Cosmic evolution of low-excitation radio galaxies in the LOFAR Two-meter Sky Survey Deep Fields

TL;DR

This study investigates the evolution of low-excitation radio galaxies (LERGs) using LOFAR survey data, revealing their changing populations and luminosity functions up to redshift 2.5, and highlighting differences based on host galaxy type.

Contribution

First measurement of LERG radio luminosity function evolution up to z~2.5, distinguishing between quiescent and star-forming host galaxies and revealing a new dominant population at high redshift.

Findings

LERGs show mild evolution in luminosity function up to z~2.5.

Star-forming host LERGs become dominant at high redshift.

Incidence of LERGs in quiescent galaxies depends strongly on stellar mass.

Abstract

Feedback from low-excitation radio galaxies (LERGs) plays a key role in the lifecycle of massive galaxies in the local Universe; their evolution, and the impact of these active galactic nuclei on early galaxy evolution, however, remain poorly understood. We use a sample of 10481 LERGs from the first data release of the LOFAR Two-meter Sky Survey Deep Fields, covering $\sim$ 25 deg$^2$, to present the first measurement of the evolution of the radio luminosity function (LF) of LERGs out to $z\sim2.5$; this shows relatively mild evolution. We split the LERGs into those hosted by quiescent and star-forming galaxies, finding a new dominant population of LERGs hosted by star-forming galaxies at high redshifts. The incidence of LERGs in quiescent galaxies shows a steep dependence on stellar-mass out to $z \sim1.5$, consistent with local Universe measurements of accretion occurring from cooling of hot gas haloes. The quiescent-LERGs dominate the LFs at $z<1$, showing a strong decline in space density with redshift, tracing that of the available host galaxies, while there is an increase in the characteristic luminosity. The star-forming LERG LF increases with redshift, such that this population dominates the space densities at most radio-luminosities by $z \sim 1$. The incidence of LERGs in star-forming galaxies shows a much weaker stellar-mass dependence, and increases with redshift, suggesting a different fuelling mechanism compared to their quiescent counterparts, potentially associated with the cold gas supply present in the star-forming galaxies.