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 ~3×10⁶ M_⊙ in the dwarf galaxy Leo I, placing the system ~50 times above the standard, local M_BH-M_* 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. At the stellar mass of Leo I, ~15% of galaxies above the M_BH-M_* relation lie >10 times above it. Leo I-like systems are rare but do exist in LCDM simulations: they occur in ~0.005% 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_* relation; (ii) the gas fraction in these galaxies experiences a significantly steeper decline over time; and (iii) >95% of satellite host galaxies in over-dense regions are located above the M_BH-M_* 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_* relation, causing them to become over-massive. However, we note that the merging histories of over-massive and under-massive systems do not differ. Hence additional environmental effects must be at play, like being located in overdense regions. In the high-z Universe, closer to the seeding epoch, the presence of central over-massive SMBHs is a signature of heavy initial black hole seeds. In contrast, we demonstrate that over-massive systems at low-z may result from more complex interactions with the environment, related to structure formation and assembly history over cosmic time, as seeding signatures are apt to be erased at late times.