Radio Quasars in the Cosmic Dawn: A Spectroscopic Sample from the DESI Survey

Anniek J. Gloudemans, Jinyi Yang, David M. Alexander, Arjun Dey, Xiaohui Fan, Victoria A. Fawcett, Martin J. Hardcastle, Stephanie Juneau, George K. Miley, Leah K. Morabito, Adam D. Myers, Swayamtrupta Panda, Huub J. A. Röttgering, Timothy Shimwell, Daniel J. B. Smith, Feige Wang, J. Aguilar, S. Ahlen, D. Bianchi, D. Brooks, T. Claybaugh, A. Cuceu, A. de la Macorra, P. Doel, S. Ferraro, J. E. Forero-Romero, E. Gaztañaga, S. Gontcho A Gontcho, G. Gutierrez, C. Hahn, M. Ishak, R. Joyce, R. Kehoe, D. Kirkby, T. Kisner, A. Kremin, M. Landriau, L. Le Guillou, M. E. Levi, M. Manera, A. Meisner, R. Miquel, J. Moustakas, S. Nadathur, W. J. Percival, F. Prada, I. Pérez-Ràfols, A. Robertson, G. Rossi, E. Sanchez, E. F. Schlafly, D. Schlegel, J. Silber, D. Sprayberry, G. Tarlé, B. A. Weaver, R. Zhou, H. Zou

First listed 2026-09-24 | Last updated 2026-09-22

Abstract

We present the largest statistical spectroscopic sample of radio quasars at 4.7<z<6.7 to date. These have been selected from a dedicated Dark Energy Survey Instrument (DESI) target program and provide a robust sample to study the influence of radio jets on early quasar evolution. Using low-frequency radio data from the LOFAR Two Metre Sky Survey Data Release 3 (LoTSS-DR3) at 144 MHz, we identify 83 high-z radio quasars, 26 of which are classified as radio-loud (5-15% of 646 confirmed high-z quasars, defined as $R = F_{5\text{GHz}}/F_{4400A} >10$). With radio luminosities of $\sim10^{25-28}$ W/Hz, the radio emission in all 83 quasars is expected to be jet-dominated rather than powered by star formation. Analysis of their rest-frame ultra-violet (UV) spectra reveals broadly similar composite spectral properties and CIV velocity shifts compared to radio non-detected quasars. The radio-detected and radio-loud quasars exhibit slightly smaller Ly$α$ and CIV equivalent widths on average, however, we find no significant evidence ($\sim1.3σ$) for a higher fraction of weak-line quasars (WLQs; ~10+/-4% versus ~4+/-1% for radio non-detected quasars). We further find that the broad absorption line (BAL) quasars are predominantly radio-quiet, and no correlation is observed between absorption index and radio-loudness, suggesting that radio emission is not produced by the BAL outflows themselves and are physically distinct phenomena. Our results demonstrate that powerful radio jets were already well-established by z~5.

Short digest

Gloudemans et al. assemble the largest spectroscopic sample of radio quasars at 4.7<z<6.7, matching 646 DESI-confirmed quasars to 144 MHz LoTSS-DR3 data and identifying 83 radio detections, including 26 radio-loud objects. Their 10^25-10^28 W Hz^-1 radio luminosities imply jet-dominated emission, establishing that powerful jets were already in place by z~5. Rest-UV composites and C IV velocity shifts are broadly indistinguishable from radio non-detections, although radio-detected sources have modestly smaller Lyα and C IV equivalent widths and no significant excess of weak-line quasars. BAL quasars are mainly radio-quiet, with no absorption-index--radio-loudness correlation, favoring physically distinct jet and BAL-outflow phenomena.

Key figures to inspect

• Figure 2. This is the core sample-definition figure: it places the DESI-LoTSS quasars in radio-loudness--redshift space, shows the R>10 threshold, and demonstrates the substantial expansion of the known z>4.7 radio-quasar population.
• Figure 5. This figure directly quantifies the weak-line-quasar comparison across radio-loud, radio-quiet, and pure subsamples, making clear that the apparent WLQ difference is limited by small-number uncertainties rather than a significant demographic excess.
• Figure 6. This is the key diagnostic for the paper's BAL conclusion, showing that BAL quasars avoid the extreme radio-loud tail and that C IV absorption strength does not significantly track radio loudness.
• Figure 10. The composite-spectrum comparison provides the strongest visual synthesis of the UV result: residuals between radio-detected versus non-detected and radio-loud versus radio-quiet stacks remain within 3σ, supporting broadly similar rest-UV spectral properties.