Galaxies with monstrous black holes in galaxy cluster environments

Massive early-type galaxies follow a tight relation between the mass of their central supermassive black hole (MBH) and their stellar mass (M⋆). The origin of observed positive outliers from this relation with extremely high MBH (> 109M⊙) remains un- clear. We present a study of such outliers in the Hydrangea/C-EAGLE cosmological hydrodynamical simulations, designed to enable the study of high-mass galaxy forma- tion and evolution in cluster environments. We find 69 MBH(M⋆) outliers at z = 0, defined as those with MBH > 107M⊙ and MBH/M⋆ > 0.01. This paper focusses on a sample of 5 extreme outliers, that have been selected based on their MBH and M⋆ values, which are comparable to the most recent estimates of observed positive out- liers. This sample of 5 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 MBH(M⋆) outliers, we find that these galaxies became outliers due to a combination of their early formation times and tidal stripping. For BMGs with MBH > 109M⊙, 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 MBH 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.