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Study inspects over 100 quasars from the MIGHTEE survey

A team of astronomers from Rhodes University and elsewhere have investigated a sample of 104 quasars detected with the MeerKAT International GHz Tiered Extragalactic Exploration (MIGHTEE) survey. The , published July 16 on the pre-print server arXiv, could help us advance our knowledge about quasars and their properties.

Quasars, or quasi-stellar objects (QSOs), 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.

In general, they are active galactic nuclei (AGN) of very high luminosity, emitting electromagnetic radiation observable in radio, infrared, visible, ultraviolet and X-ray wavelengths.

MIGHTEE is an extragalactic radio survey currently being carried out with the MeerKAT radio telescope in South Africa. Its main goal is to create deep images of the extragalactic sky to explore the cosmic evolution of galaxies, including AGNs.

Recently, a group of astronomers led by Rhodes University's Sarah V. White combed through MIGHTEE data to investigate unobscured type I (showcasing broad emission lines) quasars. All in all, they managed to investigate 104 such objects.

"We studied a sample of 104 unobscured (Type-1) quasars within the COSMOS and XMM-LSS fields of the MIGHTEE survey, selected via gJK_s color-space and reaching 1.3-GHz flux-densities of rms ≈ 3.0 µJy beam⁻¹. Thanks to the large, multiwavelength datasets that are available over these fields, the properties of radio-loud and radio-quiet quasars can be studied in a statistically robust way, with the emphasis of this work being on the AGN-related and star-formation-related contributions to the total radio emission," the researchers explained.

The study found that the quasars from the sample have redshifts between 0.6 and 3.41, while their median redshift is approximately 1.68. The radio-loudness fraction for these quasars was estimated to be 5%, which is consistent with other quasar samples described by previous studies.

It turned out that the fraction of the quasar sample, which has radio emission dominated by the AGN, is lower than that found by other studies of the MIGHTEE sample. The astronomers assume that this may be caused by the lower radio flux-density limit of the radio images, enabling a greater sensitivity to the lower radio-luminosities associated with star formation.

Moreover, the research found that the fraction of possible starburst galaxies, among the investigated quasars, increases dramatically from 31–38 to 63%, when it comes to the group of sources at highest redshifts.

According to the authors of the paper, this higher incidence of starbursts at higher redshifts means that it becomes more difficult to use the "radio-excess" method to find out whether or not the AGN is dominating the total radio emission in the system.

"As such, methods for selecting and analyzing AGN will need to be considered more carefully," the scientists conclude.