The impact of black hole feedback on the UV luminosity and stellar mass assembly of high-redshift galaxies

We employ the Delphi semi-analytical model to study the impact of black hole growth on high-redshift galaxies, both in terms of the observed UV luminosity and of the star formation rate. To do this, firstly, we assess the contribution of AGN to the total galaxy UV luminosity as a function of stellar mass and redshift. We estimate, together with their duty cycle, the observed fraction of AGN-dominated galaxies and find that they outnumber stellar-dominated galaxies at $M_{UV} \leq -24$ mag and $z \approx 5 - 6$. Secondly, we study the evolution of the AGN and stellar luminosity functions (LFs), finding that it is driven both by changes in their characteristic luminosities $M^*$ (i.e. evolution of the intrinsic brightness of galaxies) and in their normalisations $\phi^*$ (i.e. evolution of the number densities of galaxies), depending on the luminosity range considered. Finally, we follow the mass assembly history for three different halo mass bins, finding that the magnitude of AGN-driven outflows depends on the host halo mass. We show that AGN feedback is most effective when the energy emitted by the accreting black hole is approximately $1\%$ of the halo binding energy, and that this condition is met in galaxies in halos with $M_h \sim 10^{11.75} M_\odot$ at $z=4$. In such cases, AGN feedback can drive outflows that are up to 100 times more energetic than SN-driven outflows, and the star formation rate is a factor of three lower than for galaxies of the same mass without black hole activity.