Suppression of Star Formation in the Hosts of Low-Excitation Radio Galaxies

TL;DR

This study investigates how radio-loud active galactic nuclei (R-AGN) influence star formation in their host galaxies, finding suppression mainly in low-mass, non-cluster R-AGN, with high-excitation sources showing no suppression but mid-IR excess.

Contribution

It provides the first systematic analysis of the spectral energy distributions of R-AGN compared to inactive controls, revealing environment-dependent star formation suppression.

Findings

UV and mid-IR emission suppressed in non-cluster R-AGN hosts

Suppression is prominent in low-mass, non-cluster galaxies

High-excitation R-AGN show no star formation suppression but have mid-IR excess

Abstract

The feedback from radio-loud active galactic nuclei (R-AGN) may help maintain low star formation (SF) rates in their early-type hosts, but the observational evidence for this mechanism has been inconclusive. We study systematic differences of aggregate spectral energy distributions (SEDs) of various subsets of $\sim$4000 low-redshift R-AGN from Best & Heckman (2012) with respect to (currently) inactive control samples selected to have matching redshift, stellar mass, population age, axis ratio, and environment. Aggregate SEDs, ranging from the ultraviolet (UV) through mid-infrared (mid-IR, 22 $\mu$m), were constructed using a Bayesian method that eliminates biases from non-detections in GALEX and WISE. We study rare high-excitation sources separately from low-excitation ones, which we split by environment and host properties. We find that both the UV and mid-IR emission of non-cluster R-AGNs (80% of sample) are suppressed by $\sim$0.2 dex relative to that of the control group, especially for moderately massive galaxies (log $M_* \lesssim$ 11). The difference disappears for high-mass R-AGN and for R-AGN in clusters, where other, non-AGN quenching/maintenance mechanisms may dominate, or where the suppression of SF due to AGN may persist between active phases of the central engine, perhaps because of the presence of a hot gaseous halo storing AGN energy. High-excitation (high accretion rate) sources, which make up 2% of the R-AGN sample, also show no evidence of SF suppression (their UV is the same as in controls), but they exhibit a strong mid-IR excess due to AGN dust heating.