Building a Radio AGN Sample from Cosmic Morning -- The Radio High-Redshift Quasar Catalog (RHzQCat): I. Catalog from SDSS Quasars and Radio Surveys at $z > 3$

TL;DR

The paper addresses the gap in understanding radio-loud quasars during the cosmic morning ($3 \lesssim z \lesssim 5$) by constructing the largest, uniformly selected catalog (RHzQCat) through cross-matching SDSS DR16Q with four major radio surveys (FIRST, NVSS, RACS, GLEAM) using a multi-tier cross-matching framework and extensive visual validation. It delivers 2388 matched sources, of which 1629 are robust RHRQs, 315 are candidates, and 444 are rejected, spanning $3 \le z \le 5.5$ and rest-frame $L_{1.4\mathrm{GHz}}$ from $10^{25.5}$ to $10^{29.3}$ W Hz$^{-1}$. The majority are compact ($\sim$95%), with a notable subset of extended RGs (110 RGs or RG candidates) and a single dual AGN, illustrating the method's capability to capture diverse high-$z$ radio phenomena. The catalog serves as a critical resource for jet evolution, AGN feedback, and magnetism studies and provides a solid benchmark for upcoming SKA-era surveys, while explicitly acknowledging selection biases and the need for a modeled selection function for population analyses.

Abstract

Radio-loud high-redshift quasars (RHRQs) provide crucial insights into the evolution of relativistic jets and their connection to the growth of supermassive black holes. Beyond the extensively studied population at $z \ge 5$, the cosmic morning epoch ($3 \lesssim z \lesssim 5$) marks the peak of active galactic nucleus (AGN) activity and black hole accretion, yet remains relatively unexplored. In this work, we compiled the radio high-redshift quasar catalog (RHzQCat) by cross-matching the SDSS DR16Q catalog with four major radio surveys -- FIRST,NVSS, RACS, and GLEAM. Our tier-based cross-matching framework and visual validation ensured reliable source identification across surveys with diverse beam sizes. The catalog included 1629 reliable and 315 candidate RHRQs, with radio luminosities uniformly spanning $10^{25.5}$ -- $10^{29.3}$ W Hz$^{-1}$. About 95\% of the confirmed sources exhibited compact morphologies, consistent with Doppler-boosted or young AGN populations at high redshifts. Our catalog increases the number of known RHRQs at $z\ge3$ by an order of magnitude, representing the largest and most homogeneous catalog of radio quasars at cosmic morning, filling the observational gap between the early ($z>6$) and local Universe. It provides a robust reference for future statistical studies of jet evolution, AGN feedback, and cosmic magnetism with next-generation facilities such as the Square Kilometer Array (SKA).

Figures (9)

• Figure S1: Examples of SDSS spectra for different z_reliable_flag values in our sample. The black lines show the observed SDSS spectra, with their $1\sigma$ uncertainties indicated in blue. Blue dashed and red dotted lines mark the expected wavelengths of prominent emission lines at the redshifts determined by the SDSS pipeline and our visual inspection, respectively. Top left: example source with good redshift (z_reliable_flag = 1) showing agreement with our visual inspection; top right: example source with candidate high-redshift feature (z_reliable_flag = 2), showing consistency with our visual inspection; bottom left: another example source with z_reliable_flag = 2, but showing a discrepancy with our visual inspection; bottom right: example of unreliable redshifts with z_reliable_flag = 3, where no predictable redshift can be made from our visual inspection.
• Figure S2: The redshift distribution histogram of the matched catalog, with sources grouped by their redshift reliability flags.
• Figure S3: Example overlay plots of the good sample. In each panel, the background grayscale image is the optical SDSS image cutout, while the colored contours indicate the radio maps from different surveys (light blue: FIRST; green: NVSS; royal blue: RACS; orange-red: GLEAM). The contours start at the $3\sigma$ level of each survey and increase by successive factors of 2. For weak sources with fewer than three contour levels, an additional contour at $\sqrt{2}$ times the base level is inserted between the first and second contours. The inclined red cross at the image center marks the target HRQ from DR16Q. Brown crosses indicate low-redshift galaxies within the radio beam, and dark red crosses indicate low-redshift quasars within the beam. At the top of each panel, the source identification codes from the respective radio catalogs are displayed, with font colors corresponding to the contour colors.
• Figure S4: Example overlay images of the candidate sample. The explanation of the plots is the same as for Figure \ref{['fig:overlay_good']}.
• Figure S5: Example overlay images of the rejected sample. The explanation of the plots is the same as for Figure \ref{['fig:overlay_good']}.