| U.S. astronomers Chen, Hwang, Shen, Liu, Zakamska, Yang and Li (how times have changed) report searching for dual quasars using a novel technique called VODKA. As in this artist’s sketch, when massive bodies like stars fall into a supermassive black hole (SMBH), an immense amount of energy is released in extremely bright beams of radiation called quasars. Quasars are the brightest known objects in our universe, hence we can see them from the farthest distances. They highlight the locations of SMBHs. Most quasars originated during cosmic “high noon” — when the rate of star formation in our universe peaked. This was from 6 to 12 billion years ago. Thus, most of the quasars we see are 6 to 12 billion light-years away. Almost all major galaxies have a SMBH in their core. SMBHs are millions to billions of times the mass of our Sun — the most massive discovered so far is 60 billion times the mass of our Sun. When they eat voraciously, we call them active galactic nuclei (AGN). Astrophysicists are eager to study SMBHs and particularly how often they merge, which might help explain how these behemoths grow so massive so quickly. Hence astronomers strive to find SMBH pairs that are very close to one another, with “very close” meaning sub-kilo-parsec (within a few thousand light-years). This allows us to estimate their time-to-merger, and thereby their merger rate. Since most quasars are billions of light-years from us, two quasars only a few thousand light-years apart will likely appear as just one spot of light in most telescopes. When faced with such a daunting challenge, astronomers can be very creative. As the old saying goes, when life gives you potatoes, make VODKA. In this case, astronomers use one problem to solve a different problem. Each quasar’s intensity tends to be highly variable — it greatly brightens whenever a star falls into its SMBH, and dims while awaiting the next morsel. Normally that complicates observations, but this variation can also be useful. When we observe two very close quasars, we sometimes see one spot of light whose position jitters very slightly as the quasars wax and wane separately. Detecting that jitter is called varstrometry, and using varstrometry to find off-nucleus and dual sub-kilo-parsec AGN, is called VODKA. It seems many scientists spend an inordinate amount of time inventing catchy acronyms (perhaps after consuming vodka). The authors demonstrated the effectiveness of VODKA in a pilot study. They chose a small sample of 81 quasars with unusually large jitter cataloged by the European Space Agency (ESA) satellite GAIA. GAIA has precisely measured the positions and properties of more than 1 billion objects in our Milky Way galaxy and over 700,000 extragalactic bodies. The authors observed these 81 quasars with the Hubble Space Telescope, and found that 38 appeared to be single isolated sources while 43 were multiple, very close sources. In much of science, and particularly in astronomy, analyzing the data takes longer and is more difficult than taking the data. Astronomers need to determine exactly what they observed. Here are four examples that illustrate the diversity of possibilities, with green X’s marking the GAIA-determined positions. Counting from the left, picture #1 shows a quasar and an unrelated red star — while the two bodies are actually far apart, their lines-of-sight differ by only 0.18 arc-sec. Picture #2 shows 3 quasars, 2 seen by GAIA and a third, dimmer quasar below the left green X. Picture #3 shows 2 quasars with tidal tails. Picture #4 shows 4 gravitationally lensed images of the same quasar, 2 seen by GAIA, plus a central Einstein ring also gravitationally lensed. The authors demonstrated how to correctly identify dual quasars (picture #2) with spectroscopic analysis and lots of VODKA. Best Wishes, Robert April 1, 2022 Note: Previous newsletters can be found on my website. | 