Overmassive central black holes in the cosmological simulations ASTRID and Illustris TNG50

Abstract

Recent dynamical measurements indicate the presence of a central supermassive black hole (SMBH) with mass ∼3 × 106 M⊙ in the dwarf galaxy Leo I, placing the system ∼50 times above the standard, local MBH-M✶ relation. While a few overmassive central SMBHs are reported in nearby isolated galaxies, this is the first one detected in a Milky Way satellite. We used the ASTRID and Illustris TNG50 lambda cold dark matter (LCDM) cosmological simulations to investigate the assembly history of galaxies hosting overmassive SMBHs. We estimate that, at the stellar mass of Leo I, ∼15 per cent of galaxies above the MBH-M✶ relation lie >10 times above it. Leo I-like systems are rare but exist in LCDM simulations: they occur in ∼0.005 per cent of all overmassive systems. Examining the properties of simulated galaxies harbouring overmassive central SMBHs, we find that: (i) stars assemble more slowly in galaxies above the MBH-M✶ relation; (ii) the gas fraction in these galaxies experiences a significantly steeper decline over time; and (iii) > 95 per cent of satellite host galaxies in overdense regions are located above the MBH-M✶ relation. This suggests that massive satellite infall and consequent tidal stripping in a group/dense environment can drive systems away from the MBH-M✶ relation, causing them to become overmassive. As the merging histories of overmassive and undermassive 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 overmassive SMBHs are a signature of heavy BH seeds; we demonstrate, in contrast, that low-z overmassive systems result from complex environmental interactions.