Why do galaxies have extended flat rotation curves?

Summary

Galaxies have extended flat rotation curves, indicating they are isothermal spheres with particles obeying the Maxwell-Boltzmann distribution. The dark matter particles are collisional and elastic, and galaxies formed in this state due to the growing galaxy halo and universe expansion.

Highlights

• Galaxies have extended flat rotation curves, indicating isothermal spheres.
• Dark matter particles are collisional and elastic.
• Galaxies formed in an isothermal state due to the growing galaxy halo and universe expansion.
• The dark matter core radius is determined by the "warmness" of the dark matter.
• Baryons have lower thermal velocities and inelastic collisions, migrating to the bottom of the gravitational potential well.
• The measured warm dark matter adiabatic invariant is in agreement with the "no freeze-in and no freeze-out" scenario of scalar dark matter coupled to the Higgs boson.
• Warm dark matter simulations should not neglect thermal velocity if the galaxy core is of interest.

Key Insights

• The observed extended flat rotation curves of galaxies indicate that they are approximately isothermal spheres with particles obeying the Maxwell-Boltzmann distribution. This suggests that the galaxies do not have time to relax to the isothermal equilibrium state and must have formed already in this state.
• The dark matter particles are collisional and these collisions are elastic, allowing the galaxies to maintain their isothermal state. This is in contrast to baryons, which have inelastic collisions and radiate energy, causing them to migrate towards the bottom of the gravitational potential well.
• The growing galaxy halo with a density run of ρ(r) ∝ r^(-2) at large r, and the expansion of the universe, are responsible for the isothermal formation of galaxies. The particles falling into the growing galaxy halo potential well populate the tail end of the Maxwell-Boltzmann distribution.
• The dark matter core radius is determined by the "warmness" of the dark matter, which is characterized by the adiabatic invariant v(1). This adiabatic invariant is of cosmological origin and is observed to be in agreement with the "no freeze-in and no freeze-out" scenario of scalar dark matter coupled to the Higgs boson.
• Baryons have lower thermal velocities than dark matter during the formation of first-generation galaxies, causing them to migrate towards the bottom of the gravitational potential well. This results in α, the ratio of baryon to dark matter velocity dispersion, being less than 1.
• The measured warm dark matter adiabatic invariant v(1) is observed to be invariant within uncertainties and relaxation corrections, despite the core baryon density dominating the core warm dark matter density by several orders of magnitude.
• Warm dark matter simulations should not neglect the thermal velocity if the galaxy core is of interest, as it plays a crucial role in determining the core radius and density profile.

Mindmap

Citation

Hoeneisen, B. (2024). Why do galaxies have extended flat rotation curves? arXiv. https://doi.org/10.48550/ARXIV.2412.17869