Using a fairly clever, new technique, astronomers have discovered that apparently two double-quasars (and possibly a third pair) are billions of light-years from Earth. It may help to limit how many of these ferocious objects existed when the Universe was young.
A quasar is a kind of active galaxy, a galaxy that actively feeds a supermassive black hole. As I described before (in an article on binary quasars):
All large galaxies, and many smaller ones, have a supermassive black hole in their heart. As matter from the surrounding galaxy falls into the core, I can accumulate in a disk that runs around the black hole and feeds it slowly. The disk is incredibly hot and can shine incredibly bright and easily protrude from the rest of the galaxy. Sometimes, by forces that are not terribly well understood (although the magnetic properties of the disk are the probable culprit), twin beams of matter are launched up and down, away from the disk, with matter moving at a very high velocity, sometimes just a little slower than the speed of light.
Generally, such an object is called an ” active galaxy. If one of the rays is incidentally 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 it a quasar.
We know that there are extraordinarily massive black holes out there that can grow to such a great extent when two large galaxies merge. The black holes fall towards each other, and eventually revolve around each other and then after billions of years they can merge into one larger black hole. This implies that we need to find binary supermassive black holes, or at least two that are close to each other (say, within a few thousand light-years of each other). Very little is seen, however, especially because it is difficult to discover.
However, the new research has found a way to find some of them, and it’s very clever. It is known that quasars vary in brightness, and that they become brighter and dimmer on a time scale of days, weeks or months. When two quasars are close together, one can become brighter over time over the other and vice versa.
If they are so close together, it looks like one spot, this effect can betray their dual nature. Say one is to the left of the other. If the left side is brighter, the stain appears to move slightly to the left. If it then fades and the other one becomes brighter, the stain will move slightly to the right. The science of measuring the positions of objects in the air is called astrometry, and so the scientists who worked on this double-quasar research call this idea astrometric jitter.
The Gaia satellite observatory has been measuring the sky for years and measuring the positions of billion of objects with incredible accuracy. If the center of a single object is seen to move back and forth over 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 of 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 recurrent shivers. From the list, they found 15 (noting that there may be more that could be missed because they were too close to each other to see any shift in the center).
The team submitted these 15 objects that had to be observed by Hubble as part of the “Snapshot” program: very short exposure (usually 5 – 8 minutes) of targets that can be obtained between the regularly scheduled observations. Four of the 15 were observed. Of these, one is clearly a quasar with a star very close, so it is not a double quasar. A second has not even been solved by Hubble – it still looks like a single object – so its qualifications are uncertain.
But the other two pairs both appear to be double quasars, each with two clearly resolved components. However, we must be careful here. It is possible that each is actually a single quasar, but that it is gravity: the gravity of a galaxy in the foreground between us and them can distort the light and create multiple images of the same object. The team can not rule it out, even after taking spectra from one of the double quasars (and finding that it is about 11.5 billion light-years away). However, if one looks at the number of lens quasars known, they find that the chance that it is a single lens quasar is only about 5%, which means that they have 95% confidence that it is a double quasar is.
To be honest, it could also be just two active galaxies that happen to be close to each other, or a single extremely lumpy, still-forming galaxy where two clusters happen to have 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 eliminate some of these potentially confusing factors. The bigger point here, however, is that the astrometric jitter idea did it work. Over time, Gaia can also see more of this, because the longer it observes the sky, the better its measurements become. Future aerial shots can also improve this technique, though it now yields only a handful of double brushes, it may be even more useful later.
The early Universe must have more double quasars, twin monsters roaring in the dark. We hope we find more of it.