Massive Relic Galaxies Challenge the Co-evolution of Super-massive Black Holes and Their Host Galaxies

We study a sample of eight massive galaxies that are extreme outliers (3-5σ) in the {M}_\bullet - M_bulge local scaling relation. Two of these galaxies are confirmed to host extremely large super-massive black holes (SMBHs), whereas the virial mass estimates for the other six are also consistent with having abnormally large SMBHs. From the analysis of their star formation histories and their structural properties, we find that all of these extreme outliers can be considered to be relic galaxies from the early (z ∼ 2) universe: i.e., they are compact (R {}_{{e}}\lt 2 kpc) and have purely old stellar populations (t≳ 10 Gyr). In order to explain the nature of such deviations from the local relations, we propose a scenario in which the hosts of these über-massive SMBHs are galaxies that have followed a different evolutionary path than the two-phase growth channel assumed for massive galaxies. Once the SMBH and the core of the galaxy are formed at z ∼ 2, the galaxy skips the second phase, remaining structurally untouched and without further mass and size increase. We show that if the outliers had followed the normal evolutionary path by growing in size via merger activity, the expected (mild) growth in mass would place them closer to the observed local relations. Our results suggest that the SMBH growth epoch for the most massive galaxies stopped ∼10 Gyr ago.