What triggers black hole growth? Insights from star formation rates

Abstract

We present a new semi-analytic model for the common growth of black holes\n(BHs) and galaxies within a hierarchical Universe. The model is tuned to match\nthe mass function of BHs at z=0 and the luminosity functions of active galactic\nnuclei (AGNs) at z<4. We use a new observational constraint, which relates the\nluminosity of AGNs to the star-formation rate (SFR) of their host galaxies. We\nshow that this new constraint is important in various aspects: a) it indicates\nthat BH accretion events are episodic; b) it favours a scenario in which BH\naccretion is triggered by merger events of all mass ratios; c) it constrains\nthe duration of both merger-induced star-bursts and BH accretion events. The\nmodel reproduces the observations once we assume that only 4 per cent of the\nmerger events trigger BH accretion; BHs accretion is not related to secular\nevolution; and only a few per cent of the mass made in bursts goes into the BH.\nWe find that AGNs with low or intermediate luminosity are mostly being\ntriggered by minor merger events, in broad agreement with observations. Our\nmodel matches various observed properties of galaxies, such as the stellar mass\nfunction at z<4 and the clustering of galaxies at redshift zero. This allows us\nto use galaxies as a reliable backbone for BH growth, with reasonable estimates\nfor the frequency of merger events. Other modes of BH accretion, such as\ndisk-instability events, were not considered here, and should be further\nexamined in the future.\n