Stellar Dynamics and Star-Formation Histories of z $\sim$ 1 Radio-loud Galaxies

TL;DR

This study analyzes the stellar kinematics and populations of 58 radio-loud galaxies at z~1, revealing their association with old, massive galaxies and higher radio-loud AGN fractions compared to the local universe.

Contribution

First measurement of stellar velocity dispersions and age indicators in z~1 radio galaxies, linking high black-hole masses with radio loudness and galaxy quiescence.

Findings

Radio-loud AGN at z~1 are in old, high-dispersion galaxies.

Fraction of radio-loud AGN at z~1 is 5-10 times higher than today.

Massive, quiescent galaxies at z~1 frequently host radio AGN.

Abstract

We investigate the stellar kinematics and stellar populations of 58 radio-loud galaxies of intermediate luminosities (L$_{3 GHz}$ $>$ 10$^{23}$ W Hz$^{-1}$ ) at 0.6 < z < 1. This sample is constructed by cross-matching galaxies from the deep VLT/VIMOS LEGA-C spectroscopic survey with the VLA 3 GHz dataset. The LEGA-C continuum spectra reveal for the first time stellar velocity dispersions and age indicators of z $\sim$ 1 radio galaxies. We find that $z\sim 1$ radio-loud AGN occur exclusively in predominantly old galaxies with high velocity dispersions: $\sigma_*>$ 175 km s$^{-1}$, corresponding to black hole masses in excess of $10^8$ M$_{\odot}$. Furthermore, we confirm that at a fixed stellar mass the fraction of radio-loud AGN at z $\sim$ 1 is 5 - 10 times higher than in the local universe, suggesting that quiescent, massive galaxies at z $\sim$ 1 switch on as radio AGN on average once every Gyr. Our results strengthen the existing evidence for a link between high black-hole masses, radio loudness and quiescence at z $\sim$ 1.