Constraints on black hole fuelling modes from the clustering of X-ray AGN

We present a clustering analysis of X-ray selected active galactic nuclei (AGN) by compiling X-ray samples from the literature and re-estimating the dark-matter (DM) halo masses of AGN in a uniform manner. We find that moderate-luminosity AGN (L_{2-10 keV} ≃ 10⁴²-10⁴⁴ erg s^-1) in the z ≃ 0-1.3 Universe are typically found in DM haloes with masses of ∼10¹³ M_⊙. We then compare our findings to the theoretical predictions of the coupled galaxy and black hole formation model GALFORM. We find good agreement when our calculation includes the hot-halo mode of accretion on to the central black hole. This type of accretion, which is additional to the common cold accretion during disc instabilities and galaxy mergers, is tightly coupled to the AGN feedback in the model. The hot-halo mode becomes prominent in DM haloes with masses greater than ∼10^12.5 M_⊙, where AGN feedback typically operates, giving rise to a distinct class of moderate-luminosity AGN that inhabit rich clusters and superclusters. Cold gas fuelling of the black hole cannot produce the observationally inferred DM halo masses of X-ray AGN. Switching off AGN feedback in the model results in a large population of luminous quasars (L_{2-10 keV} > 10⁴⁴ erg s^-1) in DM haloes with masses up to ∼10¹⁴ M_⊙, which is inconsistent with the observed clustering of quasars. The abundance of hot-halo AGN decreases significantly in the z ≃ 3-4 universe. At such high redshifts, the cold accretion mode is solely responsible for shaping the environment of moderate-luminosity AGN. Our analysis supports two accretion modes (cold and hot) for the fuelling of supermassive black holes and strongly underlines the importance of AGN feedback in cosmological models both of galaxy formation and black hole growth.