Skip to main content
SearchLoginLogin or Signup

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

Presentation #408.09 in the session Black Holes.

Published onJul 01, 2023
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 MBH-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 MBH-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 MBH-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 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 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.

No comments here