Quasars, or short for quasi-stellar objects, are intensely luminous cores of distant galaxies powered by supermassive black holes. These objects are not only some of the brightest in the universe but also essential to our understanding of galaxy evolution and the early cosmos.
Emitting energy across the electromagnetic spectrum—from radio waves to X-rays—quasars play a key role in both astrophysics and cosmology.
One of the most ambitious efforts to study such objects is the MIGHTEE (MeerKAT International GHz Tiered Extragalactic Exploration) survey, conducted with the MeerKAT radio telescope in South Africa. The project aims to produce deep, high-resolution maps of the extragalactic sky to study the formation and evolution of galaxies, including active galactic nuclei like quasars.
Rhodes-led team explores radio signals from distant quasars
A team of astronomers led by Sarah V. White from Rhodes University recently analyzed data from the MIGHTEE survey to study a specific type of quasar known as Type 1—unobscured quasars that show broad emission lines.
In total, they examined 104 of these distant objects, located in the COSMOS and XMM-LSS regions of the sky. By using deep radio data and a wide range of multiwavelength observations, the researchers were able to better understand the differences between radio-loud and radio-quiet quasars. Their focus was on separating the radio signals coming from black hole activity and from star formation in the host galaxies.
The researchers found that the quasars in their sample span a wide range of distances, with redshifts between 0.6 and 3.41—placing some of them billions of light-years away. The median redshift was calculated to be around 1.68, indicating that most of these quasars are seen as they were when the universe was less than half its current age.
One of the key findings was that only about 5% of the quasars were classified as radio-loud, meaning they emit strong radio waves, a notion that aligns well with results from earlier studies, reinforcing the idea that most quasars are relatively quiet in the radio spectrum.
Sensitive radio data reveals more star-forming quasars
The study also found that a smaller fraction of the quasars had radio emissions primarily driven by active galactic nuclei (AGN) compared to previous analyses of the broader MIGHTEE sample. The researchers suggest this difference may be due to the enhanced sensitivity of their radio observations, which reach lower flux-density levels.
This allows for the detection of fainter radio sources, including those where the emission is more likely tied to star formation rather than AGN activity. As a result, the study provides a more nuanced view of the radio properties of quasars, especially at lower luminosities.
A striking trend also emerged: among the quasars examined, the proportion classified as potential starburst galaxies rises significantly with redshift—from about 31–38% at lower redshifts to 63% among the most distant sources.
The researchers point out that this rise in starburst activity complicates the use of the “radio-excess” method, which is typically used to determine whether an AGN is the main contributor to a galaxy’s radio emission. As a result, the authors note that future approaches to identifying and analyzing AGN will need to be more refined and sensitive to these overlapping contributions.