Illuminated gas pins a quasar pair's depth to half a million light-years in simulations

Distance comes from redshift, redshift comes from the host galaxy's recessional velocity, and that velocity is measured from light emitted by gas in the galaxy ^[7]. An active black hole moves that gas. "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 said ^[8]. He said the resulting error "can shift the inferred line-of-sight separation by as much as 10 million light-years" ^[9]. At that offset, the two black-hole-hosting galaxies would not interact strongly at all ^[10].

The alternative reads gas the quasar has already illuminated and heated, the quasar proximity effect ^[11]. "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 said ^[12]. Two bulbs at the same spot give one bright region. Put one in front of the other and a second patch appears ahead of it, and Chen said "how far ahead it sits tells you how far apart the bulbs are" ^[13].

Chen said conventional redshift measurements "can be off by tens of millions of light-years," while the new method "can pin down the physical separation of a visual quasar pair to within about half a million light-years in many cases" ^[14]. Set against the 10 million light-years he quotes for the redshift systematic, half a million is a twentieth of it ^[20].

Chen's team ran the test on simulated data with simple algorithms ^[15]. In a simulation the true separation is known by construction, so half a million light-years is the accuracy of a method measured against a separation the simulation had already specified. Chen's next step is to find out how much the precision improves when the method uses finer, smaller-scale features in real quasar spectra ^[18].

Depth matters for timing. "Being able to measure the true physical separation is an important factor in deciding how long it takes two quasar-hosting galaxies to merge," Chen said ^[5]. When those galaxies do merge, their central supermassive black holes eventually collide and form a single, larger black hole ^[6]. That timescale is the motivation for the work; the paper itself stops at the separation ^[1].

The method could also spare astronomers a wait. "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 said ^[17]. The paper is in The Astrophysical Journal Letters, with Chen, an assistant professor at the University of Alberta's Augustana Campus, as lead author ^[3][2]. Of the method, he said: "The approach is worth investigating further, because there's a lot of potential in it" ^[19].