RHzQCat: Radio High-Redshift Quasar Catalog

• RHzQCat is a comprehensive catalog defining radio-loud quasars at z>3 through systematic cross-matching of SDSS and multiple radio surveys.
• It employs tiered matching criteria and rigorous validation to yield over 1,600 reliable sources with detailed morphological and spectral data.
• The catalog underpins investigations of AGN evolution, relativistic jet dynamics, and black hole growth during the early cosmic epoch.

The Radio High-Redshift Quasar Catalog (RHzQCat) is a comprehensive resource for characterizing the population of radio-loud high-redshift quasars (RHRQs) at $z > 3$, leveraging multi-wavelength radio survey data cross-matched with optical identifications from the SDSS. RHzQCat is pivotal for investigating the evolution of relativistic jets, the connection to supermassive black hole accretion, and the nature of active galactic nuclei (AGN) during the "cosmic morning" epoch ($3 \lesssim z \lesssim 5$), an era marking the peak of AGN and black-hole growth yet previously underexplored (Zhang et al., 3 Dec 2025Sotnikova et al., 2021).

1. Construction and Cross-matching Framework

RHzQCat was built by systematically cross-matching spectroscopically confirmed quasars in the SDSS DR16Q catalog at $z \geq 3$ with four major radio surveys: FIRST (beam $\approx 5.4''$), NVSS (beam $\approx 45''$), RACS-DR1 (beam $\approx 25''$), and GLEAM (beam variable, up to $\approx 210''$). Individual survey beam sizes dictated the matching radii and morphological filters: 3$''$ (FIRST), 25$''$ (NVSS), 15$''$ (RACS), 110$''$ (GLEAM), each paired with constraints on source major axis or related parameters (Table 1 in (Zhang et al., 3 Dec 2025)).

A tier-based matching and flagging system classified associations:

• Tier 1: unambiguous single radio counterpart within the matching radius and beam-size filter.
• Tier 2: all matched radio components at $z \geq 3$ (multi-component, same-$z$).
• Tier 3: mixed-redshift group (contamination risk).

Visual validation further refined the sample using SDSS spectra for redshift-reliability (flag: $z_{\rm reliable\_flag}$), i-band plus radio overlays, and a codified morphology/positional scheme. Only sources with $z_{\rm reliable\_flag}=1$ (visual) or 2 (pipeline, $ZWARNING=0$) entered the reliable sample; candidates required at least FIRST-aligned compact emission, while those lacking central radio emission were rejected.

2. Radio and Optical Sample Properties

RHzQCat comprises 1,629 reliable and 315 candidate RHRQs after full validation, multiplying by an order of magnitude the known population at $z>3$. The redshift range is $3.0 \leq z \leq 5.5$, with a mean $z \approx 3.4$ and median $z \approx 3.3$ (Figure 1, (Zhang et al., 3 Dec 2025)). Radio luminosities at 1.4 GHz span $10^{25.5}$–$10^{29.3}$ W Hz$^{-1}$, calculated via

$L_\nu = 4\pi D_L^2 S_\nu (1+z)^{\alpha-1}$

(Equation 1 in (Zhang et al., 3 Dec 2025)), with $D_L$ the luminosity distance, $S_\nu$ the observed flux density (primarily from FIRST or NVSS), and $\alpha = 0.75$ the adopted spectral index.

Radio detection rates vary with $z$, showing a mild peak in the radio-optical detection fraction at $z \approx 4$ (Figure 2). The source count falls at $z > 4$ due to declining SDSS completeness.

3. Morphological and Spectral Characterization

A visual codebook (Table 5 in (Zhang et al., 3 Dec 2025)) was applied to all cross-matched sources. Among reliable RHRQs, $\sim$95% exhibit compact morphologies in FIRST ("U" or "U af" codes), strongly suggesting Doppler-boosted or young AGN at high redshift. Extended/jet morphologies are rare: only 64 radio galaxies in the reliable group show extended, core–jet, or Fanaroff–Riley (FRI/FRII) classes in FIRST or RACS; an additional 46 such candidates occur in the candidate group.

Spectroscopic and spectral data from RATAN-600 observations complement RHzQCat with six-frequency (1.2–22 GHz) flux densities for a flux-limited ($S_{1.4} \geq 100$ mJy) subset of 102 $z \geq 3$ quasars (Sotnikova et al., 2021). Spectral types are distributed as 46% peaked-spectrum (PS, incl. GPS and MPS), 24% flat, and 0% ultra-steep. The dominance of PS sources further evidences compact, core-dominated emission at these redshifts.

