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An international team of astronomers has discovered 25 new quasars by analyzing multiwavelength data from various surveys. All the newfound quasars were detected at redshifts greater than 4.6. The findings are detailed in a research paper May 21 on the arXiv pre-print server.

Quasars, or quasi-stellar objects (QSOs), are active galactic nuclei (AGN) in the centers of active galaxies, which showcase very high luminosity and are powered by supermassive black holes (SMBHs). They emit electromagnetic radiation observable in radio, infrared, visible, ultraviolet and X-ray wavelengths.

Finding new high-redshift quasars (at redshifts higher than 4.4) is important for astronomers as such QSOs are the most luminous and most distant compact objects in the observable universe. Their spectra can be used to estimate the mass of supermassive black holes that constrain the evolution and formation models of quasars.

Now, a group of astronomers led by Silvia Belladitta of the Max Planck Institute for Astronomy in Heidelberg, Germany, has discovered another 25 high-redshift quasars, significantly expanding the list of known QSOs of this type.

"To expand the sample of high-redshift quasars, we conducted targeted selections using optical, infrared, and radio surveys, complemented by literature-based quasar candidate catalogs," the researchers explained.

According to the paper, the 25 new quasars have redshifts ranging from 4.6 to 6.9. The majority of the newfound QSOs are located in the southern hemisphere.

Three of the quasars reported in the study, designated MQC J021+19, MQC J133−02 and PSO J200−13, are radio-loud as they show intense radio emission, with a 1.4 GHz luminosity in the range of 0.9–10 decillion erg/s/Hz.

The study found that the three radio-loud quasars are compact at all frequencies at the resolution of the available radio images. Their radio properties turned out to be within the range of values of other known radio-loud quasars at redshift greater than 4.6.

The astronomers noted that the radio-loud quasar PSO J200−13 is most likely a blazar as it exhibits properties that are typical for this subclass of quasars, like flat radio spectrum and variability at several radio frequencies at different time scales.

Moreover, the authors of the paper found that one of the newly discovered QSOs, namely PSO J041+06, is a rare weak emission-line quasar (WLQ). In general, WLQs are enigmatic objects, which have abnormally low or weak broad emission lines compared to typical quasars, and their nature is still unknown.

In concluding remarks, the researchers underlined the importance of re-analysis of the available data in the search for new quasars. "The discovery and characterization of the quasars reported in this paper came from the in-depth exploration of surveys already widely used in past years, confirming that these data sets can still be exploited for the discovery of quasars in the bright tail of their luminosity function," they said.

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