Using what is really a very 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 may help to restrict how many of these ferocious objects existed when the Universe was young.
A quasar is a kind of active galaxy, a galaxy that has a supermassive black hole that actively feeds in the center. As I described earlier (in an article on binary quasars):
All large galaxies, and many smaller ones, have a supermassive black hole in their heart. As I described about eleven billion times before, if matter from the surrounding galaxy falls to the core, it can accumulate in a disk that orbits around the black hole, feeding it slowly. The disk is incredibly hot and can glow incredibly bright, easily surpassing the rest of the galaxy. Sometimes through forces that are not very well understood (but the magnetic properties of the disk are the likely culprit), twin beams of matter are thrown up and down, away from the disk, with matter moving at an extremely high speed, sometimes only slightly slower than the speed of light.
Generally, this object is called active galaxy. If one of these beams is directed towards the Earth, we can see a lot of light from almost the entire electromagnetic spectrum, from radio waves to X-rays. We call an object like this a quasar.
We know that there are extremely massive black holes that may have grown to such a huge size when two large galaxies merge. Black holes fall into each other, ending up orbiting each other and then, after billions of years, they can fuse together into a larger black hole. This implies that we should find binary supermassive black holes, or at least two that are very close together (e.g., a few thousand light-years apart). Although very few are seen, mainly because they are difficult to discover.
New research has found a way to find some, but it is very smart. Quasars are known to fluctuate in brightness and become brighter and darker over a period of days, weeks, or months. If two quasars are together, one can be brighter than the other over time and vice versa.
If they are so close together, they appear as a single drop, this effect can betray their dual nature. Say one is to the left of the other. If the left is brighter, the blob will seem to move a little to the left. If it then fades and the other becomes brighter, the blob will shift slightly to the right. The science of measuring the positions of objects on the sky is known astrometry, and so the scientists who worked on this double quasar research call this idea astrometric fluctuation.
The Gaia satellite observatory has been inspecting the sky for years, measuring the positions of billions of objects with incredible accuracy. If you see that the center of what you think is a single object moves back and forth over time, it may be one of those rare double quasars.
The team first made a list of known quasars further about 10 billion light-years away; anything closer to that and the extended light from the stars in the galaxy surrounding the black hole can interfere with the measurements. They found about 11,000 such quasars.
They then searched the Gaia database to see if the positions of any of these quasars were experiencing recurring fluctuations. They found 15 from this list (noting that they could have lost more because they were too close together to see any change in the center).
The team sent these 15 objects for Hubble to observe as part of the “Snapshot” program: very short exposures (usually 5 to 8 minutes) of targets that can be obtained between regularly scheduled observations. Four of the 15 were observed. Of these, one is clearly a quasar with a star very close by, so it is not a double quasar. A second was not even resolved by Hubble (it still appears as a single object), so its ratings are uncertain.
But the other two pairs appear to be double quasars, with two clearly resolved components each. Still, we need to be careful here. It is possible that each is really a unique quasar, but that it has a gravitational lens: the gravity of a galaxy in the foreground between us and them can distort light, creating multiple images of the same object. The team can’t rule it out, even after taking spectra from one of the double quasars (and finding that it’s about 11.5 billion light-years away). However, looking at the number of known lens quasars, they find that the probability that it is a lens quasar is only 5%, i.e. they have 95% confidence, it really is a double quasar.
To be fair, they could only be two active galaxies that were close to each other or a single extremely lumpy galaxy that was still forming, where two lumps had massive black holes.
What is needed are more follow-up observations (including the 11 potential pairs that Hubble has not yet observed) to try to rule out some of these possible confounders. However, the most important point here is that the idea of astrometric jitter worked. Over time, Gaia can also see more, as the longer she observes the sky, the better her measurements will be. In addition, future sky surveys may improve this technique, so while it only produces a handful of double quasars, it may now be even more useful.
The primitive Universe should have more double quasars, twin monsters roaring in the dark. Here we hope to find more.