# No strong radio absorption detected in the low-frequency spectra of radio-loud quasars at z > 5.6

**Authors:** A. J. Gloudemans, A. Saxena, H. Intema, J. R. Callingham, K. J. Duncan, H. J. A. Rottgering, S. Belladitta, M. J. Hardcastle, Y. Harikane, C. Spingola  
**arXiv:** [2309.03936](https://arxiv.org/abs/2309.03936)  
**Date:** 2023-09-07  
**Summary page:** https://www.summarizepaper.com/en/paper/2309.03936v1/

## Key points

- Study presents low-frequency radio spectra of nine high-redshift quasars (redshifts: 5.6 to 6.6)
- Data obtained from Giant Metre Radio Telescope band-3, -4, and -5 observations, as well as LOFAR (144 MHz) and VLA (1.4 and 3 GHz) data
- Five quasars in sample are recently discovered and among highest redshift radio bright quasars known at low frequencies
- Objective is to model radio spectra to investigate emission mechanism and determine age of radio jets
- Results show no strong SSA or FFA absorption between 144 MHz and a few GHz, except for blazar J0309+2717 at z=6.1
- Wide range of spectral indices observed (-1.6 to 0.05), including three potential ultra-steep spectrum quasars
- Additional VLBA data confirms turnover in J0309+2717's spectral energy distribution at rest frame frequency of 0.6–2.3 GHz (90–330 MHz observed frame), indicating radiative aging in electron population within its radio lobes
- Ultra-low frequency data below 50 MHz necessary to determine absorption mechanism for J0309+2717 and accurately measure turnover frequencies for other high redshift quasars in sample
- Relation between linear radio jet size and turnover frequency established at low redshifts; if holds at high redshifts, suggests sizes must be more extended than typically observed in other radio bright quasars at similar redshifts

## Summary

In this study, the authors present the low-frequency radio spectra of nine high-redshift quasars with redshifts ranging from $5.6$ to $6.6$. The data was obtained using the Giant Metre Radio Telescope band-3, -4, and -5 observations ($\sim$300-1200 MHz), as well as archival Low Frequency Array (LOFAR; 144 MHz) and Very Large Array (VLA; 1.4 and 3 GHz) data. Five of the quasars in their sample were recently discovered and are among the highest redshift radio bright quasars known at low frequencies. The main objective of this study is to model the radio spectra of these high-redshift quasars in order to investigate their radio emission mechanism and determine the age of their radio jets. The authors achieve this by constraining the spectral turnover caused by synchrotron self-absorption (SSA) or free-free absorption (FFA). The results show that, apart from one blazar called J0309+2717 at $z=6.1$, none of the quasars exhibit a spectral flattening between 144 MHz and a few GHz, indicating that there is no strong SSA or FFA absorption within this frequency range. However, a wide range of spectral indices ranging from $-1.6$ to $0.05$ is observed, including the discovery of three potential ultra-steep spectrum quasars. To further analyze J0309+2717, additional archival VLBA data is used which confirms that its radio spectral energy distribution likely turns over at a rest frame frequency of 0.6–2.3 GHz (90–330 MHz observed frame). This turnover is indicative of radiative aging in the electron population within its radio lobes. The authors emphasize that ultra-low frequency data below 50 MHz is necessary to determine the absorption mechanism for J0309+2717 and to accurately measure the turnover frequencies for other high redshift quasars in their sample. They also mention that a relation between linear radio jet size and turnover frequency has been established at low redshifts; if this relation holds at high redshifts then limits on turnover frequency suggest that sizes must be more extended than what is typically observed in other radio bright quasars at similar redshifts.