Astronomers discover new lensed, dual and projected quasars

Chinese astronomers have investigated quasar candidates from the DESI Legacy Surveys (DESI-LS) photometry catalog. As a result, they detected 19 strongly-lensed, dual and projected quasars. The finding was reported in a paper published Jan. 15 on the arXiv pre-print server.

Quasars, or quasi-stellar objects (QSOs), are active galactic nuclei (AGN) of very high luminosity powered by supermassive black holes (SMBHs), emitting electromagnetic radiation observable in radio, infrared, visible, ultraviolet and X-ray wavelengths. They are among the brightest and most distant objects in the known universe, and serve as fundamental tools for numerous studies in astrophysics as well as cosmology.

Two quasars observed with a small separation can be, in some cases, lensed quasars—where the light from a single quasar is bent, resulting in two images of the same quasar. More often, such objects are dual quasars, which means that they are at similar redshift and physically interacting. However, the most common scenario is projected quasars—coincidentally appearing very close to each other along the line of sight, but actually at different redshifts.

A study published by a team of astronomers led by Zizhao He of the Purple Mountain Observatory in China, reports the detection of new quasars of these three types. The researchers selected high-priority observable quasar candidates from DESI-LS and investigated them with the Palomar Observatory, which resulted in the confirmation of two lensed, six dual, and 11 projected quasars.

“These observations took place on October 15–16, 2023, at the Palomar Observatory in California, utilizing the P200/DBSP instrument,” the scientists write.

Two quasars reported in the paper, designated J0746+1344 and J2121-0826, are strongly-lensed quasars at redshifts of 3.1 and 2.39, respectively. It was noted that J0746+1344 exhibits a strong flux anomaly and the lensing galaxy is located next to the brightest image, which is unusual since the opposite configuration is typically observed.

The six identified dual quasars were found at redshifts between 0.59 and 3.28. The projected physical separations of these pairs range from 50,300 to 73,500 light years. One of them, with designation J1929+6009, showcases a very small redshift difference between the two components of less than 0.0001 and a relatively small projected separation of 62,800 light years.

The remaining 11 objects are projected quasars, with separations between 35,700 and 123,400 light years. The astronomers note that one of these projected quasars, named J0422+0047, was thought to be a gravitationally lensed quasar system with the central yellow component acting as the lensing galaxy.

However, it turned out that the system may have a more complex configuration as its large line-of-sight velocity between the two components suggests a chance alignment of two projected quasars with an intervening galaxy.

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