Black hole mergers: Cooking with gas

November 13, 2019

Gravitational wave detectors are finding black hole mergers in the universe at the rate of one per week. If these mergers occur in empty space, researchers cannot see associated light that is needed to determine where they happened. However, a new study in The Astrophysical Journal Letters, led by scientists at the American Museum of Natural History and the City University of New York (CUNY), suggests that researchers might finally be able to see light from black hole mergers if the collisions happen in the presence of gas.

"With a light signature, astronomers could easily pinpoint the cosmic location of these mergers and study them in much more detail than is presently possible," said paper author Barry McKernan, a research associate in the Museum's Department of Astrophysics as well as a professor at the Borough of Manhattan Community College, CUNY, and a faculty member at CUNY's Graduate Center.

Black holes form when massive stars die. Much like dense objects sinking into a river on Earth, black holes tend to sink into regions of galaxies where gravity is strongest. It is believed that large numbers of black holes build up in the centers of galaxies, where a much larger, single, supermassive black hole lurks.

If individual small black holes pass close enough to each other as they orbit, their mutual gravity allows them to pair off and orbit each other, while also orbiting the central supermassive black hole. But a second random close encounter with another small black hole can easily break apart such a pairing.

"So the black holes dance, forming and breaking partnerships, but rarely getting close enough to each other to merge," said paper coauthor K.E. Saavik Ford, who is also a research associate in the Museum's Department of Astrophysics as well as a professor at the Borough of Manhattan Community College, CUNY, and a faculty member at CUNY's Graduate Center. "If a merger does happen, it will occur in the dark, with no associated light."

This picture changes if a large mass of gas falls onto the central supermassive black hole. This will result in a bright gas disk that envelops many of the black holes swarming around the central supermassive black hole and changes their orbits. Once inside the disk, the gas tugs on the black holes, causing them to spiral closer to the central supermassive black hole. If the smaller black holes pass close enough to each other, gas very quickly drives them together causing a merger and a burst of gravitational waves that can be detected by the Laser Interferometer Gravitational-Wave Observatory (LIGO) in the United States and the European-based Virgo detector.

The new work by McKernan, Ford, and collaborators at the California Institute of Technology, Jet Propulsion Laboratory, University of Edinburgh, Columbia University, and University of Florida, suggests that it may be possible to see the effect of black hole mergers on the gas disk. The idea: once the black holes merge, they typically experience a kick at high speed (about 50 kilometers per second/112,000 miles per hour). Nearby gas attempts to follow the merger product, but smacks into neighboring disk gas causing a shock collision. If the gas disk is thin enough to let the light escape, the shock glow may be detectable with telescope sky surveys. The likelihood of detecting the shock glow against an already bright disk is best for large mass black hole mergers, around smaller-mass central black holes. The timescale during which the glow is released may help astronomers distinguish the black hole merger from random variations in the disk gas.

"LIGO has opened up this whole new way of letting us 'hear' how two black holes merge into one; and if we are correct, there may well now be a way we can see these otherwise invisible events happen," said co-author Nicholas Ross of the University of Edinburgh. "This would have deep implications for how we study black holes and for observational cosmology."

Co-author Matthew Graham of Caltech adds: "We've got lots of telescopes, such as ZTF [Zwicky Transient Facility], now covering large regions of the sky every night. If there is an optical counterpart, we should see it; if we don't find any then that also tells us something interesting."
This work was supported in part by the U.S. National Science Foundation grant # 1831412.


The American Museum of Natural History, founded in 1869 and currently celebrating its 150th anniversary, is one of the world's preeminent scientific, educational, and cultural institutions. The Museum encompasses 45 permanent exhibition halls, including those in the Rose Center for Earth and Space and the Hayden Planetarium, as well as galleries for temporary exhibitions. It is home to the Theodore Roosevelt Memorial, New York State's official memorial to its 33rd governor and the nation's 26th president, and a tribute to Roosevelt's enduring legacy of conservation. The Museum's five active research divisions and three cross-disciplinary centers support approximately 200 scientists, whose work draws on a world-class permanent collection of more than 34 million specimens and artifacts, as well as on specialized collections for frozen tissue and genomic and astrophysical data and on one of the largest natural history libraries in the world. Through its Richard Gilder Graduate School, it is the only American museum authorized to grant the Ph.D. degree and also to grant the Master of Arts in Teaching degree. Annual visitation has grown to approximately 5 million, and the Museum's exhibitions and Space Shows are seen by millions more in venues on six continents. The Museum's website, mobile apps, and massive open online courses (MOOCs) extend its scientific research and collections, exhibitions, and educational programs to additional audiences around the globe. Visit for more information.


Become a fan of the American Museum of Natural History on Facebook at, follow us on Instagram at @AMNH, Tumblr at amnhnyc, or Twitter at @AMNH.

American Museum of Natural History

Related Black Holes Articles from Brightsurf:

The black hole always chirps twice: New clues deciphering the shape of black holes
A team of gravitational-wave scientists led by the ARC Centre of Excellence for Gravitational Wave Discovery (OzGrav) reveal that when two black holes collide and merge, the remnant black hole 'chirps' not once, but multiple times, emitting gravitational waves--intense ripples in the fabric space and time--that inform us about its shape.

Black holes? They are like a hologram
Spherical, smooth and simple according to the theory of relativity, or extremely complex and full of information as, according to quantum laws, Stephen Hawking used to say?

Under pressure, black holes feast
A new, Yale-led study shows that some supermassive black holes actually thrive under pressure.

Staining cycles with black holes
In the treatment of tumors, microenvironment plays an important role.

Black holes sometimes behave like conventional quantum systems
A group of Skoltech researchers led by Professor Anatoly Dymarsky have studied the emergence of generalized thermal ensembles in quantum systems with additional symmetries.

Scientists may have discovered whole new class of black holes
New research shows that astronomers' search for black holes might have been missing an entire class of black holes that they didn't know existed.

Are black holes made of dark energy?
Two University of Hawaii at Manoa researchers have identified and corrected a subtle error that was made when applying Einstein's equations to model the growth of the universe.

Telescopes in space for even sharper images of black holes
Astronomers have just managed to take the first image of a black hole, and now the next challenge facing them is how to take even sharper images, so that Einstein's Theory of General Relativity can be tested.

Can entangled qubits be used to probe black holes?
Information escapes from black holes via Hawking radiation, so it should be possible to capture it and use it to reconstruct what fell in: if given time longer than the age of the universe.

How black holes power plasma jets
Cosmic robbery powers the jets streaming from a black hole, new simulations reveal.

Read More: Black Holes News and Black Holes Current Events is a participant in the Amazon Services LLC Associates Program, an affiliate advertising program designed to provide a means for sites to earn advertising fees by advertising and linking to