The environments of z~1 Active Galactic Nuclei at 3.6um

TL;DR

This study analyzes the environments of z~1 AGN using stacked Spitzer 3.6um data, revealing significant galaxy over-densities around AGN and a correlation between environment density and radio luminosity, suggesting environmental influence on AGN activity.

Contribution

It provides the first large-scale analysis of AGN environments at z~1 across different AGN types and luminosities, highlighting the link between environment density and radio emission.

Findings

2-3 massive galaxies within 200-300 kpc of AGN

Environmental source density increases with radio luminosity

Over-density more significant in radio-loud AGN fields

Abstract

We present an analysis of a large sample of AGN environments at z~1 using stacked Spitzer data at 3.6um. The sample contains type-1 and type-2 AGN in the form of quasars and radio galaxies, and spans a large range in both optical and radio luminosity. We find, on average, that 2 to 3 massive galaxies containing a substantial evolved stellar population lie within a 200-300 kpc radius of the AGN, constituting a >8-sigma excess relative to the field. Secondly, we find evidence for the environmental source density to increase with the radio luminosity of AGN, but not with black-hole mass. This is shown first by dividing the AGN into their classical AGN types, where we see more significant over-densities in the fields of the radio-loud AGN. If instead we dispense with the classical AGN definitions, we find that the source over-density as a function of radio luminosity for all our AGN exhibits a positive correlation. One interpretation of this result is that the Mpc-scale environment is in some way influencing the radio emission that we observe from AGN. This could be explained by the confinement of radio jets in dense environments leading to enhanced radio emission or, alternatively, may be linked to more rapid black-hole spin brought on by galaxy mergers.