Two galaxies smashing into each other isn’t all that rare of a cosmic occurrence. Our very own Milky Way Galaxy is expected to collide with neighboring Andromeda in about four billion years. The good news for our solar system is it should come out unscathed by the collision (but the Sun will have made Earth uninhabitable before that happens). Stars are spaced out far enough to avoid any collision. Most galactic collisions are similar. But not all.
Astronomers trained the National Science Foundation’s Very Long Baseline Array (VLBA) on a cluster of galaxies designated ZwCl 8193.
James Condon, of the National Radio Astronomy Observatory, explains what they were looking for with the VLBA. “We were looking for orbiting pairs of supermassive black holes, with one offset from the center of a galaxy, as telltale evidence of a previous galaxy merger.”
But they found an object that didn’t match up with what they were looking for. B3 1715+425 is a supermassive black hole fleeing a larger galaxy in a hurry at more than 2,000 miles per second. “And leaving a trail of debris behind it,” adds Condon.
An artist’s concept of the naked supermassive black hole.
This larger galaxy sits more than 2 billion light-years away and is the largest and brightest within its cluster. Cordon dubs these galaxies ‘Jabba the Hutt’ type galaxies. They’re the largest and brightest because they hang out around the center of the cluster snacking on smaller galaxies that come their way.
But these collisions don’t tend to result in a supermassive black hole speeding away from it. Condon and his fellow astronomers haven’t seen “anything like this before.”
The galaxy surrounding the supermassive black hole is much smaller and fainter than astronomers expected to see. Plus, the black hole isn’t orbiting the other one from the larger galaxy. It’s shooting off into the blackness of space leaving a trail of ionized gas in its wake.
Condon and the other astronomers turned to the Hubble and infrared telescope like Spitzer to see what was going on. B3 1715+425 is what’s left of a smaller galaxy that collided with a larger one. The stars within the smaller galaxy didn’t come out unscathed like other galactic collisions. Its stars and gas were stripped away leaving a “nearly naked” supermassive black hole as the lone survivor. And in another billion years, it will probably be invisible. With a finite amount of gas and stars around it, we won’t be able to see its structure light up as it devours a cosmic meal.
The discovery of this “nearly naked” supermassive black hole does raise the possibility of more objects like this one lurking unseen in space according to Condon.
Astronomers will keep looking with the VLBA as a side project. And who knows, maybe the team will find some of the supermassive black holes they were originally looking for.