The New Numerical Galaxy Catalogue (ν2GC): properties of active galactic nuclei and their host galaxies

Abstract

We present the latest results of a semi-analytic galaxy formation model, "New Numerical Galaxy Catalogue", which utilises large cosmological N-body simulations. This model can reproduce many statistical properties of galaxies at z 6. 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 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 luminous 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.

Dark-ages reionization and galaxy formation simulation – XVIII. The high-redshift evolution of black holes and their host galaxies

“…Menci et al (2014) find that in their semi-analytic model disc instabilities can provide enough black hole accretion to reproduce the observed AGN luminosity functions up to z ≈ 4.5, but are not likely to be dominant for the highest luminosity AGN or at the highest redshifts. In contrast, Shirakata et al (2018) find that the primary trigger of AGN at z ≤ 4 in their semi-analytic model is mergers, whilst disc instabilities are essential for fuelling moderate luminosity AGN at higher redshifts. The hydrodynamical simulation Horizon-AGN found that only ∼ 35 per cent of black hole mass in local massive galaxies is directly attributable to merging, with the majority of black hole growth instead growing via secular processes (Martin et al 2018).…”

Slowing down of cosmic growth of supermassive black holes: theoretical prediction of the Eddington ratio distribution

“…where tstart and tacc are the starting time of the accretion and the accretion timescale per one accretion event, respectively. We use the same model of the accretion timescale as Shirakata et al (2019), tacc = α bulge t dyn,bulge +t loss . The first term of the right-hand side is proportional to the dynamical time of their host bulges, t dyn,bulge , where the values of free parameter, α bulge , is 0.58.…”

“…circumnuclear discs) and accretion disc. We define t loss as t loss,0 (MBH/M ) γ BH (∆Macc/M ) γgas , where the free parameters, t loss,0 , γBH, and γgas, are 1.0 Gyr, 3.5, and -4.0, respectively (see Shirakata et al 2019). The AGN bolometric luminosity is described withṁ ≡ MBH/Ṁ Edd (Ṁ Edd = L Edd /c 2 ) as…”

“…We also show the effect of the sample selection for obtaining the ERDFs and the dependence of the ERDFs on AGN host galaxies, which has not been studied in depth. The modelling and values of adjustable parameters are the same as Shirakata et al (2019), which present the detailed model description. This paper is organised as follows.…”

“…In Sec. 3, we show the ERDF obtained with the fiducial model (Shirakata et al 2019) and show the effect of the sample selections. In Sec.…”