The environments of active galactic nuclei at 3.6 μm

Abstract

We present an analysis of a large sample of active galactic nuclei (AGN) environments at z∼ 1 using stacked Spitzer data at 3.6 μm. 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 two to three massive galaxies containing a substantial evolved stellar population lie within a 200–300 kpc radius of the AGN, constituting a >8σ 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 overdensities in the fields of the radio-loud AGN. If instead we dispense with the classical AGN definitions, we find that the source overdensity 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 enhance radio emission or, alternatively, may be linked to more rapid black hole spin brought on by galaxy mergers.