Unveiling the weak radio quasar population at z $\geq$ 4

TL;DR

This study uses image stacking of radio survey data to detect faint radio emissions from high-redshift quasars, revealing a hidden population and providing insights into their star formation and active galactic nucleus activity.

Contribution

It introduces a stacking method to uncover faint radio signals in high-redshift quasars, estimating their radio power and star formation rates, and discusses the potential to distinguish emission origins with future observations.

Findings

Detected median radio flux density of 52 μJy in z≥4 quasars.

Estimated characteristic radio power around 10^24 W Hz^-1.

Set an upper limit on star formation rate at approximately 4200 M⊙ yr^-1.

Abstract

We applied image stacking on empty-field Faint Images of the Radio Sky at Twenty-Centimeters (FIRST) survey maps centred on optically identified high-redshift quasars at $z\geq4$ to uncover the hidden $\mu$Jy radio emission in these active galactic nuclei (AGN). The median stacking procedure for the full sample of $2229$ optically identified AGN uncovered an unresolved point source with an integrated flux density of 52$~\mu$Jy, with a signal-to-noise ratio $\sim10$. We co-added the individual image centre pixels to estimate the characteristic monochromatic radio power at $1.4~$GHz considering various values for the radio spectral index, revealing a radio population with $P_\mathrm{1.4GHz}\sim10^{24}~$W Hz$^{-1}$. Assuming that the entire radio emission originates from star-forming (SF) activity in the nuclear region of the host galaxy, we obtained an upper limit on the characteristic star formation rate, $\sim4200~$M$_\odot~$yr$^{-1}$. The angular resolution of FIRST images is insufficient to distinguish between the SF and AGN origin of radio emission at these redshifts. However, a comparison with properties of individual sources from the literature indicates that a mixed nature is likely. Future very long baseline interferometry radio observations and ultra-deep Square Kilometre Array surveys are expected to be sensitive enough to detect and resolve the central $1-10~$kpc region in the host galaxies, and thus discriminate between SF and AGN related emission.