Scientists have explained the release of a supermassive black hole from the galaxy

Scientists have reconstructed the probable history of RBH-1, the first convincing candidate for the role of a supermassive black hole ejected from the center of its galaxy. According to calculations, the object was formed after the merger of two rapidly rotating black holes, and the resulting gravitational recoil accelerated it to almost 1000 km/s. This was reported in the journal Physical Review Letters (PRL).

What is known about the supermassive black hole RBH-1

The unusual object is located about 7.5 billion light-years from Earth. In 2022, astronomer Peter van Dokkum noticed a thin straight band pointing away from a compact galaxy in an archive image of the Hubble telescope. At first, the researcher mistook it for an image defect, similar to a scratch.

Further analysis showed that the structure exists in reality. Its length reaches 62 kiloparsecs, or more than 202 thousand light-years, which is about twice the diameter of the Milky Way. The band consists of gas and young blue stars, and at its far end there is a compact region without a noticeable cluster of stars.

In 2023, scientists suggested that the structure could have emerged following a supermassive black hole moving through the near-galactic gas. The object compresses and heats the matter in front of it, and in the wake left behind, conditions are created for the formation of stars. At that time, RBH-1 was considered only as a candidate: an alternative version suggested that astronomers were observing an unusual galaxy facing the Earth.

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How James Webb helped confirm the existence of RBH-1

New observations using the James Webb NIRSpec spectrograph have reinforced the theory of a runaway black hole. The researchers found a sharp drop in gas velocity of about 600 km/s at the end of the stellar wake in a region about 1 kiloparsec in size. The ratio of spectral lines also corresponded to a fast shock front.

Based on the shape and movement of the gas, scientists estimated the speed of RBH-1 at 954 km/s. Taking into account the error, it can range from about 828 to 1064 km/s. The mass of the object has not been directly measured, but energy calculations indicate that it should reach at least several tens of millions of solar masses.

At the same time, Hubble and James Webb did not get a direct image of the black hole. As the authors of the observational work admit, all available signals are created by the shock wave and the surrounding gas. RBH-1 itself remains invisible and is determined by its effect on the substance.

Why was the black hole thrown out of the galaxy

The authors of the new article tried to find out what event could have given such a massive object such a speed. To do this, the researchers modeled hundreds of thousands of possible combinations of masses and rotation directions of two supermassive black holes, and then selected configurations capable of reproducing the observed motion of RBH-1.

When two black holes get closer, they lose energy and merge, emitting gravitational waves. If the masses of objects and the directions of their rotation are distributed asymmetrically, the waves carry away an unequal amount of momentum in different directions. The resulting black hole receives a push, or gravitational recoil, capable of throwing it out of the center of the galaxy.

Tejasvi Venumadhav, a co-author of the study and an associate professor of physics at the University of California, Santa Barbara, compared this effect to "recoil when firing a cannon." Calculations have shown that the difference in mass alone is not enough for the speed of RBH-1. Without rapid rotation, the maximum recoil would be about 200 km/s — almost five times less than the observed value.

What were the black holes like before the merger

According to scientists, the mass of one of the original objects could exceed the mass of the second by no more than six times. The larger black hole was rotating at a speed corresponding to about 70-75% of the maximum value allowed by the general theory of relativity. The axis of its rotation was tilted relative to the orbit of the system, which is why the direction of motion gradually changed before the merger.

The researchers also concluded that the two original black holes probably met after the collision of their parent galaxies. It should have been a large merger with a large amount of gas, in which one galaxy exceeded the other in mass by no more than four times. The compact galaxy formed after the collision was designated GX in the study.

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The merger of black holes and the release of RBH-1 occurred about 70 million years before the moment at which astronomers observe this system. Since the galaxy is located about 7.5 billion light-years from Earth, the event occurred in the distant past, when the universe was about half its current age.

Why is the discovery of RBH-1 important for astronomy

The authors emphasize that the proposed scenario is based on the assumption of gravitational recoil after the merger. It explains well the object's speed, its estimated mass, and the absence of a supermassive black hole at the center of the galaxy. However, it is no longer possible to register the direct gravitational wave signal from this ancient event.

RBH-1 may be the first observed example of a mechanism long predicted by the general theory of relativity. Previously, scientists had found other candidates for the role of ejected black holes, but their displacement relative to the centers of galaxies allowed alternative explanations.

Studying such objects can help in preparing for the operation of the LISA space observatory. Unlike LIGO and Virgo, which register mergers of stellar-mass black holes, LISA will look for low-frequency gravitational waves from much more massive systems.

The Hubble and James Webb observations will allow us to compare the visible effects of ancient collisions with gravitational wave signals that LISA will be able to detect in the future. This will help scientists better understand how galaxy mergers affect the evolution of supermassive black holes located at their centers.