What triggers black-hole growth? Insights from star formation rates

TL;DR

This paper introduces a semi-analytic model linking black hole growth to star formation rates, emphasizing the role of mergers and episodic accretion in galaxy evolution.

Contribution

It presents a new model that connects AGN luminosity to host galaxy star formation, highlighting merger-triggered black hole accretion and constraining its duration and frequency.

Findings

BH accretion is episodic and merger-triggered

Minor mergers predominantly trigger low/intermediate luminosity AGNs

Model successfully reproduces galaxy stellar mass functions and clustering

Abstract

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