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What do cosmological dark matter structures look like: simulations, theory and observations. Steen H. Hansen Dark Cosmology Center Niels Bohr Institute Copenhagen University. Spåtind, January 2008. Dark Matter structures. Dark Matter density profile (a fairly non-weird thing)

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## What do cosmological dark matter structures look like: simulations, theory and observations

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**What do cosmological dark matter structures look like:**simulations, theory and observations Steen H. Hansen Dark Cosmology Center Niels Bohr Institute Copenhagen University Spåtind, January 2008**Dark Matter structures**Dark Matter density profile (a fairly non-weird thing) Dark Matter velocity anisotropy profile (the weird thing)**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?**Dark Matter density profile**How to form DM structures in numerical simulations: Initial conditions known from observations**Dark Matter density profile**Repeat: The central slope is important For indirect detection of DM For understanding gravity = slope**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!**Dark Matter density profile**Needed for Anne`s lectures: We just saw that everything shows central slope of about -1 But … that is for pure dark matter! When baryons (stars, gas, dust) are included, then virtually anything may happen: Sometimes a core appears (like in dwarf galaxies?), probably because of the effects of stars (slope = 0). When gas cools (inside large galaxies?) then a steep slope may appear (slope = -2).**Dark Matter structures**Dark Matter density profile (a fairly non-weird thing) Dark Matter velocity anisotropy profile (the weird thing)**Dark Matter anisotropy**Velocity anisotropy = “different temperature”**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)**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 such an anisotropy? (Hansen & Moore, 2006)**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!**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! (see Anne’s lecture)**Can we measure the velocity anisotropy**of a distant galaxy cluster - already today? How to measure this weird thing Sersic 159-3 X-ray observations give us both temperature and density as functions of radius**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)**beta_DM = function(T_gas, density_gas)**How to measure this weird thing We conclude: these large DM structure have not reached equilibrium through the same processes as baryonic (star or gas) structures do. (Hansen & Piffaretti, 2008) Beta_gas = 0**Conclusions**Observational proofs of the existence of Dark Matter Universal properties of Dark Matter structures How to measure weird properties directly**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**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**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**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**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…**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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