A new way to accurately measure the distance between quasar pairs - the blazing, black-hole-powered centres of distant galaxies that appear side by side in space - is showing promise through University of Alberta research.
The feasibility study looked at a method of gauging the line-of-sight distance between double quasars, to help address a depth-perception problem that makes it difficult to measure how far apart these pairs actually are, says astrophysicist Dr. Huanqing Chen, assistant professor at Augustana Campus and lead author on the paper.
"When we spot two quasars close together in an image, we're looking at a projection of a three-dimensional universe, so we know their separation across the sky very precisely, but their separation along our line of sight carries a large uncertainty," Chen says.
"Being able to measure the true physical separation is an important factor in deciding how long it takes two quasar-hosting galaxies to merge."
When they do merge, their central supermassive black holes eventually collide and form a single, even larger black hole.
The new method could help scientists reduce the uncertainty that comes with measuring these distances using telescopes, Chen says.
Space scientists use the relationship between distance and redshift - recessional velocity, or how fast space expansion moves a galaxy away from an observer. To determine that velocity, they measure light emitted by gas in the galaxy.
"But active supermassive black holes often push that gas outward so fast that the velocity we measure deviates significantly from the galaxy's actual recessional velocity," Chen notes.
"That systematic error can shift the inferred line-of-sight separation by as much as 10 million light-years - far enough apart that the two black-hole-hosting galaxies would not interact strongly at all."
The study tested an alternative approach called the quasar proximity effect, looking at the regions of gas around a quasar that are lit up and heated by its light.
"A quasar lights up its surroundings the way a light bulb lights up a dark room, and we can see that lit-up region in the quasar's spectrum because the gas there has become transparent," Chen explains.
"If you picture two light bulbs that sit at exactly the same spot, you see a single bright region. If one is placed in front of the other, you see a second bright patch out ahead - and how far ahead it sits tells you how far apart the bulbs are. With quasars, we use those transparent regions the same way, to work out the separation between the two."
The results revealed that the new method offers more precise measurements.
"While conventional redshift measurements can be off by tens of millions of light-years, this new method can pin down the physical separation of a visual quasar pair to within about half a million light-years in many cases."
Though the study used simulated data and simple algorithms, the method shows promise as a way to help astronomers study quasar pairs using their own data, without having to also rely on highly specialized equipment that can take many years to access, Chen adds.
"Those observations can take years to obtain, if they can be obtained at all. This method could let astronomers characterize a quasar pair's true separation without that second observation."
Chen next plans to investigate how much the precision of the method improves if using finer, smaller-scale features in real quasar spectra.
"The approach is worth investigating further, because there's a lot of potential in it."
