Free Astronomy Magazine July-August 2021

40 JULY-AUGUST 2021 ASTRO PUBLISHING ideas about how supermassive black holes come together to eventually form a binary,” elaborated team member Nadia Zakamska of Johns Hopkins University. Finding the two quasar pairs was a daunting challenge, requiring a new method that combined data from several space and ground-based tel- escopes, including the international Gemini Observatory, a Program of NSF’s NOIRLab. Quasar pairs at such large distances can only be resolved by sharp-eyed telescopes such as Hubble or Gemini, but observing time on these telescopes is too valu- able to use it to sweep through large areas of the night sky in search of rare astronomical objects. To focus their search, the re- searchers first identified 15 quasars for further investigation using the Sloan Digital Sky Survey, a three-di- mensional map of objects in the night sky. From this list of 15 quasars, they then used observa- tions from the Gaia spacecraft to identify four potential quasar pairs. Finally, these candidates were im- aged with the Hubble Space Tele- scope, which visually resolved two quasar pairs, giving this novel method a success rate of 50%. The team then used the Gemini Multi-Object Spectrograph (GMOS) on Gemini North (located on Mau- nakea in Hawai’i) to verify the dis- covery and further investigate one of the quasar pairs. The combina- tion of the sensitivity of GMOS and superb observing conditions al- lowed the team to resolve individual spectra from both quasars in the pair. These spectra provided the team with independent measure- ments of the distance to the quasars and their composition, as well as confirming that the two quasars are indeed a pair rather than a chance alignment of a single quasar with a foreground star. “The Gemini observations were crit- ically important to our success be- cause they provided spatially re- solved spectra to yield redshifts and spectroscopic confirmations simulta- neously for both quasars in a dou- ble,” explained Yu-Ching Chen, a graduate student at the University of Illinois who is on the discovery team. “This method unam- biguously rejected interlopers due to chance superpositions such as from unassociated star-quasar systems.” While the team members are confident in their discovery, there is a small possibility that they have actually observed double images of single quasars. These astronomical doppelgängers can be formed by gravitational lensing, which occurs when an inter- vening massive galaxy distorts and splits the light from a dis- tant object, often resulting in multiple images of that ob- ject. The researchers are con- vinced that this is highly unlikely, however, as they could not detect any fore- ground galaxies in their observa- tions. With their method success- fully demonstrated, the researchers now plan to search for more quasar pairs, building up a census of double quasars in the early Universe. “This proof of concept really demonstrates that our targeted search for dual quasars is very ef- ficient,” concluded Hsiang-Chih Hwang, a graduate student at John Hopkins University and the princi- pal investigator of the Hubble ob- servations. “It opens a new direction where we can accumulate a lot more interesting systems to follow up, which astronomers weren’t able to do with previous techniques or datasets.” “This exciting investigation illus- trates yet again the discovery poten- tial of combining archived survey data with new, focused observations from state-of-the-art facilities,” said Martin Still, Gemini Program Officer at NSF. “The international Gemini Observatory proved to be the ideal instrument to confirm the identity of these black holes and character- ize their environment.” C osmoView Episode 26: Black hole pairs found in distant merging galaxies. [Internatio- nal Gemini Obs./NOIRLab/NSF/AURA/J. da Silva/J. Pollard. Music: Stellardrone-Airglow] !