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Dark matter - existence, and other strange properties

Dark matter - existence, and other strange properties. Steen H. Hansen Dark Cosmology Center Niels Bohr Institute Copenhagen University. DFS annual meeting Nyborg Strand , June 20 2007. What is dark matter?. Overview. Observational proofs of the existence of Dark Matter

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Dark matter - existence, and other strange properties

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  1. Dark matter - existence,and other strange properties Steen H. Hansen Dark Cosmology Center Niels Bohr Institute Copenhagen University DFS annual meeting Nyborg Strand, June 20 2007

  2. What is dark matter?

  3. Overview Observational proofs of the existence of Dark Matter Universal properties of Dark Matter structures How to measure weird properties directly

  4. Dark Matter in galaxies

  5. Dark Matter in galaxies Rotation curves in galaxies are too flat

  6. Dark Matter in the Universe - CMB A snapshot of the Universe at 300 kyr

  7. Dark Matter in the Universe - SN

  8. Dark Matter in the Universe - SN

  9. One method to measure total mass in galaxy clusters is weak lensing DM in galaxy clusters

  10. DM in galaxy clusters Visible stars and galaxies + total mass (from weak lensing) X-ray gas + total mass (Clowe et al, 2006)

  11. Observation: Red is gas, blue is total mass Artists impression: Red is gas, blue is dark matter DM in galaxy clusters (Clowe et al, 2006)

  12. Dark Matter structures Dark Matter density profile (a fairly non-weird thing) Dark Matter velocity anisotropy profile (the weird thing)

  13. Dark Matter density profile Why is the central density profile important ? 1) Many dark matter candidates can self-annihilate Then the annihilation rate goes like density squared, 2 If the central density slope is steep, then those dark matter candidates will be easy to observe 2) We would like to understand something fundamental about self-gravitating dark matter. Maybe the profile is a good (easy) first step?

  14. Dark Matter density profile How to form DM structures in numerical simulations: Initial conditions known from observations

  15. Dark Matter density profile Repeat: The central slope is important For indirect detection of DM For understanding gravity = slope

  16. Dark Matter density profile High resolution simulation of DM cluster formation Conclusion: central DM density slope is -1(NFW 1997), -1.5(Moore et al 1998), -1.2(Diemand et al, 2004), -0.8(Hansen & Stadel, 2006), …..

  17. Temperature Dark Matter density profile X-ray observation of a galaxy cluster gives gas temperature and gas density profile together with assumption (hydrostatic equilibrium) it gives total mass profile Radius Density Conclusion: central DM density slope is close to -1 (Pointcouteau et al, 2005)

  18. Mass reconstruction using strong lensing of a galaxy cluster Dark Matter density profile Limousin et al, 2007 Conclusion: central DM density slope is close to -1 (Limousin et al, 2006)

  19. First analytical solution found that the central density slope is -1(Hansen 2004) Refined analytical solutions find the central density slope is -0.8(Austin et al, 2006; Dehnen & McLaughlin 2006) Analytical solution to the problem implies solving the fundamental collisionless equation (the Jeans equation) This equation has too many free parameters, so some assumptions must be made Dark Matter density profile Input from numerical simulations - we do not understand why this relation exists (Taylor & Navarro, 2001)

  20. Dark Matter density profile Status concerning central density steepness: Numerical simulations find slope about -1 Various observations find slope about -1 Analytical solutions (under certain assumptions) find slope about -1 However, we do not understand the physics behind these results!

  21. Dark Matter structures Dark Matter density profile (a fairly non-weird thing) Dark Matter velocity anisotropy profile (the weird thing)

  22. Dark Matter anisotropy Velocity anisotropy = “different temperature”

  23. Dark Matter anisotropy A non-zero anisotropy implies that DM behaves very different from a classical gas Beta_gas = 0 Why do we care about the velocity anisotropy ? What does it normally look like ? The non-zero anisotropy in a typical cosmological structure (DM + baryons) (Hansen & Moore, 2006)

  24. Dark Matter anisotropy Spherical collapse simulations = “rather artificial” Could the anisotropy be specific to initial condition, or evolution of the universe? We want to test the anisotropy in “non-cosmological” structures

  25. Dark Matter anisotropy Spherical collapse simulations = “rather artificial” Results...

  26. Dark Matter anisotropy Create funny initial conditions, and then slam them together

  27. Dark Matter anisotropy Structures quickly forget their initial conditions, when perturbed strongly (Hansen & Stadel, 2006)

  28. Dark Matter anisotropy All dark matter structures, normal or crazy origin, have the same connection with anisotropy We have no idea why Can we measure this? (Hansen & Moore, 2006)

  29. How to measure this weird thing Reminder: this is really a strange thing beta_gas = 0 (entropy does not know about direction) If beta_DM is really different from zero, then the equilibration of dark matter structures is not achieved through point-like interactions!

  30. Velocity anisotropy of our own galaxy How to measure this weird thing Host & Hansen, 2007 Build a HUGE directional sensitive detector…. Many years into the future!

  31. Velocity anisotropy of a distant galaxy cluster? How to measure this weird thing Sersic 159-3 X-ray observations give us both temperature and density as functions of radius

  32. How to measure this weird thing The equation of hydrostatic equilibrium (gas equation, valid if nice and relaxed cluster) Total Mass= function(T_gas, density_gas) The Jeans equation (DM equation) Total Mass= function(T_DM, density_DM, beta_DM)

  33. We can now solve the two equations to get T_gas = T_DM (function of beta_DM) How to measure this weird thing

  34. If we assume energy equipartition (something fancy that makes really good sense) beta_DM = function(T_gas, density_gas) How to measure this weird thing (Hansen & Piffaretti, 2007) Beta_gas = 0

  35. Conclusions Observational proofs of the existence of Dark Matter Universal properties of Dark Matter structures How to measure weird properties directly

  36. Conclusions Observational proofs of the existence of Dark Matter The existence of Dark Matter has been established beyond any doubt Universal properties of Dark Matter structures How to measure weird properties directly

  37. Conclusions Observational proofs of the existence of Dark Matter The existence of Dark Matter has been established beyond any doubt Universal properties of Dark Matter structures How to measure weird properties directly

  38. Conclusions Observational proofs of the existence of Dark Matter The existence of Dark Matter has been established beyond any doubt Universal properties of Dark Matter structures The phenomenological universalities have been established, however, we basically don’t understand their origin How to measure weird properties directly

  39. Conclusions Observational proofs of the existence of Dark Matter The existence of Dark Matter has been established beyond any doubt Universal properties of Dark Matter structures The phenomenological universalities have been established, however, we basically don’t understand their origin How to measure weird properties directly

  40. The End Conclusions Observational proofs of the existence of Dark Matter The existence of Dark Matter has been established beyond any doubt Universal properties of Dark Matter structures The phenomenological universalities have been established, however, we basically don’t understand their origin How to measure weird properties directly We have probably already started measuring it…

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