# The difficult path to coalescence: massive black hole dynamics in merging low mass dark matter haloes and galaxies

**Authors:** Christian Partmann, Thorsten Naab, Antti Rantala, Anna Genina, Matias Mannerkoski, Peter H. Johansson  
**arXiv:** [2310.08079](https://arxiv.org/abs/2310.08079)  
**Date:** 2023-10-12  
**Summary page:** https://www.summarizepaper.com/en/paper/2310.08079v1/

## Key points

- High-resolution numerical study of sinking and merging of massive black holes (MBHs) in multiple minor mergers
- MBH mass range: $10^3 - 10^7 \, \mathrm{M}_\odot$
- Dark matter halo mass range: $4\times 10^8 \, \mathrm{M}_\odot \lesssim \mathrm{M}_{\mathrm{halo}} \lesssim 2\times 10^{10} \, \mathrm{M}_\odot$
- Utilization of simulation code Ketju, an extension of Gadget-3
- Incorporation of unsoftened forces to account for dynamical friction and scattering effects caused by MBH binaries or multiples
- Complex evolution paths observed for sinking MBHs including binaries, triplets, free-floating MBHs, and dynamically or recoil ejected MBHs
- Difficulty for low-mass MBHs ($\lesssim 10^5 M_\odot$) to sink or merge; collisional interactions with dark matter can drive coalescence
- Higher mass MBHs ($\gtrsim 10^6 M_\odot$) tend to sink to the center more frequently and trigger scouring process leading to a transformation from baryon-dominated system to dark matter-dominated system
- Paper structure: Introduction (simulation framework Ketju and initial conditions), simulations with dark matter halos (sinking binary/triplet formation and impact on density distributions), simulations with galaxies included in dark matter halos (same discussions as previous section), results analysis and discussion, conclusion
- Study sheds light on challenges associated with bringing/maintaining low mass MBHs in centers of low mass halos/galaxies

## Summary

In this paper, the authors present a high-resolution numerical study of the sinking and merging of massive black holes (MBHs) in multiple minor mergers of low-mass dark matter halos with and without galaxies. The mass range of the MBHs considered is $10^3 - 10^7 \, \mathrm{M}_\odot$, while the mass range of the dark matter halos is $4\times 10^8 \, \mathrm{M}_\odot \lesssim \mathrm{M}_{\mathrm{halo}} \lesssim 2\times 10^{10} \, \mathrm{M}_\odot$. To accurately model the dynamics of MBHs and their interaction with their stellar and dark matter environment, the authors utilize the simulation code Ketju. Ketju is an extension of the tree gravity solver Gadget-3 which has been previously introduced in other works. The code incorporates unsoftened forces between star/dark matter components and MBHs to accurately account for dynamical friction and scattering effects caused by MBH binaries or multiples. The simulations conducted in this study reveal various complex evolution paths for sinking MBHs including binaries, triplets, free-floating MBHs and dynamically or recoil ejected MBHs. It is found that low-mass MBHs ($\lesssim 10^5 M_\odot$) have difficulty sinking to the center or merging; however collisional interactions with dark matter alone can drive these low-mass MBHs to coalesce. On the other hand higher mass MBHs ($\gtrsim 10^6 M_\odot$) tend to sink to the center more frequently and trigger a process known as scouring which leads to a transformation from a baryon-dominated system to a dark matter-dominated system as both cores are affected. The paper is structured as follows: Section 2 provides an introduction to the simulation framework Ketju and initial conditions used in this study; Section 3 discusses simulations with dark matter halos focusing on sinking binary/triplet formation and impact on density distributions; Section 4 repeats these discussions for simulations with galaxies included in dark matter halos; results are then analyzed and discussed in section 5 followed by conclusion in section 6. This high resolution study sheds light on challenges associated with bringing/maintaining low mass MBHs in centers of low mass halos/galaxies which remains a hurdle for merger assisted seed growth mechanisms.