Prospects for Observing High-redshift Radio-loud Quasars in the SKA Era: Paving the Way for 21-cm Forest Observations

TL;DR

This paper forecasts the potential for detecting high-redshift radio-loud quasars with SKA to enable 21-cm forest studies, highlighting the importance of quasar abundance and radio-loud fraction evolution.

Contribution

It introduces a physics-driven model for predicting high-redshift radio-loud quasar abundance and assesses SKA's capability to detect them for 21-cm forest observations.

Findings

Average quasar lifetime at z~6 is about 200,000 years.

Evolving radio-loud fraction reduces observable quasars significantly.

SKA-LOW could detect around 20 quasars at z~9 in one year.

Abstract

The 21-cm forest is a sensitive probe for the early heating process and small-scale structures during the epoch of reionization (EoR), to be realized with the upcoming Square Kilometre Array (SKA). Its detection relies on the availability of radio-bright background sources, among which the radio-loud quasars are very promising, but their abundance during the EoR is still poorly constrained due to limited observations. Here, we use a physics-driven model to forecast future radio-loud quasar observations. We fit the parameters of the model using observational data of high-redshift quasars. Assuming Eddington accretion, the model yields an average lifetime of $t_{\rm q} \sim 10^{5.3}$yr for quasars at $z\sim6$, consistent with recent results obtained from quasar proximity zone pre-study. We show that if the radio-loud fraction of quasars evolves with redshift, it will significantly reduce the abundance of observable radio-loud quasars in the SKA era, making 21-cm forest studies challenging. With a constant radio-loud fraction, our model suggests that a one-year sky survey conducted with SKA-LOW has the capability to detect approximately 20 radio-loud quasars at $z\sim 9$, with sufficient sensitivity to resolve individual 21-cm forest lines.