Using Hubble, researchers accidentally discover ejected black hole forcing star creation

by Haygen Warren

Throughout the universe, supermassive black holes can typically be found at the center of massive galaxies that stretch tens of light years across. However, using Hubble, a team of researchers has discovered a supermassive black hole that appears to have been ejected from its galaxy. The black hole, which is around 20 million times more massive than our Sun, is traveling incredibly fast — so fast that it could travel the distance between Earth and the Moon in just 14 minutes.

What’s more, as the black hole travels through space it is plowing into gas ahead of it. As the black hole compresses the gas, star formation is triggered, and new hot blue stars are created — leaving a 200,000 light-year-long trail of stars behind the black hole. The trail of stars is very bright as it likely houses an extremely high amount of young stars, with the trail’s brightness being nearly half as bright as the black hole’s host galaxy. Nothing like this has ever been observed by scientists, and Hubble’s observations are helping scientists understand the characteristics of black holes and how they affect their surrounding environments.

In Hubble’s observations, the trail of stars appears to be a bright, thin singular string of light extending from a galaxy. The black hole is barrelling through space at one end of the column, where an extremely bright spot of ionized oxygen also lies; the team of researchers believes this bright spot of gas is created by the motion of the black hole hitting the gas, rapidly shocking and heating the gas to extreme temperatures. The spot of gas could also be radiation produced by an accretion disk surrounding the black hole.

Hubble’s image of the runaway black hole and the 200,000 light-year-long trail of stars. (Credit: NASA/ESA/Pieter van Dokkum (Yale)/Joseph DePasquale (STScI))

“We think we’re seeing a wake behind the black hole where the gas cools and is able to form stars. So, we’re looking at star formation trailing the black hole,” explained Pieter van Dokkum of Yale University in New Haven, Connecticut.

“Gas in front of it gets shocked because of this supersonic, very high-velocity impact of the black hole moving through the gas. How it works exactly is not really known,” said van Dokkum.

As mentioned, this phenomenon has never been observed before by scientists, van Dokkum et al. just happened to stumble across the trail of stars while searching for globular clusters within a dwarf galaxy near the trail of stars when they noticed the trail in one of the Hubble images.

“This is pure serendipity that we stumbled across it. I was just scanning through the Hubble image and then I noticed that we have a little streak. I immediately thought, ‘Oh, a cosmic ray hitting the camera detector and causing a linear imaging artifact.’ When we eliminated cosmic rays we realized it was still there. It didn’t look like anything we’ve seen before,” van Dokkum said.

When van Dokkum et al. noticed the trail in their images, they chose to perform follow-up spectroscopic observations using the ground-based W. M. Keck Observatory in Hawaii. Through these new observations, the team determined that the trail was likely the result of a supermassive black hole being ejected through the outer gas halo of its host galaxy.

However, how does an object as massive and intense as a supermassive black hole get ejected from its host galaxy?

The ejected black hole’s host galaxy is the result of a collision between two galaxies approximately 50 million years ago. Given that the two galaxies had supermassive black holes at their centers, the two black holes began orbiting around one another, becoming a binary black hole system.

Then, a third galaxy collided with the remains of the first collision, bringing a third black hole into the binary black hole system. As with most three-body systems, the introduction of this third black hole created a huge shift in the balance of the system, causing it to become extremely unstable and chaotic. One of the three black holes then gained an extreme amount of momentum from the other two black holes, leading to the black hole being ejected from the galaxy. A binary black hole system may still remain even after the ejection of one of the black holes, with the third black hole possibly taking the place of one of the black holes in the original binary system.

However, when one of the black holes was ejected, the other two black holes were ejected in the opposite direction, meaning the host galaxy may no longer have (a) supermassive black hole(s) in its center. Hubble’s observations support this assumption, as no signs of an active black hole or binary black hole system have been seen in the core of the host galaxy.

Research teams are currently planning to perform follow-up observations of the ejected black hole, its trail of stars, and the host galaxy using the joint NASA/ESA/CSA James Webb Space Telescope and NASA’s Chandra X-ray Observatory. The teams are hoping that the follow-up observations will provide further support for their assumptions and show more characteristics of the star trail.

Additionally, NASA’s upcoming Nancy Grace Roman Space Telescope, which has a massive wide field of view for deep sky surveys with the resolution of Hubble, will allow scientists to search for more of these star trails and possible runaway black holes. According to van Dokkum, scientists will likely have to enlist the help of machine-learning algorithms that are specifically designed to search for weirdly shaped trails, as identifying these star trails amongst a massive sea of stars and galaxies will be extremely hard for a small group of scientists.

van Dokkum et al.’s research was published on April 6 in The Astrophysical Journal Letters.

(Lead image: artist’s depiction of the ejected black hole leaving behind a trail of stars with its host galaxy in the background. Credit: NASA/ESA/Leah Hustak (STScI))

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