Over-massive Central Black Holes in the Cosmological Simulations ASTRID and Illustris TNG50

Recent dynamical measurements indicate the presence of a central SMBH with mass $\sim 3\times {10}^6{\mathrm{M}}_{\odot }$ in the dwarf galaxy Leo I, placing the system $\sim 50$ times above the standard, local $M_{BH}-M_{\star }$ relation. While a few over-massive central SMBHs are reported in nearby isolated galaxies, this is the first detected in a Milky Way satellite. We used the ASTRID and Illustris TNG50 LCDM cosmological simulations to investigate the assembly history of galaxies hosting over-massive SMBHs. We estimate that, at the stellar mass of Leo I, $\sim 15\mathrm{\%}$ of galaxies above the $M_{BH}-M_{\star }$ relation lie $>10$ times above it. Leo I-like systems are rare but exist in LCDM simulations: they occur in $\sim 0.005\mathrm{\%}$ of all over-massive systems. Examining the properties of simulated galaxies harboring over-massive central SMBHs, we find that: (i) stars assemble more slowly in galaxies above the $M_{BH}-M_{\star }$ relation; (ii) the gas fraction in these galaxies experiences a significantly steeper decline over time; and (iii) $>95\mathrm{\%}$ of satellite host galaxies in over-dense regions are located above the $M_{BH}-M_{\star }$ relation. This suggests that massive satellite infall and consequent tidal stripping in a group/dense environment can drive systems away from the $M_{BH}-M_{\star }$ relation, causing them to become over-massive. As the merging histories of over-massive and under-massive systems do not differ, we conclude that additional environmental effects, such as being in overdense regions, must play a crucial role. In the high-$z$ Universe, central over-massive SMBHs are a signature of heavy black hole seeds; we demonstrate, in contrast, that low-$z$ over-massive systems result from complex environmental interactions.