The New Numerical Galaxy Catalogue (\nu^2 GC): Properties of Active Galactic Nuclei and Their Host Galaxies

TL;DR

This paper introduces a semi-analytic galaxy formation model that incorporates a variable accretion timescale for AGNs, improving the match with observed AGN luminosity functions at redshifts below 6.

Contribution

The study proposes a new accretion timescale model based on black hole and gas properties, enhancing the accuracy of AGN population predictions in galaxy formation simulations.

Findings

Underestimation of AGN number density at z<1.5 with traditional models.

Longer accretion timescales for less luminous AGNs improve model fit.

Model successfully reproduces observed AGN luminosity functions at z<6.

Abstract

We present the latest results of a semi-analytic model of galaxy formation, "New Numerical Galaxy Catalogue", which is combined with large cosmological N-body simulations. This model can reproduce statistical properties of galaxies at z < 6.0. We focus on the properties of active galactic nuclei (AGNs) and supermassive black holes, especially on the accretion timescale onto black holes. We find that the number density of AGNs at z < 1.5 and at hard X-ray luminosity 10^{ 44 }< erg/s is underestimated compared with recent observational estimates when we assume the exponentially decreasing accretion rate and the accretion timescale 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 timescale of such less luminous AGNs instead should be a function of the black hole mass and the accreted gas mass. This timescale 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 timescale for less luminous AGNs at z < 1.0 than bright QSOs whose accretion timescale would be 10^{ 7-8 } yr. With this newly introduced accretion timescale, our model can explain the observed luminosity functions of AGNs at z < 6.0.