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