The link between galaxy and black hole growth in the EAGLE simulation

TL;DR

This study uses the EAGLE simulation to explore how galaxy star formation rates and black hole accretion rates are related, revealing complex, halo-dependent growth patterns that challenge simple universal models.

Contribution

It provides a detailed analysis of the SFR-BHAR relationship across different halo masses, highlighting the non-universality and complex evolution of galaxy and black hole growth.

Findings

Different halo masses show distinct SFR-BHAR growth patterns.

No universal SFR-BHAR relationship exists across all galaxy types.

The primary driver of observed trends is sampling different regions of the growth plane.

Abstract

We investigate the connection between the star formation rate (SFR) of galaxies and their central black hole accretion rate (BHAR) using the EAGLE cosmological hydrodynamical simulation. We find, in striking concurrence with recent observational studies, that the <SFR>--BHAR relation for an AGN selected sample produces a relatively flat trend, whilst the <BHAR>--SFR relation for a SFR selected sample yields an approximately linear trend. These trends remain consistent with their instantaneous equivalents even when both SFR and BHAR are time-averaged over a period of 100~Myr. There is no universal relationship between the two growth rates. Instead, SFR and BHAR evolve through distinct paths that depend strongly on the mass of the host dark matter halo. The galaxies hosted by haloes of mass M200 $\lesssim 10^{11.5}$Msol grow steadily, yet black holes (BHs) in these systems hardly grow, yielding a lack of correlation between SFR and BHAR. As haloes grow through the mass range $10^{11.5} \lesssim$ M200 $\lesssim 10^{12.5 }$Msol BHs undergo a rapid phase of non-linear growth. These systems yield a highly non-linear correlation between the SFR and BHAR, which are non-causally connected via the mass of the host halo. In massive haloes (M200 $\gtrsim 10^{12.5}$Msol) both SFR and BHAR decline on average with a roughly constant scaling of SFR/BHAR $\sim 10^{3}$. Given the complexity of the full SFR--BHAR plane built from multiple behaviours, and from the large dynamic range of BHARs, we find the primary driver of the different observed trends in the <SFR>--BHAR and <BHAR>--SFR relationships are due to sampling considerably different regions of this plane.