Massive relic galaxies challenge the co-evolution of SMBHs and their host galaxies

TL;DR

This study identifies and analyzes a rare class of relic galaxies with extremely large SMBHs that deviate from local scaling relations, suggesting they are remnants from the early universe that skipped typical growth phases.

Contribution

It presents evidence that some massive galaxies with oversized SMBHs are relics from z~2, challenging the standard two-phase galaxy growth model.

Findings

Relic galaxies host abnormally large SMBHs.

These galaxies are compact and have old stellar populations.

They likely skipped the second growth phase, remaining structurally unchanged.

Abstract

We study a sample of eight massive galaxies that are extreme outliers (3-5$\sigma$) in the M$_{\bullet}$-M$_\mathrm{bulge}$ 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 these extreme outliers can be considered as relic galaxies from the early (z$\sim$2) Universe: i.e. they are compact (R$_{\mathrm{e}}$$<$2 kpc) and have purely old stellar populations (t$\gtrsim$10 Gyr). In order to explain the nature of such deviations from the local relations, we propose a scenario in which the hosts of these \"uber-massive SMBHs are galaxies that have followed a different evolutionary path than the two-phase growth channel assumed for massive galaxies. Once the SMBH and the core of the galaxy are formed at z$\sim$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 $\sim$10Gyr ago.