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

Abstract

We investigate the stellar kinematics and stellar populations of 58 radio-loud galaxies of intermediate luminosities (L${}_{3GHz}$ $>$ 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: ${\sigma }_{\ast }>$ 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.

10 pages, 8 figures, 1 table. Accepted for publication in The Astrophysical Journal

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