The New Numerical Galaxy Catalogue (ν2 GC): properties of active galactic nuclei and their host galaxies

We present the latest results of a semi-analytic galaxy formation model, 'New Numerical Galaxy Catalogue', which utilizes large cosmological N-body simulations. This model can reproduce many statistical properties of galaxies at z less than or similar to 6. We focus on the properties of active galactic nuclei (AGNs) and supermassive black holes, especially on the accretion timescale on to black holes (BHs). We find that the number density of AGNs at z < 1.5 and at hard X-ray luminosity < 10(44) erg s(-1) is underestimated compared with recent observational estimates when we assume the exponentially decreasing accretion rate and the accretion time-scale which is proportional to the dynamical time of the host halo or the bulge, as is often assumed in semi-analytic models. We show that to solve this discrepancy, the accretion time-scale of such less luminous AGNs should be a function of the BH mass and the accreted gas mass. This time-scale can be obtained from a phenomenological modelling of the gas angular momentum loss in the circumnuclear torus and/or the accretion disc. Such models predict a longer accretion time-scale for less luminous AGNs at z < 1.0 than luminous QSOs whose accretion time-scale would be 10(7-8) yr. With this newly introduced accretion time-scale, our model can explain the observed luminosity functions of AGNs at z < 6.0.