4. Catalog Structure and Public Release

The RHzQCat release consists of two principal machine-readable tables (Zhang et al., 3 Dec 2025):

• Classification table ("Tabs1:class1"): SDSS identifier, precise coordinates, category (good/candidate/rejected), redshift details, match tier, survey source names, separations, morphology/positional codes, and comments.
• Parameter table ("Tabs1:parm2"): Includes all above plus radio flux densities, uncertainties, and survey-specific quality metrics.

Overlay images (SDSS plus radio contours) and the full catalog are deposited at Zenodo (doi:10.5281/zenodo.17541810) and GitHub (https://github.com/ykzhang112233/highz-catalogue). RATAN-600 six-band spectra, indices ($\alpha_{\rm low}$, $\alpha_{\rm high}$), radio loudness ($R = S_{5\,{\rm GHz}}/S_{4400\,\text{Å}}$, typical $\log R \sim 3.5$ for most sources), and turnover frequencies are provided for the RATAN sample via VizieR and the BL Lac/Quasar database (Sotnikova et al., 2021).

Survey Contribution Table

Survey	Number (Total)	Reliable	Candidate	Rejected
FIRST	1,607	1,406	201	0
NVSS	1,614	1,218	257	139
RACS	955	805	129	21
GLEAM	370	91	54	225

5. Statistical Completeness and Reliability

The catalog's reliability is ensured through tiered cross-match criteria, redshift validation including visual-inspection and pipeline flags ($z_{\rm reliable\_flag}$), and strict morphological checks using independent radio survey overlays. Candidates exhibit at least partial alignment, while rejected sources lack robust central radio emission or possess unreliable redshifts ($z_{\rm reliable\_flag}=3$). This conservative validation, together with redshift and morphology analysis, yields a high-fidelity sample for statistical analysis (categories detailed in Sect. 3 and 2.2.2 of (Zhang et al., 3 Dec 2025)).

The RATAN-600 sample attains complete detection at 4.7 GHz, with declining completeness at higher frequencies (89% at 11.2 GHz, 46% at 22 GHz), providing quasi-simultaneous spectral energy distributions for cross-matched high-$z$ RHRQs (Sotnikova et al., 2021).

6. Implications for Astrophysics and Future Surveys

RHzQCat provides a baseline for studies of jet speeds, brightness temperatures, and population demographics of RHRQs. The dominance of compact morphologies supports interpretations of Doppler boosting or a youthful, dense AGN population. Extended FR I/II sources enable investigation of early intergalactic medium properties via lobe–CMB inverse-Compton scattering.

As the most uniform and extensive compilation at $3 \leq z \leq 5.5$, RHzQCat fills a significant observational gap between the early ($z>6$) and local Universe, offering a crucial training and cross-validation dataset for source-finding algorithms suited to next-generation radio arrays, such as SKA1-LOW and SKA1-MID. The catalog’s architecture and validation workflow directly inform strategies for large, multi-survey AGN census projects, including dual AGN identification and VLBI astrometry campaigns (Zhang et al., 3 Dec 2025).

7. Spectral Index, Radio Loudness, and Population Trends

Spectral indices ($\alpha$), measured at various frequency segments using RATAN-600 and literature data, reveal a median $\log R = 3.5$: the majority of high-redshift quasars in RHzQCat are strongly radio-loud ($\log R > 2.5$ in $>90\%$ of measured cases) (Sotnikova et al., 2021). No significant trend is observed between redshift and spectral index ($r_{\rm low}$ vs. $z = +0.07$, $r_{\rm high}$ vs. $z = +0.01$). The multifrequency approach also identifies new MPS and GPS candidates, highlighting a substantial fraction of core-dominated, potentially young or evolving jet systems in the early Universe.

Representative RATAN-600 Spectral Types (Sotnikova et al., 2021)

Type	Fraction (%)	Description
PS	46	Peaked-spectrum (incl. GPS/MPS)
Flat	24	$-0.5 \leq \alpha \leq 0$
Steep	15	$-1.1 < \alpha < -0.5$
Inv/Up	10	Inverted/Upturn/Complex
US	0	Ultra-steep ($\alpha \leq -1.1$)

These catalogs (RHzQCat by Zhang et al. (Zhang et al., 3 Dec 2025) and the RATAN-600 sample by Sotnikova et al. (Sotnikova et al., 2021)) together form the statistical foundation for investigations of AGN jet evolution, black hole growth, and the physical conditions of radio quasars at early cosmic epochs, and serve as public resources for the extragalactic community.