Radio-AGN Feedback: When the Little Ones were Monsters

TL;DR

This study investigates how the fraction of radio-loud AGN varies with galaxy mass and redshift, revealing a significant increase in lower mass galaxies at high redshift, which impacts galaxy evolution models.

Contribution

It provides new observational evidence of the evolving radio-loud AGN fraction across different galaxy masses and redshifts, highlighting the role of cold gas supply in AGN activity.

Findings

Significant increase in radio-loud AGN fraction in low-mass galaxies at z ~ 1-2.

The radio-loud fraction for high-mass galaxies remains unchanged over redshift.

The increase is linked to higher cold gas supply and radiative mode accretion at earlier epochs.

Abstract

We present a study of the evolution of the fraction of radio-loud active galactic nuclei (AGN) as a function of their host stellar mass. We make use of two samples of radio galaxies: one in the local universe, $0.01 < z < 0.3$, using a combined SDSS-NVSS sample and one at higher redshifts, $0.5 < z < 2$, constructed from the VLA-COSMOS_DEEP Radio Survey at 1.4 GHz and a K$_s$-selected catalogue of the COSMOS/UltraVISTA field. We observe an increase of more than an order of magnitude in the fraction of lower mass galaxies ($M_* < 10^{10.75}$ M$_{\odot}$) which host Radio-Loud AGN with radio powers $P_{1.4GHz} > 10^{24}$ W/Hz at z ~ 1-2 while the radio-loud fraction for higher mass galaxies ($M_* > 10^{11.25}$ M$_{\odot}$) remains the same. We argue that this increase is driven largely by the increase in cold or radiative mode accretion with increasing cold gas supply at earlier epochs. The increasing population of low mass Radio-Loud AGN can thus explain the upturn in the Radio Luminosity Function (RLF) at high redshift which is important for understanding the impact of AGN feedback in galaxy evolution.