Obscured active galactic nuclei triggered in compact star-forming galaxies

Abstract

We present a structural study of 182 obscured active galactic nuclei (AGNs) at z ≤ 1.5, selected in the Cosmic Evolution Survey field from their extreme infrared to X-ray luminosity ratio and their negligible emission at optical wavelengths. We fit optical to far-infrared spectral energy distributions and analyse deep Hubble Space Telescope imaging to derive the physical and morphological properties of their host galaxies. We find that such galaxies are more compact than normal star-forming sources at similar redshift and stellar mass, and we show that it is not an observational bias related to the emission of the AGN. Based on the distribution of their UVJ colours, we also argue that this increased compactness is not due to the additional contribution of a passive bulge. We thus postulate that a vast majority of obscured AGNs reside in galaxies undergoing dynamical compaction, similar to processes recently invoked to explain the formation of compact star-forming sources at high redshift.