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

Abstract

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. We estimate that, at the stellar mass of Leo I, $\sim 15~{{\ \rm per\ cent}}$ 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 $\sim 0.005~{{\ \rm per\ cent}}$ 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) $>95~{{\ \rm per\ cent}}$ 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. 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.

Overmassive Black Holes at Cosmic Noon: Linking the Local and the High-redshift Universe

“…In particular, the role of outflows in modulating asynchronous BH-galaxy growth and/or growth in tandem is urgently needed. In addition, as noted in recent analysis of simulations (e.g., Weller et al 2023), environment is also likely to play an important role in determining where accreting sources fall in relation to the local M BH −M * relation. A recent study by Inayoshi & Ichikawa (2024) suggests that dust-rich environments in the high-redshift Universe could create conditions prone to generate intrinsically overmassive populations of BHs, with a distribution similar to what our data at z < 3 is showing.…”

“…The finding that these BHs are overmassive with respect to the local M BH -M * relation is not a consequence of survey selection effects but is rather expected from some theoretical simulations (e.g., Agarwal et al 2013;Inayoshi et al 2022;Trinca et al 2022), especially if the SMBHs arise from heavy seeds (e.g., Scoggins et al 2023). Recent results from the ASTRID, Illustris TNG50 simulation suites also predict the existence of such an overmassive SMBH population in the local Universe (Weller et al 2023). At these late epochs, environmental effects like tidal stripping of the stellar component of host galaxies are implicated in causing the central SMBHs in dwarf galaxies to be overmassive compared to observed scaling relations (e.g., Ferré-Mateu et al 2018.…”

Extreme Tidal Stripping May Explain the Overmassive Black Hole in Leo I: A Proof of Concept

“…If the presence of the SMBH is confirmed, and the tidal stripping hypothesis holds, Leo I could be a system halfway through the process of becoming a wandering black hole. Based on cosmological simulations, several recent studies show that tidal stripping is a primary mechanism for producing wandering black holes (Di Weller et al 2023).…”