Effect of the interactions and environment on nuclear activity

TL;DR

This study investigates how galaxy environment and interactions influence nuclear activity, revealing that dense environments suppress optical AGN but promote radio AGN, with galaxy interactions playing a significant role in fueling nuclear activity.

Contribution

It introduces a comprehensive analysis of environmental and interaction effects on nuclear activity using new parameters and statistical methods, highlighting the complex interplay between gas supply and galaxy surroundings.

Findings

Optical AGN are less common in dense environments.

Galaxy interactions increase the prevalence of optical and radio AGN.

Radio AGN are more frequent in dense environments and are influenced by galaxy interactions.

Abstract

We present a study of the prevalence of optical and radio nuclear activity with respect to the environment and interactions in a sample of SDSS galaxies. We defined a local density parameter and a tidal forces estimator and used a cluster richness estimator from the literature. The possible correlations between these parameters were removed using a principal component analysis. We applied a stratified statistical method that takes into account the effect of possible confounding factors like the galaxy mass. We found that the prevalence of optical AGN is a factor 2-3 lower in the densest environments, but increases by a factor of ~2 in the presence of strong one-on-one interactions. The importance of galaxy interactions decreases from star-forming nuclei (SFN) to Seyferts to LINERs to passive galaxies, in accordance with previous suggestions of an evolutionary time-sequence. The fraction of radio AGN increases strongly towards denser environments, and is enhanced by galaxy interactions. Overall, the results agree with a scenario in which the mechanisms of accretion into the black hole are determined by the presence and nature of a supply of gas, which in turn is controlled by the local density of galaxies and their interactions. A plentiful cold gas supply is required to trigger SFN, optical AGN and radiatively-efficient radio AGN. This is less common in the cold-gas-poor environments of groups and clusters, but is enhanced by one-on-one interactions which result in the flow of gas into nuclear regions; these two factors compete against each other. In the denser environments where cold gas is rare, cooling hot gas can supply the nucleus at a sufficient rate to fuel low-luminosity radiatively-inefficient radio AGN. However, the increased prevalence of these AGN in interacting galaxies suggests that this is not the only mechanism by which radiatively-inefficient AGN can be triggered.