Constraints on black hole fuelling modes from the clustering of X-ray AGN. (arXiv:1305.2200v1 [astro-ph.CO])

Constraints on black hole fuelling modes from the clustering of X-ray AGN. (arXiv:1305.2200v1 [astro-ph.CO]):
We present a clustering analysis of X-ray selected 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 (Lx(2-10
keV)=10^42-10^44 erg/sec) in the z=0-1.3 Universe are typically found in DM
haloes with masses of ~10^13 Msun. 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 onto the central black hole. This type of accretion, which is
additional to the common cold accretion during disk 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 Msun, 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 (Lx(2-10 keV) > 10^44 erg/sec) in DM haloes with
masses up to ~10^14 Msun, 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.