A stellar black hole hurtling through space

Fast Giant: Astronomers have detected a supermassive black hole hurtling through space at supersonic speed – trailing a 200,000 light-year-long trail of stars in its wake. The 20-million-solar-mass giant was ejected from a nearby galaxy – most likely during a galaxy merger. This is the first time that astronomers have been able to observe and detect such an ejected black hole.

When galaxies collide, their central black holes usually merge. But there are instances where something goes wrong: massive turbulence and emitted gravitational waves
sling
Then one of the two supermassive black holes from its orbit ejects it from the galaxy. Astronomers have already detected the first indications of this in the form of supermassive black holes
offset laterally
lying in their galaxy.

Astronomers discover randomly ejected giants

Now astronomers led by Peter van Dokkum of Yale University have discovered a more extreme case: a supermassive black hole that has been completely ejected from its galaxy. This was evident because the ejected giant traced a long trail of gas and stars that stretched all the way back to its ancient galaxy. “It was quite a coincidence that we found her,” says Van Dokkum.

Astronomers were actually looking for globular star clusters in nearby dwarf galaxy RCP28 in images from the Hubble Space Telescope. “When I was looking at one of the Hubble images, I noticed a bright little line. It looked unlike anything we had known before,” says van Dokkum. It showed that this line, about 200,000 light-years across, was composed of young stars and gas and appeared to be It emerges from a galaxy about 7.6 billion light-years away.

A long-tailed spotlight hurtles through space at supersonic speeds

But what caused this star’s tail? Astronomers report “Having never encountered such a phenomenon before, neither in astronomical photographs nor in the literature, we decided to record the observation time at the Keck Observatory.” With the help of the Keck telescopes’ high-resolution spectrograph and Echelle spectrograph, they were able to get more details of the mysterious bar of stars.

The analysis revealed a bright ionized oxygen spot at the tip of the star’s tail, drifting away from the galaxy at a good 1,000 miles per second — that’s about 350,000 miles per hour. Behind the rim, which races through space at supersonic speed, first very young stars appear, and then quite old ones.

Expelled from the galaxy 39 million years ago

According to astronomers, a supermassive black hole of about 20 million solar masses is hiding at the bright end of this star’s tail. This must have been ejected from the galaxy at the end of the tail some 39 million years ago. Van Dokkum and his colleagues suspect that this occurred as a result of a double merger: the first, about 50 million years ago, saw the two central black holes form a tight orbiting pair.

Then another galaxy collision occurred, adding a third supermassive black hole. According to the old adage, “Three is one too many,” this created a gravitational perturbation that shot one of the three black holes out of the galaxy. Its gravity blew a large amount of gas into space. “Behind it, we see a kind of awakening of gas that is slowly cooling and stars are forming,” van Dokkum explains.

not noticed before

“Nothing comparable has been observed anywhere else in the universe,” says the astronomer. “It has been predicted for 50 years that supermassive black holes can sometimes be ejected from their galaxies. But only now have we been able to find the first direct evidence of this.” However, astronomers have not yet been able to directly detect the black hole at the tip of the star’s tail. .

The bright spot at the tip of the star’s tail could either be from the activity of the black hole itself — for example, because it’s actively swallowing matter — or from some sort of arc wave from intergalactic gas shocks colliding with the black hole and being. hot.

Have the remaining black holes been ejected, too?

The research team hopes that further observations at other wavelengths and at higher resolutions will provide greater clarity. “The X-ray images could explain the physics of the shocking bow wave or even show the accretion disk of the supermassive black hole,” the astronomers say. Images taken with the James Webb Telescope and in the ultraviolet range can also provide more information.

In addition, additional observations could indicate whether the two remaining black holes remained in the galaxy or whether they were also expelled. Because on the other side of the galaxy there is a much lighter and shorter “tail” whose nature is still unclear. Theoretically, it would be conceivable that the remaining pair of black holes were also blasted out of the galaxy by the recoil. (The Astrophysical Journal Letters, 2023; doi: 10.3847/2041-8213/acba86)

Source: Space Telescope Science Institute, W.M. Keck Observatory

This article was written by Nadja Podbregar