The origin of compact galaxies with anomalously high black hole masses

Observations of local galaxies harbouring supermassive black holes (BH) of anomalously high mass, qM(BH), relative to their stellar mass, M-*, 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 I > cold dark matter context using the EAGLE cosmological, hydrodynamical simulation. We find 15 'M-BH(M-*)-outlier' galaxies, defined as having M-BH more than 1.5 dex above the median M-BH(M-*) relation in the simulation, M-BH,M- med(M-*). All M-BH(M-*)-outliers are satellite galaxies, typically with M-* similar to 10(10) M-aS (TM) and M-BH similar to 10(8) M-aS (TM). 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-*)-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,M- med(M-*)) a [1.2, 1.5]) grow rapidly in M-BH to lie well above the present-day M-BH - M-* relation at early times (z a parts per thousand(3) 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-*)-outliers with extended dark matter haloes and undisturbed morphologies.