New Radio-Loud QSOs at the end of the Re-ionisation Epoch

TL;DR

This paper reports the discovery of two high-redshift radio-loud QSOs near the end of re-ionisation, suggesting a higher fraction of weak emission-line QSOs and providing insights into early universe jet activity.

Contribution

First identification of high-redshift radio-loud QSOs using combined radio and optical/near-infrared surveys, revealing potential prevalence of weak emission-line QSOs at z~6.

Findings

Confirmed two QSOs at z~6.1 with no prominent Lyα emission

Estimated space density of RL QSOs at z~6 consistent with lower-redshift trends

Suggests an increased fraction of weak emission-line QSOs at high redshift

Abstract

We present the selection of high-redshift ($z\gtrsim5.7$) radio-loud (RL) quasi-stellar object (QSO) candidates from the combination of the radio Rapid ASKAP Continuum Survey (RACS; at 888 MHz) and the optical/near-infrared Dark Energy Survey (DES). In particular, we selected six candidates brighter than $S_{\rm 888MHz}>1$ mJy beam$^{-1}$ and ${\rm mag}(z_\mathrm{{DES}})<21.3$ using the dropout technique (in the $i$-band). From this sample, we were able to confirm the high-$z$ nature ($z\sim6.1$) of two sources, which are now among the highest-redshift RL QSOs currently known. Based on our Gemini-South/GMOS observations, neither object shows a prominent Ly$\alpha$ emission line. This suggests that both sources are likely to be weak emission-line QSOs hosting radio jets and would therefore further strengthen the potential increase of the fraction of weak emission-line QSOs recently found in the literature. However, further multiwavelength observations are needed to constrain the properties of these QSOs and of their relativistic jets. From the discovery of these two sources, we estimated the space density of RL QSOs in the redshift range $5.9<z<6.4$ to be 0.13$^{+0.18}_{-0.09}$ and found it to be consistent with the expectations based on our current knowledge of the blazar population up to $z\sim5$.