Two new high-redshift red quasars discovered

Using the Subaru telescope, astronomers have identified two new quasars reddened by dust (red) in the high redshifts. The finding, detailed in an article published on July 16 on the arXiv prepress server, could improve understanding of these rare but interesting objects.

Quasars, or quasi-stellar objects (QSOs), are extremely luminous active galactic nuclei (AGNs) that contain supermassive central black holes with accretion disks. Its redshifts are measured from the strong spectral lines that dominate its visible and ultraviolet spectra. Some QSOs are reddened by dust, which is why they are called red quasars. These objects have a not inconsiderable amount of dust extinction, but are not completely obscured.

Astronomers are especially interested in finding new high-redshift quasars (at redshift greater than 5.0) as they are the most luminous and most distant compact objects in the observable universe. The spectra of such QSOs can be used to estimate the mass of supermassive black holes that constrain quasar evolution and formation models. Therefore, high redshift quasars could serve as a powerful tool for exploring the early universe.

To date, a red quasar has not been identified at a redshift of more than 5.0. This is mainly due to the weakness of such sources in the ultraviolet wavelengths of the resting frame, and due to the lack of a large sample of quasars at high redshifts. Now, a team of astronomers led by Nanako Kato of Ehime University in Japan reports a breakthrough in the search for high-redshift red quasars.

“We present the first discovery of dust-reddened quasars (red quasars) in the high-z universe,” the astronomers wrote in the article.

The find was made as part of the Subaru High-z Exploration of Low-Luminosity Quasars (SHELLQs) project. The observations that led to the discovery were made using the Hyper Suprime-Cam (HSC) at the 8.2m Subaru Telescope in Hawaii. The study also used data from NASA’s Wide Field Infrared Survey Explorer (WISE).

Kato’s team has analyzed the SHELLQ data set containing 93 high-redshift quasars. As a result, they identified four red quasar candidates and two of them, designated HSC J120505.09−000027.9 (with a redshift of 6.7) and HSC J023858.09−031845.4 (with a redshift of 5.83), were confirmed as red quasars with dust redness of 0.115 and 0.127, respectively.

“Two of the four candidates were found to be red quasars with dust redness of E (B – V)> 0.1,” the newspaper says.

The researchers stressed that their discovery demonstrates how important the HSC instrument is in identifying new high-redshift QSOs, especially reddened by dust. The team plans to continue its search for high-redshift red quasars using HSC, WISE, and also data obtained with NASA’s Spitzer spacecraft. They added that follow-up observations of such sources with instruments such as the Thirty Meter Telescope (TMT) or the James Webb Space Telescope (JWST) would be crucial in studying their individual nature in detail.

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