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Investigating Thermodynamic Activity in Dark Matter Halos: Coincidence or Universal Feature?

This study explores the potential thermodynamic behavior of dark matter in galactic halos, challenging traditional models that use equilibrium assumptions. While the standard treatment acknowledges rotation curves and density profiles, it often overlooks the implications of self-interaction among dark matter particles. By analyzing various galaxies and their rotation characteristics, we suggest that dark matter may exhibit properties reminiscent of a self-gravitating isothermal Boltzmann gas. This could provide insights into dark matter's thermal equilibrium, self-interactions, and the discrepancies in simulation predictions.

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Investigating Thermodynamic Activity in Dark Matter Halos: Coincidence or Universal Feature?

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  1. Possible Evidence of Thermodynamic Activity in Dark Matter Haloes

  2. A coincidence? Universal feature of galactic haloes : Flat rotation curves Flat rotation curves naturally appear if source is a self-gravitating isothermal Boltzmann gas. Circular orbits + Spherical symmetry +

  3. Now recall basic thermo undergrad homework problem: find density profile of the atmosphere

  4. Standard treatment of dark matter haloesN particle simulations

  5. Standard treatment of dark matter haloes N particle simulations Simulation-inspired density profiles: Navarro-Frenk-White Einasto ...and many others give reasonable fits to rotation curve data

  6. Standard treatment of dark matter haloes N particle simulations Thermodynamics Simulation-inspired density profiles: Navarro-Frenk-White Einasto ...and many others give reasonable fits to rotation curve data

  7. Equilibrium thermodynamics not considered in standard treatment of dark matter haloes • Relaxation times arising from gravitational interactions alone are too long (compared with the Hubble time) for thermodynamic equilibrium to be established. • If interactions other than gravity are present among the dark matter particles, they are too weak to establish thermal equilibrium. Bull%&t cluster • Rotation curves not exactly flat

  8. But issues remain for simulations eg., cusp at r=0, missing satellite problem • Strong bounds on dark matter-baryon interactions, -- not so for dark matter self-coupling • Self-couplings ( with interaction times < 1/H ) can cure simulation issues Observational evidence for self-interacting cold dark matter, David N. Spergel, Paul J. Steinhardt Phys.Rev.Lett. 84 (2000) 3760-3763 Beyond Collisionless Dark Matter: Particle Physics Dynamics for Dark Matter Halo StructureSean Tulin, Hai-Bo Yu, Kathryn M. Zurek,  arXiv:1302.3898 • Is equilibrium possible?

  9. A closer look at rotation curves: Using simplifying assumptions, • both density and potential can be determined directly. Newton Poisson

  10. NGC 2841 • Disk distance scale =3.5 kpc, data available up to 51.6 kpc (THINGS) • H gas mass is approximately 4% of the disk mass Series fit using

  11. NGC 5055 • Disk distance scale =3.622 kpc, data available up to 44.4 kpc (THINGS) • H gas mass is approximately 12.6% of the disk mass

  12. NGC 3521 • Disk distance scale =3.3 kpc, data available up to 35.5 kpc (THINGS) • H gas mass is approximately 11% of the disk mass

  13. NGC 7331 • Disk distance scale =3.2 kpc, data available up to 24 kpc (THINGS) • H gas mass is approximately 7% of the disk mass

  14. NGC 2403 • Disk distance scale =2.75 kpc, data available up to 24 kpc (THINGS) • H gas mass is approximately 19% of the disk mass

  15. NGC 2903 • Disk distance scale =3 kpc, data available up to 31 kpc (THINGS) • H gas mass is approximately 7% of the disk mass Series fit from 1 kpc to 31 kpc

  16. Disk distance scale = 2.68 kpc, data available up to 38 kpc • H gas mass is approximately 29% of the disk mass NGC 3198

  17. Moral: coincidence with Boltzmann gas for large portions of haloes - even though rotation curves not exactly flat. Breakdown of Boltzmann description at small and large distances • A simple model for Boltzmann region • Assume: • spherical symmetry • dynamics given by Emden eq. • gravitational attraction to • inner (baryonic) region • determined by boundary • conditions at r=R_g • leads to three-parameter family of solutions • get fits for

  18. Boltzmann fits NGC 2841 NGC 5055 NGC 7331 NGC 2903 NGC 2403 NGC 3521 NGC 3198

  19. CONCLUDING REMARKS • many improvements possible: • drop spherical symmetry • include H gas, disk contributions • extend to galaxy interior • – test quantum statistics If similarity with a Boltzmann gas not a coincidence, appears to indicate dark matter in thermal equilibrium. What can this tell us about dark matter self-interactions?

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