The origin of compact galaxies with anomalously high black hole masses

TL;DR

This study uses cosmological simulations to investigate the origins of galaxies with supermassive black holes that are unusually large relative to their stellar mass, identifying key formation mechanisms and evolutionary paths.

Contribution

It reveals that tidal stripping and early high-redshift formation are primary causes of black hole mass outliers in satellite galaxies within a cosmological context.

Findings

Outliers are mostly satellite galaxies with high black hole to stellar mass ratios.

Tidal stripping and early formation lead to the outlier status and compactness.

High-redshift formation explains some black hole outliers with undisturbed morphologies.

Abstract

Observations of local galaxies harbouring supermassive black holes (BHs) of anomalously high mass, M_BH, relative to their stellar mass, M_star, appear to be at odds with simple models of the co-evolution between galaxies and their central BHs. We study the origin of such outliers in a Lambda cold dark matter context using the EAGLE cosmological, hydrodynamical simulation. We find 15 'M_BH(M_star)-outlier' galaxies, defined as having M_BH more than 1.5 dex above the median M_BH(M_star) relation in the simulation, M_{BH,med}. All M_BH(M_star)-outliers are satellite galaxies, typically with M_star ~ 10^10 M_sun and M_BH ~ 10^8 M_sun. They have all become outliers due to a combination of tidal stripping of their outer stellar component acting over several Gyr and early formation times leading to rapid BH growth at high redshift, with the former mechanism being most important for 67 per cent of these outliers. The same mechanisms also cause the M_BH(M_star)-outlier satellites to be amongst the most compact galaxies in the simulation, making them ideal candidates for ultracompact dwarf galaxy progenitors. The 10 most extreme central galaxies found at z=0 (with log_{10}(M_BH/M_{BH,med}) in [1.2, 1.5]) grow rapidly in M_BH to lie well above the present-day M_BH-M_star relation at early times (z > 2), and either continue to evolve parallel to the z=0 relation or remain unchanged until the present day, making them 'relics' of the high-redshift universe. This high-z formation mechanism may help to explain the origin of observed M_BH(M_star)-outliers with extended dark matter haloes and undisturbed morphologies.