The Case of the Nearly Naked Black Hole

supermassive black hole is nearly naked
Artist's conception of how the "nearly naked" supermassive black hole originated. Bill Saxton, NRAO/AUI/NSF

Every galaxy has a supermassive black hole at its heart. Our galaxy has one, Andromeda has one, and even the most distant large galaxies that astronomers can observe sport these mysterious monsters  hidden away among their stars and clouds of gas and dust. These giant black holes sit in the galactic cores, sometimes quietly ticking away. Other times, they're busy voraciously eating anything that gets too close and sending out massive amounts of radiation.

Such black holes are fearsome and it's hard to think of anything that would affect them. As it turns out, there are some events and processes that can affect a supermassive black hole. 

Collision!

Billions of years ago two galaxies that are part of a cluster 2 billion light-years away had a very close encounter of the dangerous kind. One galaxy punched through the heart of a smaller one. The action stripped nearly all the stars and gas away from the little one. The only thing left behind was its active supermassive black hole and a little remnant of the former galaxy. Supermassive black holes usually have huge disks of material accreting around them, feeding gas and dust (and planets and stars) into their unforgiving traps. Stripped of its feeding disk, the remaining supermassive black hole is nearly naked, although it still has some stars traveling along with it. Called B3 1715+425, it is providing an intrigue look at what happens when collisions go weird.

How Did Astronomers Spot this Object?

Since black holes themselves aren't easily "seen" in optical wavelengths of light, astronomers look at them using radio telescopes or instruments that are sensitive to x-rays and other radiation given off by the material around a black hole. This one seems to have lost its accretion disk, so there's not much to see there.

However, there's still a jet emanating from it, and the whole thing is giving radio waves that can be detected here on Earth. So, how did astronomers find it? The answer is easy: they used a set of radio telescopes to look for something out of the ordinary: orbiting pairs of black hole holes.

Searching out such pairs is one way to figure out if galaxy mergers have taken place. Typically, in a busy cluster, even with mergers, there should be supermassive black holes sitting at the centers of galaxies. So, some astronomers put together an observing program using the Very Large Baseline Array in New Mexico to detect supermassive black holes that are millions or billions of times more massive than the Sun. They wanted to trace how galaxies gobble each other up in mergers, and see what happens to their central supermassive black holes when such collisions take place.

This odd couple jumped out at them in the radio emissions they detected from the region of the galaxy collision. The black hole and its little remnant of a galaxy shred are racing away from the scene of the merger at a rate of a couple of thousand kilometers per second. It's leaving behind a trail of hot gas behind it. As the remnant runs away, it will probably leave gehind more of its gas.

That's unfortunate, because gas is what galaxies need to form new stars. So, the remnant will slowly dim away to invisibility. In a billion years, there will be nothing left to see.

How Did This Merger End So Badly For the Black Hole?

In giant clusters of galaxies, mergers take place fairly frequently. They build ever-larger galaxies with growing black holes in their cores. Somehow, this merger ended badly for the small galaxy and its black hole. The galaxy itself was shredded apart, and the black hole is now doomed to wander intergalactic space in the cluster. Perhaps someday it will be part of another merger in the cluster.

This kind of discovery highlights just how complex the process of galaxy mergers can get. When galaxies do merge (as the Milky Way and Andromeda will do in the far distant future), they mingle stars and clouds of gas and dust.

Their central black holes eventually merge. What's left is a huge elliptical galaxy that future astronomers will look at and try to figure out what the two original galaxies were like. In the case of the tiny galaxy and its naked black hole, now that astronomers know their story, they can look for other events like this one to see if other nearly naked black hole exist out there — somewhere in the depths of space.