Dynamical friction in self-interacting ultralight dark matter (2312.07684v1)

Abstract: We explore how dynamical friction in an ultralight dark matter (ULDM) background is affected by dark matter self-interactions. We calculate the force of dynamical friction on a point mass moving through a uniform ULDM background with self-interactions, finding that the force of dynamical friction vanishes for sufficiently strong repulsive self-interactions. Using the pseudospectral solver $\texttt{UltraDark.jl}$, we show with simulations that reasonable values of the ULDM self-interaction strength and particle mass cause $\mathcal{O}(1)$ differences in the acceleration of an object like a supermassive black hole (SMBH) traveling near the center of a soliton, relative to the case with no self-interactions. For example, repulsive self-interactions with $\lambda = 10^{-90}$ yield a deceleration due to dynamical friction $\approx70\%$ smaller than a model with no self-interactions. We discuss the observational implications of our results for SMBHs near soliton centers and for massive satellite galaxies falling into ultralight axion halos and show that outcomes are dependent on whether a self-interaction is present or not.

• The paper reveals that strong repulsive self-interactions can effectively nullify dynamical friction by preventing gravitational wake formation.
• It utilizes the Gross-Pitaevskii-Poisson framework and UltraDark.jl simulations to explore friction across varying self-interaction strengths.
• Findings imply modified orbital decay for astrophysical bodies, opening new observational pathways to constrain ultralight dark matter models.

Essay on "Dynamical friction in self-interacting ultralight dark matter"

The paper "Dynamical friction in self-interacting ultralight dark matter" by Glennon et al. investigates the effects of dynamical friction on massive objects moving through an ultralight dark matter (ULDM) background, particularly considering the role of dark matter self-interactions. This paper extends previous analyses of ULDM dynamics by incorporating the self-interaction term, a feature present in many plausible ULDM scenarios.

Theoretical Framework

The authors begin by detailing the ultralight dark matter model, wherein the constituent particles possess an extremely small mass, approximately on the order of $10^{-22}\text{ eV}$. These particles are described by a classical field minimally coupled to gravity. The dynamics of the system are governed by the Gross-Pitaevskii-Poisson (GPP) equations, which account for both the quantum nature of ULDM and classical gravitational interactions.

A significant contribution of this paper is examining scenarios where quartic self-interactions introduce a non-negligible term in the dynamics equations. This adds complexity not considered in non-interacting ULDM analyses, necessitating novel analytical and computational techniques to solve the equations of motion.

Analytical Insights

Underpinning the research is an analytic treatment that provides insights into how self-interactions modify dynamical friction forces. Particularly, the authors find that, for sufficiently strong repulsive self-interactions, the gravitational wake typically responsible for slowing down a massive object via dynamical friction can be significantly suppressed, effectively nullifying the dynamical friction. This nullification occurs when the sound speed in the self-interacting ULDM medium exceeds the relative velocity of the massive object, preventing wake formation.

Numerical Simulations

To validate and extend their theoretical predictions, Glennon et al. performed extensive numerical simulations using the UltraDark.jl pseudospectral solver. These simulations explore a wide parameter space of possible self-interaction strengths and ULDM configurations, testing the theoretical limits. Three primary regimes emerge: negligible self-interactions, where traditional dynamical friction is observed; moderate repulsive self-interactions causing a reduction in frictional force; and strong repulsive interactions leading to negligible dynamical friction.

The dynamical friction forces are seen to decrease significantly as the self-interaction strength increases, reaching near-zero for the boundary beyond which gravitational wakes do not form. Numerical results provide general agreement with the expected analytic trends, though with noted quantitative differences in intermediate regimes requiring further investigation.

Implications and Applications

This paper's implications are both theoretical and observational. Theoretically, it highlights an important dynamical mechanism by which self-interactions can influence structure formation in ULDM models. Practically, the findings suggest altered orbital decay rates for astrophysical objects like supermassive black holes (SMBHs) and massive satellite galaxies (like the Large Magellanic Cloud) when traveling through self-interacting ULDM media. The orbital lifetime for an SMBH near a soliton core, for example, could be significantly increased with plausible ULDM self-interactions. Additionally, these dynamics might offer alternative observational signatures to constrain the strength of dark matter self-interactions.

Future Directions

Conclusively, the authors open pathways for new research directions. Understanding discrepancies between analytic theories and simulations in certain regions of parameter space is a natural next step. Furthermore, extending the simulation setup to include self-consistent, large-scale ULDM structures could offer deeper insights into cosmological and galactic dynamics. Observationally, the effects on dynamical processes such as SMBH mergers or satellite galaxy infall offer promising avenues for linking underlying dark matter physics with astrophysical observations.

Overall, this paper offers a comprehensive extension to ULDM dynamical studies by incorporating self-interactions, providing essential insights and laying foundational work for subsequent theoretical and observational explorations in the field.