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

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 active galactic nucleus (AGN)-selected sample produces a relatively flat trend, whilst the < BHAR >-SFR relation for an 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 M-200 less than or similar to 10(11.5) M-circle dot 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) less than or similar to M-200 less than or similar to 10(12.5) M-circle dot 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 (M-200 greater than or similar to 10(12.5) M-circle dot), both SFR and BHAR decline on average with a roughly constant scaling of SFR/BHAR similar to 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.