Abstract
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.
Highlights
The theory of galaxy formation is one of the most active topics of modern-day cosmology
The subhalo in the Friends of Friends (FOF)-group that contains the particle with the deepest gravitational potential is defined as the central galaxy of that group, the other subhaloes are labelled as satellites
The median relation, shown as a solid red line, is lower for satellite galaxies than the median relation for centrals at M ≈ 1012M. This is caused by a small subset (∼ 20) of ‘undermassive’ BH galaxies that are most likely caused by the ejection of black holes through tidal encounters of similar-mass black holes
Summary
The theory of galaxy formation is one of the most active topics of modern-day cosmology. Barber et al (2016, hereafter B16) studied positive outliers of the MBH(M ) relation in the (100 Mpc)3-volume EAGLE simulation (Schaye et al 2015; Crain et al 2015) They confirmed that such outliers do exist, and arise due to their early formation time and/or the tidal stripping of stars. This work aims to study the origin and properties of the MBH(M ) outliers in the cluster environments of the Hydrangea simulations (Bahe et al 2017, a subset of the C-EAGLE simulations, for which the high-resolution zoom-in region extends to 10 times the virial radius) in a stellar mass regime closer to that of observed galaxies with overly-massive BHs. We compare our results to those from B16 for the (100 Mpc) EAGLE volume and investigate the physical origin of galaxies with extreme BH masses.
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