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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) - PowerPoint PPT Presentation
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Steen H. Hansen
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
Steen H. Hansen
Dark Matter structures
Dark Matter density profile(a fairly non-weird thing)
Dark Matter velocity anisotropy profile(the weird thing)
Steen H. Hansen
Why is the central density profile important ?
Dark Matter density profile
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 darkmatter 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?
Steen H. Hansen
QuickTime™ and aYUV420 codec decompressor
are needed to see this picture.
How to form DM structures in numerical simulations:Initial conditions known from observations
Dark Matter density profile
Steen H. Hansen
Dark Matter density profile
Repeat: The centralslope is important
For indirectdetection of DM
For understandinggravity
= slope
Steen H. Hansen
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 behindthese results!
Steen H. Hansen
Dark Matter density profile
Needed for Anne`s lectures:We just saw that everything shows central slope of about
-1But … 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).
Steen H. Hansen
Dark Matter structures
Dark Matter density profile(a fairly non-weird thing)
Dark Matter velocity anisotropy profile(the weird thing)
Steen H. Hansen
Dark Matter anisotropy
Velocity anisotropy = “different temperature”
€
β =1−σ tan
2
σ rad2
Steen H. Hansen
Dark Matter anisotropy
(Hansen & Moore, 2006)
A non-zero anisotropyimplies that DM behavesvery different from aclassical gas
Beta_gas = 0
The non-zero anisotropyin a typical cosmologicalstructure (DM + baryons)
What does it normallylook like ?
Why do we careabout the velocityanisotropy ?
Steen H. Hansen
Dark Matter anisotropy
All dark matter structures, normal orcrazy origin, havethe same connectionwith anisotropy
We have no idea why
(Hansen & Moore, 2006)Can we measure suchan anisotropy?
Steen H. Hansen
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 structuresis not achieved through point-like interactions!
Steen H. Hansen
How to measure this weird thing
Build a HUGE directional sensitive detector….Many years into the future! (see Anne’s lecture)
Velocity anisotropy of our own galaxy
Host & Hansen, 2007
Steen H. Hansen
How to measure this weird thingCan we measure the velocity anisotropy of a distant galaxy cluster - already today?
Sersic 159-3
X-ray observationsgive us both temperature anddensity as functionsof radius
Steen H. Hansen
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)
Steen H. Hansen
How to measure this weird thing
beta_DM = function(T_gas, density_gas)
(Hansen & Piffaretti, 2008)
Beta_gas = 0
We conclude:these large DM structure havenot reachedequilibrium through the same processesas baryonic (star or gas) structures do.
Steen H. Hansen
Conclusions
Observational proofs of the existence of Dark Matter
Universal properties of Dark Matter structures
How to measure weird properties directly
Steen H. Hansen
Conclusions
Observational proofs of the existence of Dark MatterThe existence of Dark Matter has been established beyond any doubt
Universal properties of Dark Matter structures
How to measure weird properties directly
Steen H. Hansen
Conclusions
Observational proofs of the existence of Dark MatterThe existence of Dark Matter has been established beyond any doubt
Universal properties of Dark Matter structures
How to measure weird properties directly
Steen H. Hansen
Conclusions
Observational proofs of the existence of Dark MatterThe existence of Dark Matter has been established beyond any doubt
Universal properties of Dark Matter structuresThe phenomenological universalities have been established,however, we basically don’t understand their origin
How to measure weird properties directly
Steen H. Hansen
Conclusions
Observational proofs of the existence of Dark MatterThe existence of Dark Matter has been established beyond any doubt
Universal properties of Dark Matter structuresThe phenomenological universalities have been established,however, we basically don’t understand their origin
How to measure weird properties directly
Steen H. Hansen
Conclusions
Observational proofs of the existence of Dark MatterThe existence of Dark Matter has been established beyond any doubt
Universal properties of Dark Matter structuresThe phenomenological universalities have been established,however, we basically don’t understand their origin
How to measure weird properties directlyWe have probably already started measuring it…
Steen H. Hansen
Conclusions
Observational proofs of the existence of Dark MatterThe existence of Dark Matter has been established beyond any doubt
Universal properties of Dark Matter structuresThe phenomenological universalities have been established,however, we basically don’t understand their origin
How to measure weird properties directlyWe have probably already started measuring it…
The End
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