🔭 astrophysics

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

This paper presents the discovery and spectroscopic confirmation of 39 high-redshift (z>3.5) radio quasars identified through a multi-wavelength survey combining eROSITA X-ray, DECam optical/NIR, and ASKAP radio data, which significantly expands the known population of z>4 radio quasars and provides a valuable sample for studying early Universe jet and accretion evolution.

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

Published 2026-08-06

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CC BY 4.0

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

Original paper licensed under CC BY 4.0 (http://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

Imagine the universe as a giant, bustling city where most buildings are quiet, steady houses. But scattered throughout are a few wild, neon-lit skyscrapers that shoot massive beams of light and energy straight at us. These are called blazars. They are a special type of super-massive black hole at the center of a galaxy, but with a twist: their "jets" of energy are pointed directly at Earth, like a laser pointer aimed right into your eye. Because they are aimed at us, they look incredibly bright and powerful, even though they are billions of light-years away.

Now, imagine trying to find these skyscrapers when the city is very young, just a few hundred million years after the Big Bang. This is the "high-redshift" universe, a time when everything was different. Scientists have a theory about why these ancient blazars might be even brighter than their modern cousins. They think that as the universe expands, the background "fog" of leftover heat from the Big Bang (called the Cosmic Microwave Background) gets denser and hotter. When the fast-moving electrons in a blazar's jet crash into this denser fog, they get a massive energy boost, making the blazar shine even brighter in X-rays. It's like a surfer catching a wave that gets bigger and bigger the further out they go. The big question is: does this actually happen? Do we see more of these super-bright ancient blazars than we expect, or is our theory wrong?

This paper is like a cosmic treasure hunt. A team of astronomers used three different "flashlights" to scan a huge chunk of the sky: one for X-rays (the eRASS:1 survey), one for radio waves (the RACS survey), and one for visible and infrared light (the DELVE survey). By combining these views, they built a list of 68 new candidates for these ancient, high-speed black holes. They then pointed powerful telescopes at 46 of these candidates to take their "fingerprints" (spectra) and confirm how far away they really are.

The hunt was a huge success. They confirmed 39 new quasars (the parent galaxies of blazars) with redshifts greater than 3.5, and 14 of them are at redshifts greater than 4. This discovery is a big deal because it increases the number of known radio quasars at these extreme distances by about 65% in that specific area of the sky. It's as if they found a whole new neighborhood of skyscrapers that nobody knew existed.

When the team looked closely at these new finds, they found something fascinating. These ancient blazars are incredibly bright in X-rays compared to their visible light. This matches the prediction that the "fog" of the early universe is boosting their energy. The authors compared their new list of blazars to a mathematical model that predicts how many should exist if this "fog boost" (called the IC/CMB effect) is real. The results show a pretty good match, suggesting the model is on the right track. However, the match isn't perfect; there are slightly more blazars at the very highest distances than the model predicts, and the team admits they might have missed some faint ones because they couldn't get a clear "fingerprint" for every single candidate.

The paper also explicitly rules out a simpler idea: that the ratio of X-ray to radio light stays the same no matter how far back in time you look. The data shows that the X-rays get much brighter relative to the radio waves as we look further back, which means the "fog boost" is definitely happening. While the authors are confident that the boost is real, they are careful to say that the exact numbers might shift a bit once they get more data and confirm the remaining candidates. They have found the most extreme, high-energy systems in the early universe, and they are now the perfect test subjects to help us understand how black holes and their jets evolved when the cosmos was young.

Technical Summary: The Most Extreme High- $z$ Radio Quasars in the eRASS:1 X-ray Survey

Problem and Motivation
Blazars, a subset of jetted active galactic nuclei (AGN) with relativistic jets oriented close to the line of sight, serve as unique probes for studying relativistic jet emission and the statistical evolution of jetted AGN populations. A key theoretical driver for studying high-redshift ( $z > 4$ ) blazars is the Inverse Compton scattering of Cosmic Microwave Background (CMB) photons by jetted electrons (IC/CMB). As CMB energy density scales with $(1+z)^4$ , this mechanism is predicted to significantly enhance the X-ray luminosity of high- $z$ blazars relative to their optical emission. However, existing statistical studies are limited by the scarcity of known high- $z$ blazars and the difficulty in disentangling observational biases from genuine population evolution. Previous detections, such as those by SWIFT-BAT, have been sparse, and existing samples from the first eROSITA All-Sky Survey (eRASS:1) often suffer from heterogeneous selection functions, hindering robust statistical analysis.

