Using some really pretty clever new technique, astronomers have discovered what appear to be two double quasars (and possibly a third pair as well) billions of light years from Earth. This can help limit how many of these fierce objects existed when the Universe was young.
A quasar is a type of active galaxy, a galaxy with an actively feeding supermassive black hole in the center. As I described earlier (in an article on binary quasars):
All major galaxies, and many smaller ones, have supermassive black holes in their hearts. As I have described about 50 billion times before, matter from the surrounding galaxy can fall into the core, accumulate in a disk that orbits the black hole and slowly feeds it. The disk is incredibly hot and can shine incredibly bright, easily outpacing the rest of the galaxy. Sometimes, by forces not very well understood (though the drive’s magnetic properties are the likely culprit), dual beams of matter are launched up and down away from the disk, moving matter at an extremely high speed, sometimes just a little bit slower than the speed of light.
In general, such an object becomes one active galaxyIf one of those rays happens to be aimed at Earth, we can see a lot of light from almost the entire electromagnetic spectrum, from radio waves to X-rays. We call such an object a quasar.
We know that there are extremely massive black holes that could have grown to such an enormous size when two large galaxies converge. The black holes converge, eventually orbit each other and after billions of years can merge into one larger black hole. This means that we need to find binary supermassive black holes, or at least two that are close together (for example, within a few thousand light years of each other). However, very few are seen, mainly because they are difficult to detect.
However, the new research has figured out a way to find a few, and it’s super smart. Quasars are known to fluctuate in brightness and become brighter and dimmer over a timescale of days, weeks, or months. If two quasars are close together, one can get brighter than the other over time and vice versa.
When they are so close together they appear as one blob, this effect can betray their dual nature. Suppose one is to the left of the other. When the left one is brighter, it looks like the blob has shifted a little bit to the left. Then when it fades out and the other gets brighter, the blob will shift a bit to the right. The science of measuring the positions of objects in the sky is mentioned astrometry, and that’s what the scientists who worked on this double quasar research call this idea astrometric jitter
The Gaia satellite observatory has been studying the sky for years and measuring the positions of billions of objects with incredible accuracy. If the center of what it thinks is a single object moves back and forth over time, it might be one of these rare double quasars.
The team first listed known quasars that were more distant than about 10 billion light years – anything closer than that, and the expanded light from the stars in the galaxy surrounding the black hole could disrupt the measurements. They found about 11,000 such quasars.
Then they searched the Gaia database to see if the positions of any of these quasars suffered from recurring jitter. Of that list, they found 15 (noting that more could be missed because they were too close together to see any shift in the middle).
The team submitted these 15 objects to be observed by Hubble as part of the “Snapshot” program: Very short exposures (typically 5 – 8 minutes) of targets that can be obtained between regularly scheduled observations. Four of the 15 were observed. Of this, there is clearly a quasar with a star very close, so it is not a double quasar. One second was not resolved even by Hubble – it still appears as a single object – so the ratings are uncertain.
But the other two pairs both appear to be double quasars, each with two clearly resolved components. However, we have to be careful here. It’s possible that each is in fact a single quasar, but is lensed by gravity: The gravitational pull of a foreground galaxy between us and them can distort light, creating multiple images of the same object. The team cannot rule this out, even after taking spectra from one of the double quasars (and found it to be about 11.5 billion light years away). Still, if we look at the number of known quasars with lens, they find that the probability of it being a single quasar with lens is only about 5%, meaning they have 95% confidence that it is really a double quasar.
To be fair, it could also be just two active galaxies that just happen to be close together, or a single extremely lumpy, still-forming galaxy where two clumps have huge black holes inside them.
What is needed are more follow-up observations (including the 11 potential pairs not yet seen by Hubble) to try to rule out some of these potential confounders. The bigger point here, however, is the idea of the astrometric jitter workedOver time, Gaia can also see more of this, because the longer he observes the sky, the better the readings become. Also, future aerial surveys could improve this technique, so while it only yields a handful of double quasars now, it may prove to be even more useful later.
The early universe should have more double quasars, twin monsters that roar in the dark. I hope we will find more.