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SYFY WIRE Bad Astronomy

A binary quasar caught by accident!

By Phil Plait
Artwork depicting binary quasars, two actively galaxies orbiting one another. Credit: NASA/ESA/Hubble/ESO/M. Kornmesser, adapted by Phil Plait

What's more amazing than a supermassive black hole greedily gobbling down material and expelling radiation and vast amounts of matter in twin beams moving away at very nearly the speed of light?

Why, two of them orbiting each other. I mean, duh.

A team of astronomers recently discovered just such an incredible system. What's even better is: They found it by accident*.

All big galaxies, and many smaller ones, have a supermassive black hole at their heart. As I have described approximately eleventy bazillion times before, if matter from the surrounding galaxy is falling into the core, it can pile up into a disk that orbits the black hole, slowly feeding it. The disk is incredibly hot and can shine incredibly luminously, easily outshining the rest of the galaxy. Sometimes, through forces not terribly well understood (though the magnetic properties of the disk are the likely culprit), twin beams of matter are launched up and down, away from the disk, with the matter moving at extremely high velocity, sometimes just a smidge slower than the speed of light.

Generically, such an object is called an active galaxy. If one of those beams happens to be pointed at Earth, we can see lots of light from across almost the entire electromagnetic spectrum, from radio waves to X-rays. We call an object like that a quasar.

These tend to be pretty far away, billion of light years, so identifying them can be tricky. Astronomers looking to confirm some possible quasar candidates selected the object GQ 1114+1549 for follow-up, and took a spectrum of it using the Gran Telescopio Canaris, a 10.4-meter telescope on the island of La Palma in the Canary Islands. It has an instrument that breaks the light up into hundreds or thousands of individual colors, allowing astronomers to determine a lot about the object, including its distant, brightness, temperature, and more.

This part is what kills me. If you just point the telescope at your object and take a spectrum, every object in your field of view will have its light spread out according to color. That's confusing, and will mess up the observations. To prevent that, astronomers use a slit: Literally a long rectangular slot cut out of a metal plate in the path that light takes through the telescope. The object is placed in the middle of the slit, and hopefully most if not all other things in the telescope's field of view are blocked.

In this case, when they got their spectrum they saw three objects. One was their quasar, another was a star coincidentally close by in the same field, and the third was… another quasar!

The serendipitous discovery of a binary quasar. The image shows the two objects (A and B) with two stars nearby

Mind you, this was seen wholly by accident. Look at the diagram above; had the slit been rotated even a little bit more clockwise they would've missed the second quasar altogether! As it was they just nicked it, getting enough light to see that it was there.

Knowing they were on to something cool, they followed up with another telescope, this time orienting the slit so that both quasars were fully observed. And what they saw confirmed it: Both objects were quasars, and located at the same distance! Given their tight separation, they may be as close as 250,000 light years apart. For galaxies, that's pretty close (the Andromeda galaxy is literally ten times farther away from the Milky Way than that). It's extremely likely these two galaxies orbit each other as a binary pair.

Artwork depicting binary quasars, two actively galaxies orbiting one another. Credit: NASA/ESA/Hubble/ESO/M. Kornmesser, adapted by Phil Plait

Over a hundred binary quasars are known, but very few this close together. It's possible their interaction affects how they behave, too. One quasar, 3C321, is blasting out a jet that is slamming into a nearby galaxy, for example. That can affect the direction of the jet, the way it emits light, and even its ultimate fate. Certainly the galaxies affect each other; the jets can blow around gas in the other galaxy, affecting how stars form.

Having another example of such a rare system is a lovely gift for astronomers. We don't know that much about what happens in these circumstances, so this is a great chance. I hope they get observations with other 'scopes, like Hubble and in radio (where the jets from quasars can be very bright and easy to see).

It's not even clear how many of these binary quasars there are in the Universe. They're not all that easy to find, as this paper makes clear! Sometimes, you just have to be lucky.

*Even better than that? Amazingly, they found a similar binary quasar system in 2016, also by accident!