2019MNRAS.485..396V

Massive early-type galaxies follow a tight relation between the mass of their central supermassive black hole (M_BH) and their stellar mass (M_*). The origin of observed positive outliers from this relation with extremely high M_BH (> 10⁹ M_☉) remains unclear. We present a study of such outliers in the Hydrangea/C-EAGLE cosmological hydrodynamical simulations, designed to enable the study of high-mass galaxy formation and evolution in cluster environments. We find 69 M_BH(M_*) outliers at z = 0, defined as those with M_ BH_> 10⁷ M_☉ and M_BH/M_* > 0.01. This paper focuses on a sample of five extreme outliers, that have been selected based on their M_BH and M_* values, which are comparable to the most recent estimates of observed positive outliers. This sample of five outliers, classified as 'black hole monster galaxies' (BMGs), was traced back in time to study their origin and evolution. In agreement with the results of previous simulations for lower mass M_BH(M_*) outliers, we find that these galaxies became outliers due to a combination of their early formation times and tidal stripping. For BMGs with M_ BH_> 10⁹^M_☉, major mergers (with a stellar mass ratio of µ > 0.25) at early times (z > 2) precede the rapid growth of their supermassive BHs. Furthermore, the scatter in the relation between M_BH and stellar velocity dispersion, σ, correlates positively with the scatter in [Mg/Fe](σ). This indicates that the alpha enhancement of these galaxies, which is closely related to their star formation history, is related to the growth of their central BHs.