Linking X-ray AGN with dark matter halos: a model compatible with AGN luminosity function and large-scale clustering properties

¹ Max-Planck-Institut für Astrophysik, Karl-Schwarzschild-Str. 1, 85741 Garching, Germany
e-mail: gert@mpa-garching.mpg.de
² Tartu Observatory, 61602 Tõravere, Estonia
³ Space Research Institute of Russian Academy of Sciences, Profsoyuznaya 84/32, 117997 Moscow, Russia

Received: 12 April 2013
Accepted: 25 October 2013

Abstract

Aims. Our goal is to find a minimalistic model that describes the luminosity function and large-scale clustering bias of X-ray-selected active galactic nuclei in the general framework of the concordance ΛCDM model.

Methods. We assume that a simple population-averaged scaling relation between the AGN X-ray luminosity L_X and the host dark matter halo mass M_h exists. With such a relation, the AGN X-ray luminosity function can be computed from the halo mass function. Using the concordance ΛCDM halo mass function for the latter, we obtain the M_h − L_X relation required to match the redshift-dependent AGN X-ray luminosity function known from X-ray observations.

Results. We find that with a simple power-law-scaling M_h ∝ L^Γ(z), our model can successfully reproduce the observed X-ray luminosity function. Furthermore, we automatically obtain predictions for the large-scale AGN clustering amplitudes and their dependence on the luminosity and redshift, which seem to be compatible with AGN clustering measurements. Our model also includes the redshift-dependent AGN duty cycle, which peaks at the redshift z ≃ 1, and its peak value is consistent with unity, suggesting that on average there is no more than one AGN per dark matter halo. For a typical X-ray-selected AGN at z ~ 1, our best-fit M_h − L_X scaling implies low Eddington ratio L_X/L_Edd ~ 10^-4 − 10^-3 (2–10 keV band, no bolometric correction applied) and correspondingly high mass-growth e-folding times, suggesting that typical X-ray AGN are dominantly fueled via relatively inefficient “hot-halo” accretion mode.