Methodology
The authors present a multi-wavelength selection strategy to identify high- $z$ radio quasars in the German half of the eRASS:1 survey (covering $\sim$ 14,000 deg $^2$ ). The methodology proceeds in four stages:

Cross-Matching: The team cross-matched the eRASS:1 point-source X-ray catalogue with the Rapid ASKAP Continuum Survey (RACS-mid) radio catalogue at 1.37 GHz. To minimize spurious associations, they employed flux-dependent matching radii (ranging from 3.25 $^{\prime\prime}$ to 14.45 $^{\prime\prime}$ ) and restricted the search to extragalactic regions ( $|b| > 20^\circ$ ).
Optical/NIR Photometric Selection: The resulting X-ray/radio sources were cross-matched with the DECam Local Volume Exploration (DELVE) survey. The authors applied a Ly $\alpha$ dropout technique to select candidates in the redshift range $3.6 < z < 6.4$ , utilizing color cuts in $g-r$ , $r-i$ , and $i-z$ bands. They further refined the sample by incorporating data from Gaia, Pan-STARRS, DES, SkyMapper, and IR surveys (WISE, VHS, VIKING, CatWISE) to filter out low-redshift contaminants and variable systems.
Spectral Energy Distribution (SED) Screening: Visual inspection of optical-to-NIR SEDs was performed to exclude sources consistent with low- $z$ elliptical galaxies, reducing the candidate list from 486 to 68.
Spectroscopic Follow-up: The authors conducted a spectroscopic campaign (October 2024 – January 2026) using multiple telescopes (TNG, LBT, NTT, Gemini South, Magellan Clay) covering the $\sim$ 5000–9000 Å range to confirm redshifts.

Key Results

Sample Confirmation: Of the 46 candidates observed spectroscopically, 39 were confirmed as high-redshift quasars ( $z > 3.5$ ).
Population Increase: Among the confirmed sources, 14 are at $z > 4$ . This represents a $\sim$ 65% increase in the known population of $z > 4$ radio quasars within the same flux limits and sky area covered by eRASS:1.
Source Properties: The selected sources occupy the bright X-ray and UV end of the general quasar distribution. They exhibit high X-ray-to-UV luminosity ratios ( $\alpha_{ox}$ ), lying significantly above the mean relation for non-blazar quasars.
Blazar Identification: Using the X-ray-to-UV intensity ratio criterion ( $\tilde{\alpha}_{ox} < 1.355$ ) proposed by Ighina et al. (2019), the authors identified 26 blazars in their sample, 14 of which are at $z > 4$ .
Model Comparison: The observed number of blazars as a function of redshift was compared against predictions from a fractional IC/CMB model (Ighina et al. 2021) and a constant X-ray-to-radio luminosity ratio model.
- The data rule out a scenario where the X-ray-to-radio ratio does not evolve with redshift; the observed counts at $z \sim 4$ are $\sim$ 5 times higher than the constant-ratio prediction.
- The observations show "relatively good agreement" with the IC/CMB model, though the total number of observed $z > 4$ blazars is slightly higher than the model prediction (a $\sim$ 2.6 $\sigma$ discrepancy).
- The model over-predicts the number of blazars at $z > 4.7$ , though this is limited by very low statistics (only two sources detected).

Significance and Claims
The paper claims that the identified sample represents the most extreme high- $z$ systems currently known, well-suited for investigating the evolution of relativistic jets and accretion properties in the early Universe. The authors assert that their results provide strong evidence that the X-ray properties of high- $z$ blazars evolve with redshift, consistent with the IC/CMB mechanism, while noting that the slight excess in observed counts may be due to incompleteness in spectroscopic follow-up or classification biases.

The authors remain modest regarding the definitive nature of their blazar classification, acknowledging that without dedicated VLBI or deeper X-ray observations, individual classifications rely on statistical proxies. They conclude that once the full candidate sample is spectroscopically confirmed and deeper eRASS scans become available, significantly tighter constraints can be placed on the cosmological evolution of blazars and alternative models for their high-energy emission.

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