AGN Selection Methods Have Profound Impacts on the Distributions of Host-galaxy Properties

Abstract

We present a comparative study of X-ray and IR active galactic nuclei (AGNs) at z ≈ 2 to highlight the important AGN selection effects on the distributions of host-galaxy properties. Compared with non-AGN star-forming galaxies (SFGs) on the main sequence, X-ray AGNs have similar median star formation (SF) properties, but their incidence ( q AGN ) is higher among galaxies with either enhanced or suppressed SF, and among galaxies with a larger stellar-mass surface density, regardless if it is measured within the half-light radius (Σ e ) or central 1 kpc (Σ 1kpc ). Unlike X-ray AGNs, IR AGNs are less massive and have enhanced SF and similar distributions of colors, Σ e and Σ 1kpc , relative to non-AGN SFGs. Given that Σ e and Σ 1kpc strongly correlate with M * , we introduce the fractional mass within the central 1 kpc ( M 1 kpc / M ∗ ), which only weakly depends on M * , to quantify galaxy compactness. Both AGN populations have similar M 1 kpc / M ∗ distributions compared to non-AGN SFGs’. While q AGN increases with Σ e and Σ 1kpc , it remains constant with M 1 kpc / M ∗ , indicating that the trend of increasing q AGN with Σ is driven by M * more than morphology. While our findings are not in conflict with the scenario of AGN quenching, they do not imply it either, because the incidence of AGNs hosted in transitional galaxies depends crucially on AGN selections. Additionally, despite the relatively large uncertainty of AGN bolometric luminosities, their very weak correlation, if any, with SF activities, regardless of AGN selections, also argues against a direct causal link between the presence of AGNs and the quenching of massive galaxies at z ∼ 2.