[Artist concept of a black hole in a starry background. Credit: NASA, ESA, D. Coe, G. Bacon (STScI)]
In astronomy, you deal with a lot of ridiculously violent cosmic phenomena. Stars explode, asteroids collide, whole galaxies smash together. When you look at the math and physics, when you actually grasp the levels of power involved, it’ll make the hair on the back of your neck stand up. It’s chaos wielded on a mind-crushing scale.
And then there’s the “two supermassive black holes colliding and merging and then launching the resulting even larger billion-solar-mass black hole out of a galaxy at nearly 8 million kilometers per hour due to gravitational waves” scale of immensity.
The story here starts with a relatively innocuous-looking galaxy. Called 3C 186, it’s a fuzzy blob even in the most powerful telescopes, but don’t be fooled: It’s 8 billion light-years away, a distance vast enough to shrink even the mightiest galaxy to a smear of light.
Even using Hubble, it doesn’t look like much:
Ah, but appearances deceive. In this case, a lot. What you’re seeing is a galaxy, a collection of billions of stars together with huge clouds of gas and dust. We’ve known for some time that every big galaxy, ours included, has a supermassive black hole in its center. These can be a million times the mass of our Sun, or even billions. But in case after case, we see them right smack dab in the center of the galaxy.
3C 186 is what’s called a quasar, a galaxy with an abnormally bright core. These are caused by the black hole residing there! Although you might think of black holes as the ultimate cosmic disposal, from which not even light cannot escape, that’s only true for the black hole, itself. Just outside the black hole, light can most certainly get out.
As matter falls in —gas clouds, stars, and more— it gets ripped apart and churned up. This stuff piles up in a flat swirl called an accretion disk that races madly around the black hole as the inner material falls in. Friction and other forces heat this material to millions of degrees, so hot it blasts out light across the electromagnetic spectrum, from radio waves to super-high-energy gamma rays. This light can be intense, so bright that it can outshine the entire rest of the galaxy combined, and be visible across billions of light years.
That bright “star” you see in 3C 186 is the central supermassive black hole eagerly gobbling down material, and blasting out that radiation. But wait. Did I say “central”? Yeah, not so much. It appears to be significantly offset from the galaxy’s core, by about 40,000 light years. That’s a long haul.
That’s very, very weird. And not only that, the galaxy, itself, looks disturbed. It's asymmetric, and with some evidence of shell structure or possible what are called “tidal tails”: long streams of gas and stars that form when galaxies collide.
Putting all this together, the astronomers who investigated this object came up with a scenario that, frankly, gives me the willies. Here’s the story:
Something like one or two billion years ago (in their frame of reference; remember, it takes 8 billion years for the light to reach us), two galaxies collided. Both were fairly large, and both had supermassive black holes in their cores. We don’t know just how massive, but probably something like a billion times the Sun’s mass each. The galaxies eventually merged, forming the tidal tails and shells we see.
In the meantime the two black holes circled each other like experienced fighters taking in the measure of the other. Over billions of years, they slowly approached, and finally, like the galaxies hosting them, they collided and merged.
When two black holes collide, they do two things: They create a single, more massive black hole, and they also emit a colossal amount of energy in the form of gravitational waves, literally a shaking of the fabric of spacetime. This blasts out ripples in the structure of space, like flapping a sheet up and down. These were mostly theoretical until LIGO detected gravitational waves directly from two merging black holes in 2015 (and from a second merger just months later), and now, we know this does, in fact, occur.
The strength of the blast from the merger depends on many things, including the masses of the black holes colliding. Some of that mass is converted directly into the energy of the gravitational waves. In this case, they were truly huge, so the energy released was beyond staggering: The equivalent of more than a hundred million stars exploding all at once.
See? That’s why I get the heebie-jeebies about stuff like this. Cripes!
But we’re not done. That energy was not sent outwards in all directions equally. Black holes can spin, and if the two black holes don’t have their spins aligned, then the resulting blast of gravitational waves can be asymmetric, more powerful in one direction than another. This creates a vast recoil, a tremendously powerful kick. That acts like a rocket, and the newly formed black hole can be hurtled out of the galactic center at phenomenally high speeds, up to thousands of kilometers per second.
Imagine something that can toss around an object a billion times the mass of the Sun at speeds thousands of times faster than a rifle bullet!
Yet the observations of 3C 186 fit all this. The speed of the black hole can be measured via its relative redshift to the galaxy itself, and it comes out to over 2000 kilometers per second, or nearly 8 million kilometers per hour. Given its distance from the galaxy’s center, that implies the merger happened about 5 million years ago.
Now, I have a couple of comments. One is that, if I were just looking at the image data, I might not be 100% convinced the black hole is off-center; there’s a lot of fuzz to the right, so the galaxy, itself, might be asymmetric, distorted. That makes finding the center problematic. However, spectral observations make the picture (literally) much clearer. By breaking the light up into individual colors, a lot of information can be gleaned. And the killer observation is that low-speed gas is still seen near the galaxy’s core, but much faster moving gas is seen near the black hole.
The low-speed material is hot gas left behind after the black hole was ejected. The high-speed stuff is gas that stayed with the black hole, held by the object’s tremendous gravity, as it flew away. That material is still swirling around the black hole, still hot, and still emitting lots of light. It’s the redshift measured from that that indicates the black hole is screaming away from the galaxy.
The second comment is that there are other possible scenarios besides this one of a black hole launched away from its parent galaxy. It’s possible that the central black hole really is central, but blasting out a fast and furious wind, and that’s behind these peculiar observations, or that the accretion disk is peculiar, asymmetric. The astronomers who did this work looked into those ideas, and while they fit some of the data, a lot of the specific observations they made make either or both of these explanations unlikely. As incredible as it is, the cannonball black hole explanation is the likeliest.
When people ask me why I love science so much, well. Many of the ideas behind this event were unknown until pretty recently, and even the existence of black holes in the centers of galaxies has only been known for a few decades!
That we can even understand such things at all is truly remarkable. Yet, here, we have a magnificent tale made up of many incredible pieces, and while this is one of the most amazing events I can imagine, it’s all created and woven together by science.
And even if this explanation turns out to be incorrect, we’ll still have learned something amazing about an amazing object. It’s win-win.
Why do I love science? That’s why.