The most extreme high-z radio quasars in the eRASS:1 X-ray survey

L. Ighina, L. Palmieri, L. N. Martínez-Ramírez, A. Caccianiga, T. Connor, A. Moretti, B. Arsioli, Y. Beletsky, E. Marini, A. Rossi

First listed 2026-08-06 | Last updated 2026-08-04

Abstract

We present the selection and identification of high-z radio quasars from a combination of X-ray (eROSITA All-Sky Survey), optical/NIR (DECam Local Volume Exploration), and radio (Rapid ASKAP Continuum Survey) surveys. After building a sample of 68 new high-z quasar candidates, we performed follow-up spectroscopic observations on 46 sources, confirming 39 to have redshifts z>3.5. For the subset of radio quasars at z>4, the 14 new sources identified here represent an increase of $\sim$65% in the known population of objects above the same flux limits in the same area. Using X-ray-to-optical relative intensity, we identified blazars in our sample, which we then used to compare the total number of z>4, X-ray selected blazars to the predictions of a fractional IC/CMB model previously proposed in the literature. Overall we find a relatively good agreement, even considering the potential biases in the redshift distribution from the incompleteness of the spectroscopic follow-up and the blazar classification criteria. The sources presented in this work represent a sample well suited for investigating the evolution of the most extreme systems in the early Universe, including their relativistic jets and accretion properties

Short digest

Ighina et al. construct an eRASS:1-selected sample of extreme radio quasars by cross-matching eROSITA X-ray sources with RACS radio detections and DELVE optical/NIR dropout candidates, then spectroscopically following up 46 of 68 candidates. They confirm 39 quasars at z>3.5, including 14 new z>4 radio quasars, increasing the population above the same flux limits and area by about 65%. Their X-ray-to-optical and radio-loudness diagnostics identify a predominantly blazar-like subset, whose cumulative z>4 counts agree reasonably well with a fractional IC/CMB X-ray-enhancement model. The resulting uniformly selected sample provides a stronger basis for testing early jetted-SMBH growth and relativistic-jet evolution, although the redshift distribution remains affected by incomplete spectroscopy and blazar-classification choices.

Key figures to inspect

• Figure 2. This figure makes the multiwavelength selection tangible by showing the DELVE magnitude versus RACS radio surface-brightness plane, the colour-selected parent sample, and which targets were previously known, rejected, or spectroscopically observed. It also demonstrates that the optically and radio-bright candidates driving the survey yield were largely identified.
• Figure 4. This is the key physical diagnostic for the sample: the X-ray-to-optical relative intensity versus UV luminosity places the new quasars above the standard alpha_ox-L_2500 relation and alongside known high-redshift blazars. It directly motivates the interpretation that many selected sources are X-ray-enhanced jetted systems, while showing the overlap with highly accreting radio quasars.
• Figure 5. The redshift trends in X-ray-to-optical intensity and radio loudness show how the authors operationally identify blazar candidates using alpha_ox=1.355 and R=80 thresholds. It is the clearest figure for seeing that most new discoveries satisfy the adopted blazar-like criteria.
• Figure 6. This conclusion-driving comparison confronts the observed cumulative z>4 blazar counts with predictions for no X-ray-to-radio evolution and for fractional IC/CMB enhancement. The closer agreement with the IC/CMB track is the paper's central population-level result, while the high-redshift upper limit conveys the remaining small-number limitation.
• Figure 7. The discovery spectra provide the direct observational confirmation behind the new high-redshift sample, with expected redshifted emission-line positions marked for the newly identified quasars. This figure is useful as the spectroscopic anchor for the selection and population analysis.