Bad Astronomy

Binary black holes terrorize quasar nucleus

Astronomers Todd Boroson and Tod Lauer have found what appears to be the best candidate seen for a tightly bound binary black hole, a close-knit duo of death in the center of a quasar. They sifted through over 17,000 quasar signatures to find this rare pair, sitting in the center of quasar SDSS 153636.22+044127.0 at a distance of about 4 billion light years from Earth.

Although many double black holes are known (for example, NGC 6240 and 3C 75) – remnants of ancient galactic mergers, two galaxies colliding and eventually forming a single, bigger galaxy – these are widely-spaced pairs (30,000 and 10,000 light years apart, respectively, for NGC 6240 and 3C 75), and while it’s likely that in those cases the black holes orbit each other, it’s not completely certain.

In this new case, while it’s still not 100% rock-solid certain the two holes orbit one another, it’s very, very likely. First, by examining the redshifts carefully, Boroson and Lauer determined the two black holes are at the same distance from Earth. Second, they are separated by a very small amount: only about 1/3 of a light year separates the two monsters, making it incredibly unlikely they are unaffiliated.

And unlike other black hole binary candidates seen before, this pair’s authenticity is testable! They are so close and moving so rapidly – 6000 kilometers per second, an appreciable fraction of the speed of light – that the orbital period of the two is only about 100 years. Examining them again in just a few years should reveal their orbital motion around each other, clinching their marriage.

I’ll note that the two black holes are so close together that even the most powerful telescopes in the world would see them as a single object (in fact, the astronomers plan to take Hubble observations to see if they can resolve the pair or not; they expect they cannot which will further support their claim that this is a single system). We know there are two there because of their spectra. Imagine matter swirling around a black hole, and imagine further it emits a very narrow color range of, say, yellow light. As it orbits the black hole, it will sometimes be headed toward us, and sometimes away. The Doppler shift changes the color of the light, red and blue-shifting it over the course of the orbit. There are animations on an ESA page that may help you understand this. In those videos, sound is used as an analogy for color – lower pitch is redder light, and higher pitch is bluer light.

But there’s a lot of matter circling the drain of a black hole, forming a disk. We see red shifted light from the all the material in disk heading away from us, and blue shifting from the material in the disk headed toward us. We see all of this at the same time, so instead of a single color of light from the disk, it gets smeared out by that motion, forming essentially a bell curve (no, you won’t be graded on this). That curve, called a broadened line, is the signature of the black hole.

What Boroson and Lauer saw were two such broad line regions in the spectra, indicating two black holes. What’s cool too is that the mass of the black hole affects how broad the line is; the more massive it is, the more powerful its gravity, the faster material moves, so the broader the Doppler shift is. By carefully examining the broadening, they were able to determine that the black holes had masses of 20 million and 1 billion times the mass of the Sun.

That’s pretty hefty; the black hole in the center of our Milky Way galaxy is only about 4 million solar masses. These guys are real bruisers.

So it’s a pretty safe call that this really is a binary pair of black holes, two gaping maws of death circling each other at a distance of about 3 trillion kilometers, each supermassive and actively consuming material. They formed their bond when their parent galaxies collided eons ago, and eventually, billions or possibly hundreds of billions of years from now, these two giant black holes will merge themselves, swallowing each other and forming a single object with their combined mass. That’s one reason these beasts are so rare: they eat each other, leaving behind a single black hole. It was only through foresight, planning, and hard work that this pair was found at all. There may not be many more like them in the whole Universe, so every one we find gives us precious insight into the clockwork process of the cosmos itself.

Image credit: p. Marenfeld and NOAO/AURA/NSF.