with a black hole nearly ten times more massive. These mergers raise fundamental questions about the way black hole mergers happen.

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