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

Author:

Emma Jane Weller, Fabio Pacucci, Priyamvada Natarajan, Tiziana Di Matteo

Keyword:

Astrophysics, Astrophysics of Galaxies, Astrophysics of Galaxies (astro-ph.GA), Cosmology and Nongalactic Astrophysics (astro-ph.CO), High Energy Astrophysical Phenomena (astro-ph.HE)

journal:

Monthly Notices of the Royal Astronomical Society, 2023, Volume 522, Issue 4, pp.4963-4971

date:

2023-05-02 16:00:00

Abstract

Recent dynamical measurements indicate the presence of a central SMBH with mass $\sim 3\times 10^6 \, \rm M_\odot$ in the dwarf galaxy Leo I, placing the system $\sim50$ 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, $\sim15\%$ 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 $\sim0.005\%$ 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 $M_{BH} - M_\star$ 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 $M_{BH} - M_\star$ 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.

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