Dynamical Friction and Black Holes in Ultralight Dark Matter Solitons (2403.09038v1)

Abstract: We numerically simulate the motion of a black hole as it plunges radially through an ultralight dark matter soliton. We investigate the timescale in which dynamical friction reduces the kinetic energy of the black hole to a minimum, and consider the sensitivity of this timescale to changes in the ULDM particle mass, the total soliton mass, and the mass of the black hole. We contrast our numerical results with a semi-analytic treatment of dynamical friction, and find that the latter is poorly suited to this scenario. In particular, we find that the back-reaction of the soliton to the presence of the black hole is significant, resulting in oscillations in the coefficient of dynamical friction which cannot be described in the simple semi-analytical framework. Furthermore, we observe a late-time reheating effect, in which a significant amount of kinetic energy is transferred back to the black hole after an initial damping phase. This complicates the discussion of ULDM dynamical friction on the scales relevant to the final parsec problem.

• The paper reveals how ULDM particle mass and black hole mass critically influence the dynamical friction timescale during radial plunges.
• The paper demonstrates a reheating effect where soliton oscillations partially restore the black hole's kinetic energy after friction.
• The paper shows that standard semi-analytic models fall short of capturing the complex, time-dependent interactions in ULDM solitonic cores.

Dynamical Friction and Black Holes in Ultralight Dark Matter Solitons

The paper of dynamical friction within ultralight dark matter (ULDM) models has emerged as a pivotal element in understanding the non-trivial interactions between supermassive black holes (SMBHs) and dark matter in galactic cores. This paper conducts a numerical investigation into these interactions by simulating the trajectory of a black hole through a ULDM solitonic core, providing a nuanced analysis of the dynamical friction forces at play and their contribution to the intricate dynamics of such systems.

Overview and Methodology

The paper's primary focus is the behavior of a black hole as it plunges radially through a ULDM soliton, a scenario motivated by the ULDM hypothesis where wave-like phenomena on astrophysical scales are exhibited due to the minuscule mass of dark matter particles, typically around $10^{-23}$ to $10^{-20}$ eV. This contrasts with classical cold dark matter (CDM), potentially resolving discrepancies observed in small-scale galactic phenomena that are inconsistent with CDM predictions.

The simulation framework implemented here uses the {\sc PyUltraLight} code, solving the Schrödinger-Poisson equations to model ULDM field dynamics. The black holes are represented as Plummer spheres, simplifying the complex gravitational interactions within the soliton. The simulations capture essential factors such as ULDM particle mass, core mass, and black hole mass to discern their influence on dynamical friction.

Key Findings

1. Timescale Dependence: The paper elucidates the dependence of the dynamical friction timescale on several parameters. Both ULDM particle mass and the black hole mass significantly influence the energy dissipation rate as the black hole traverses the ULDM soliton. Specifically, higher ULDM particle masses contribute to more efficient friction, primarily due to changes in solitonic core density.
2. Reheating Phenomenon: A notable finding is the reheating effect observed at later times where the kinetic energy, initially reduced due to dynamical friction, is partially restored to the black hole. This results from complex oscillations in the soliton structure itself, which are not predicted by conventional semi-analytic models.
3. Semi-analytic Models: The paper provides a critical contrast between numerical results and the semi-analytic treatments based on established models like Chandrasekhar's. These models inadequately capture the full complexity of the interactions in ULDM fields due to their failure to account for soliton backreaction and time-dependent dynamical friction coefficients. Despite this, certain adapted versions of these models can approximate the stopping time of black holes in the soliton context.

Implications and Future Directions

The results carry significant implications for astrophysical scenarios involving black holes within galactic cores, particularly concerning the final parsec problem where SMBH binaries fail to close the gap to the point where gravitational wave emission becomes dominant. Dynamical friction in ULDM cores may contribute to a resolution by providing an alternative mechanism for energy dissipation.

The observed phenomena invite further exploration into ULDM-solition-black hole dynamics, particularly considering solitonic excitation and non-linear backreaction effects. Future research might focus on more sophisticated models incorporating these dynamics or experimental verification through gravitational wave observations, complementing the numerical framework presented.

Conclusion

This research adds a valuable perspective to the discourse on dark matter and its interactions with baryonic matter, carving out a role for ULDM in potential cosmological and astrophysical phenomena. The findings underscore the need for a reconciled understanding of dynamical friction in exotic dark matter frameworks and set the stage for deeper inquiry into the evolving landscape of dark matter research.