Host galaxy morphologies of X-ray selected AGN: assessing the significance of different black hole fuelling mechanisms to the accretion density of the Universe at z ~ 1.

Abstract

We use morphological information of X-ray selected active galactic nuclei (AGN) hosts to set limits on the fraction of the accretion density of the Universe at z ~ 1 that is not likely to be associated with major mergers. Deep X-ray observations are combined with high-resolution optical data from the Hubble Space Telescope in the All-wavelength Extended Groth strip International Survey, Great Observatories Origins Deep Survey (GOODS) North and GOODS South fields to explore the morphological breakdown of X-ray sources in the redshift interval 0.5 < z < 1.3. The sample is split into discs, early-type bulge-dominated galaxies, peculiar systems and point sources in which the nuclear source outshines the host galaxy. The X-ray luminosity function and luminosity density of AGN at z ~ 1 are then calculated as a function of morphological type. We find that disc-dominated hosts contribute 30 +/- 9 per cent to the total AGN space density and 23 +/- 6 per cent to the luminosity density at z ~ 1. We argue that AGN in disc galaxies are most likely fuelled not by major merger events but by minor interactions or internal instabilities. We find evidence that these mechanisms may be more efficient in producing luminous AGN compared to predictions for the stochastic fuelling of massive black holes in disc galaxies.