Galaxies with monstrous black holes in galaxy cluster environments

Abstract

Massive early-type galaxies follow a tight relation between the mass of their central supermassive black hole (${\mathrm{M}}_{\mathrm{BH}}$) and their stellar mass (${\mathrm{M}}_{\star }$). The origin of observed positive outliers from this relation with extremely high ${\mathrm{M}}_{\mathrm{BH}}$ ($>10^9{M}_{\odot }$) 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 $M_{\mathrm{BH}}(M_{\star })$ outliers at $z=0$, defined as those with ${\mathrm{M}}_{\mathrm{BH}}>10^7{\mathrm{M}}_{\odot }$ and ${\mathrm{M}}_{\mathrm{BH}}/{\mathrm{M}}_{\star }>0.01$. This paper focusses on a sample of 5 extreme outliers, that have been selected based on their ${\mathrm{M}}_{\mathrm{BH}}$ and ${\mathrm{M}}_{\star }$ 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 ${\mathrm{M}}_{\mathrm{BH}}({\mathrm{M}}_{\star })$ outliers, we find that these galaxies became outliers due to a combination of their early formation times and tidal stripping. For BMGs with ${\mathrm{M}}_{\mathrm{BH}}>10^9{\mathrm{M}}_{\odot }$, 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 ${\mathrm{M}}_{\mathrm{BH}}$ 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.

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• How Ram Pressure Drives Radial Gas Motions in the Surviving Disk
• Active Galactic Nuclei: Boon or Bane for Biota?
• Quantifying the role of ram pressure stripping of galaxies within galaxy groups
• A search for active galactic nuclei in low-mass compact galaxies
• The (black hole mass)-(color) relations for early- and late-type galaxies: red and blue sequences
• Low-mass compact elliptical galaxies: spatially-resolved stellar populations and kinematics with the Keck Cosmic Web Imager
• The formation of compact massive relic galaxies in MOND
• Astrophysics with the Laser Interferometer Space Antenna
• Distinguishing the formation paths of massive compact ETGs through their internal dynamical structures
• Newcomers and suburbanites can drive the evolution of the size-stellar mass relation of early type galaxies in galaxy clusters