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Galaxies with monstrous black holes in galaxy cluster environments.

van Son, Lieke A.C. and Barber, Christopher and Bahé, Yannick M. and Schaye, Joop and Barnes, David J. and Crain, Robert A. and Kay, Scott T. and Theuns, Tom and Vecchia, Claudio Dalla (2019) 'Galaxies with monstrous black holes in galaxy cluster environments.', Monthly notices of the Royal Astronomical Society., 485 (1). pp. 396-407.


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 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 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 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 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.

Item Type:Article
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Publisher statement:© 2019 The Author(s) Published by Oxford University Press on behalf of the Royal Astronomical Society.
Date accepted:06 February 2019
Date deposited:19 February 2019
Date of first online publication:11 February 2019
Date first made open access:No date available

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