# The nature of faint radio galaxies at high redshifts

## TL;DR

This study investigates faint high-redshift radio galaxies, revealing their lower luminosities, stellar masses, and different ionization properties compared to brighter counterparts, and explores their evolution and role in galaxy formation.

## Contribution

It provides the first detailed analysis of faint HzRGs at high redshifts, including redshift measurements, stellar mass estimates, and insights into their ionization and evolutionary states.

## Key findings

- Faint HzRGs have lower Lyα luminosities and narrower emission lines.
- Stellar masses of faint HzRGs are lower than those of brighter ones.
- Faint HzRGs at z>4 are dominated by star formation and cold gas accretion.

## Abstract

We present spectra and near-infrared images of a sample of faint radio sources initially selected as promising high-redshift radio galaxy (HzRG) candidates. We have determined redshifts for a total of 13 radio galaxies with redshifts ranging from $0.52\le z \le 5.72$. Our sample probes radio luminosities that are almost an order of magnitude fainter than previous large samples at the highest redshifts. We use near-infrared photometry for a subsample of these galaxies to calculate stellar masses using simple stellar population models, and find stellar masses to be in the range $10^{10.8} - 10^{11.7} M_\odot$. We then compare our faint radio galaxies with brighter radio galaxies at $z\ge2$ from the literature. We find that fainter radio galaxies have lower Ly$\alpha$ luminosities and narrower full width at half maxima compared to the bright ones, implying photoionisation by weaker AGN. We also rule out the presence of strong shocks in faint HzRGs. The stellar masses determined for faint HzRGs are lower than what has been observed for brighter ones. We find that faint HzRG population in the redshift range $2-4$ forms a bridge between star-forming and narrow-line AGN, whereas the ones at $z>4$ are dominated by star-formation, and may be building up their stellar mass through cold accretion of gas. Finally, we show that the overall redshift evolution of radio sizes at $z>2$ is fully compatible with increased inverse Compton scattering losses at high redshifts.