Using what is truly a fairly clever new technique, astronomers have discovered what appear to be two double quasars (and possibly the third pair) billions of light years from Earth. This can help you constrain how many of these ferocious objects existed when the Universe was young.
A quasar is a kind of active galaxy, a galaxy centered on a supermassive black hole that feeds actively. As described above (in an article on binary quasars):
All large galaxies and many smaller ones have a supermassive black hole at the heart. As we described about ten trillion times before, if matter from the surrounding galaxy falls to the core, it can accumulate in a disk that orbits the black hole, feeding it slowly. The disk is incredibly hot and can shine incredibly bright, easily surpassing the rest of the galaxy. Sometimes by forces that are not very well understood (however the magnetic properties of the disk are probably the culprit), the twin beams of matter are launched up and down, away from the disk, with matter moving at an extremely high speed, sometimes only with a slidge slower than the speed of light.
Generically, such an object is called active galaxy. If one of these rays goes to 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 merge. Black holes fall into each other, eventually 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 close together (say, a few thousand light-years apart). However, very few are seen, mainly because they are difficult to discover.
New research has found a way to find some of them, but it is very smart. Quasars are known to fluctuate in brightness, becoming brighter and weaker over a period of days, weeks, or months. If two quasars are close, one can become brighter than the other in time and vice versa.
If they are so close that they appear as a single spot, this effect can betray their dual nature. Say that one is to the left of the other. If the left one is brighter, the blob will appear to be moved slightly to the left. If it then fades and the other becomes brighter, the blob will move slightly to the right. It is called the science of measuring the positions of objects in the sky astrometryand so the scientists who worked on this double quasar research call this idea astrometric jitter.
The Gaia satellite observatory has been researching the sky for years, measuring positions billions of objects with incredible accuracy. If the center of what is believed to be a single object is thought to move back and forth in time, it may be one of those rare double quasars.
The team first made a list of known quasars beyond about 10 billion light-years – anything closer to that and the extended light from the stars in the galaxy surrounding the black hole could 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 suffering from recurring fluctuations. From this list they found 15 (mentioning that there could be several that were missed because they were too close to see any change in the center).
The team sent these 15 objects to be observed by Hubble as part of the “Snapshot” program: very short exposures (usually 5-8 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 very close star, so it is not a double quasar. One second has not been resolved by Hubble – it still appears as a single object – so its qualifications are uncertain.
But the other two pairs seem to be almost double, with two components clearly solved each. However, we must be careful here. It is possible that each is actually a single quasar, but has a gravitational lens: the gravity of a galaxy in the foreground between us and them can distort light, creating several images of the same object. The team can’t rule this out, even after taking the spectrum of one of the double quasars (and found it to be about 11.5 billion light-years away). However, looking at the number of known lens quasars, I find that the chances of it being a lens quasar is only about 5%, which means they have 95% confidence, it is truly a double quasar.
To be fair, it could be just two active galaxies approaching each other or a single extremely crowded galaxy that is still forming, if two clusters have massive black holes in them.
Several further observations (including the 11 potential pairs that have not yet been observed by Hubble) are needed to try to rule out some of these confounders. However, the most important point here is the idea of astrometric jitter worked. Over time, Gaia can see even more of these, because the more she observes the sky, the better her measurements become. Also, future sky surveys can be improved with this technique, so that even if it produces only a handful of double quasars now, it may prove even more useful later.
The early universe should have several double quasars, twin monsters screaming in the dark. Here we hope to find more of them.