Wandering Supermassive Black Holes in Milky Way Mass Halos (1802.06783v2)

Abstract: We present a self-consistent prediction from a large-scale cosmological simulation for the population of `wandering' supermassive black holes (SMBHs) of mass greater than $10^6$ M$_{\odot}$ on long-lived, kpc-scale orbits within Milky Way (MW)-mass galaxies. We extract a sample of MW-mass halos from the Romulus25 cosmological simulation (Tremmel et al. 2017), which is uniquely able to capture the orbital evolution of SMBHs during and following galaxy mergers. We predict that such halos, regardless of recent merger history or morphology, host an average of $5.1 \pm 3.3$ SMBHs, including their central black hole, within 10 kpc from the galactic center and an average of $12.2 \pm 8.4$ SMBHs total within their virial radius, not counting those in satellite halos. Wandering SMBHs exist within their host galaxies for several Gyrs, often accreted by their host halo in the early Universe. We find, with $>4\sigma$ significance, that wandering SMBHs are preferentially found outside of galactic disks.

• The paper shows that Milky Way-mass halos host an average of 5 SMBHs within 10 kpc and 12 within the virial radius.
• It employs the high-resolution Romulus25 simulation with advanced sub-grid physics to accurately model SMBH seeding, growth, and orbital dynamics.
• Findings suggest wandering SMBHs likely originate from early accreted dwarf galaxies and tend to reside outside the galactic disk with strong statistical significance.

Wandering Supermassive Black Holes in Milky Way-Mass Halos

The paper by Tremmel et al. examines the presence of wandering supermassive black holes (SMBHs) within Milky Way (MW)-like halos using data from the {\sc Romulus25} cosmological simulation. This simulation is notable for its ability to accurately track SMBH orbital evolutions on sub-kiloparsec scales, making it a useful tool for investigating these phenomena.

Key Findings

The authors predict that MW-mass halos host an average of $5.1 \pm 3.3$ SMBHs within 10 kpc of their centers, including the central black hole, and $12.2 \pm 8.4$ SMBHs within the virial radius, excluding those in satellite halos. Notably, wandering SMBHs have a preferential tendency to be located outside the galactic disk with more than 4σ significance. This distribution suggests that these SMBHs arise predominantly from the accretion of smaller galaxies in the early universe, lending insight into hierarchical galaxy formation and black hole growth models.

Simulation Details

The {\sc Romulus25} simulation employs a $\Lambda$CDM cosmology and features advanced sub-grid physics for SMBH formation and evolution. SMBHs are seeded in high-density, gas-rich regions capable of rapid collapse, allowing them to exist in modest halos of $10^8$-$10^{10}$ M$_{\odot}$ shortly after the Big Bang. Once formed, SMBHs grow via gas accretion modeled through a modified Bondi-Hoyle formula, accounting for the angular momentum in their vicinity. This detailed physics resolution ensures a high degree of reliability in tracking SMBH dynamics, particularly in the evolution of those off the central galactic plane.

Orbital Dynamics and Growth

Analysis of the simulation reveals complex orbital dynamics, with most wandering SMBHs maintaining stable kpc-scale orbits over several billion years. Their dynamics suggest they predominantly originated from smaller progenitor galaxies that were assimilated into the larger structure. Surprisingly, a significant fraction of these black holes (over 50%) have grown minimally beyond their initial seed mass. In contrast, a smaller subset has experienced significant growth, either due to advantageous orbital paths or consequent close SMBH interactions.

Implications

This research underscores the importance of understanding SMBHs beyond those at galactic centers. The presence and dynamics of wandering SMBHs can significantly influence our comprehension of early galaxy assembly processes, particularly regarding the retention and ejection mechanisms for black holes during galaxy mergers. The identification of such SMBHs presents observational challenges yet holds promise for corroborating simulation-based predictions.

Future research may leverage strategies like gravitational wave astronomy or high-resolution X-ray observations to detect such wandering black holes, thereby enhancing our understanding of SMBH populations in relation to galactic evolution. Furthermore, extending such simulations to include potential scenarios like three-body interactions or gravitational wave recoil could refine our models of SMBH retention or ejection processes in disrupted photogenic environments.

Conclusion

Tremmel et al.'s investigation provides a well-grounded prediction about wandering SMBHs, reinforcing the cosmological narrative of hierarchical galaxy formation. The work presents a compelling case for ongoing exploration into these dynamics, offering a potential avenue for revising galactic evolution models as observational techniques advance.