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 ($\rm M_{BH}$) and their stellar mass ($\rm M_{\star}$). The origin of observed positive outliers from this relation with extremely high $\rm M_{BH}$ ($> 10^{9} M_{\odot}$) 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 $M_{\rm BH}(M_{\star})$ outliers at $z=0$, defined as those with $ \rm M_{BH} >10^{7} M_{\odot}$ and $\rm M_{BH}/\rm M_{\star}> 0.01$. This paper focusses on a sample of 5 extreme outliers, that have been selected based on their $\rm M_{BH}$ and $\rm M_{\star}$ values, which are comparable to the most recent estimates of observed positive outliers. 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 $\rm M_{BH}(\rm M_{\star})$ outliers, we find that these galaxies became outliers due to a combination of their early formation times and tidal stripping. For BMGs with $\rm M_{BH} > 10^9 M_{\odot}$, major mergers (with a stellar mass ratio of $\mu > 0.25$) at early times ($z>2$) precede the rapid growth of their supermassive BHs. Furthermore, the scatter in the relation between $\rm M_{BH}$ and stellar velocity dispersion, $\sigma$, correlates positively with the scatter in [Mg/Fe]($\sigma$). 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.