Dichotomy in host environments and signs of recycled AGN

TL;DR

This study investigates how active galactic nuclei (AGN) host properties relate to their large-scale environments, revealing environmental influences on galaxy activity and morphology, especially distinguishing red and blue host galaxies.

Contribution

It provides a comparative analysis of AGN and non-active galaxies' environments, highlighting differences in density profiles and neighbor compositions based on galaxy color and activity.

Findings

Red AGN inhabit less dense environments than non-active red galaxies.

Blue AGN environments are similar to non-active blue galaxies, but with more blue, star-forming neighbors.

Red AGN environments show an excess of blue, star-forming galaxies compared to non-active red galaxies.

Abstract

We analyse the relation between AGN host properties and large scale environment for a representative red and blue AGN host galaxy sample selected from the DR4 SDSS. A comparison is made with two carefully constructed control samples of non-active galaxies, covering the same redshift range and color baseline. The cross-correlation functions show that the density distribution of neighbours is almost identical for blue galaxies, either active, or non-active. Although active red galaxies inhabit environments less dense compared to non-active red galaxies, both reside in environments considerably denser than those of blue hosts. Moreover, the radial density profile of AGN, relative to galaxy group centres is less concentrated than galaxies. This is particularly evident when comparing red AGN and non-active galaxies. The properties of the neighbouring galaxies of blue and red AGN and non active galaxies reflect this effect. While the neighbourhood of the blue samples is indistinguishable, the red AGN environs show an excess of blue-star forming galaxies with respect to their non-active counterpart. On the other hand, the active and non-active blue systems have similar environments but markedly different morphological distributions, showing an excess of blue early-type AGN, which are argued to be late stage mergers. This comparison reveals that the observable differences between active red and blue host galaxy properties including star formation history and AGN activity depends on the environment within which the galaxies form and evolve.