Imagine, if you will, the moment. Pieter van Dokkum of Yale University was examining images from the Hubble Space Telescope. He was looking for globular clusters, spherical aggregations of hundreds of thousands of stars congregating in most galaxies’ outer regions.
His gaze lit upon RCP 28, a tiny dwarf galaxy 7.8 billion light-years away.
He saw no globular clusters. Instead, he spied a faint streak. At first, he thought it was a “linear imaging artifact” caused by a common-as-dirt cosmic ray, a charged particle ejected from a distant supernova.
But it wasn’t a cosmic ray. Van Dokkum and his team realized that “it didn’t look like anything we’ve seen before.”
You can see the image at https://www.nasa.gov/feature/goddard/2023/hubble-sees-possible-runaway-black-hole-creating-a-trail-of-stars.
Consequently, van Dokkum’s research team scheduled time on the Keck Telescope in Hawaii. Spectrographic analysis of the steak revealed a trail of hot, young stars 200,000 light-years long, twice the width of our Milky Way galaxy. It pointed straight at the galactic center of RCP 28.
At the front end of the streak was a supermassive black hole weighing in at 20 million solar masses. Most galaxies, including our Milky Way, have such black holes at their center.
Those accumulations of material are so gravitationally powerful that even light cannot escape their surfaces.
As a galaxy forms, gas and dust swirl together to form stars, giving many galaxies their whirlpool shape.
The region at the center of a galaxy is dense with dust and gas but does not collapse into stars. Instead, the material condenses into a black hole of enormous size.
The black hole at the center of the Milky Way consists of two million solar masses.
Black holes at the centers of galaxies start relatively small but gather mass by devouring stars, dust, and gas in their gravitational vicinity.
But black holes can gather mass in even more violent ways.
Early in the universe, galaxies were much closer together. Their mutual gravities caused them to merge. As their central black holes collided, they formed a brand new black hole that combined the masses of the originals. Multiple galactic mergers can produce black holes of billions of solar masses.
But even stranger things can happen. If the black holes pass at just the proper distance, they can go into orbit around one another. It is much like your thumbs twiddling — if each thumb consisted of millions of solar masses.
Even more rarely, as two galaxies merge, the close pass of one galactic black hole to another one can slingshot one or both of them out of their galaxies. Astronomers have speculated about such “rogue” black holes for 50 years but have never observed one — until now.
Significantly, the galaxy near Van Dokkum’s streak contains no black hole.
A black hole of about 20 million solar masses leads Van Dokkum’s streak. It travels at 3.6 million miles per hour, or 1,000 miles per second, away from its former host galaxy, RCP 28.
Without further study, astronomers cannot be sure how the black hole left its host galaxy or how it is forming the stars that trail in its wake.
They speculate that a galaxy, probably RCP 28, once had two supermassive black holes orbiting at its center. About 7.8 billion years ago, another galaxy with its central black hole passed through RCP 28. The invading galaxy’s black hole did not come close enough for the black holes to merge.
Instead, one or more black holes got slingshotted out of the galaxy. So far, van Dokkum’s single streak of light represents the only black hole detected.
Fifty million years after that fateful encounter, the black hole still zips along at 1,000 miles per second through the thin veil of gas and dust surrounding RCP 28. The gas in front of the black hole is compressed and heated in the shock wave in front of the black hole. As the gas and dust pass around and behind the black hole, they cool sufficiently to form into the hot, young stars in the black hole’s wake.
If those events seem unlikely, consider that none violate physical laws. It’s a big universe. If something can happen, it probably will. As our telescopes get more powerful, stranger things are waiting to be discovered.
Tom Burns is the former director of the Perkins Observatory in Delaware.
