Highly-accreting quasars and nuclear and host galaxy star formation

We report on a multi-wavelength study of extreme Population A quasars (xA) — high–Eddington ratio AGN marked by extremely strong optical Fe II — to identify the physical processes powering their radio emission. The construction of a spectral energy distribution (SED) for a sample of ~ 150 xA quasars confirms the presence of a pronounced Big Blue Bump and steep soft X-ray slope consistent with high accretion rates. Additionally, we revealed enhanced far-infrared and radio components indicating a predominance of emission ultimately associated with star formation. To further explore this results, we obtained new VLA L-, C-, and X-band observations, complemented by LOFAR, VLASS, and mid-IR data, and we constructed radio spectra spanning 0.15–10 GHz for a sample of 18 xA quasars. Approximately one-third of these quasars exhibit steep, unbroken power-law spectra consistent with synchrotron emission and FIR power from star formation; another third show composite SEDs while only a small fraction display signatures of compact jets. These results demonstrate that star formation can dominate the radio emission even at radio powers typically attributed to AGN jets. The concurrent presence of enhanced star formation and extreme accretion appears to be a hallmark of a spectral type along the quasar Eigenvector 1 sequence distinguished by extreme metal abundances and high-velocity outflows in both broad and narrow emission lines.