A cosmic sling-shot mechanismJohan Samsing

DARK, Niels Bohr Institute, University of Copenhagen

• The Dynamical Sling-Shot Mechanism.

• Previous work and motivations.

• Movie of a DM halo merger!

• An ejected particle in an expanding universe.

• Modeling of the mass ejection history.

• The field outside the virial radius.

• Phase-space distribution of ejected particles.

Outline

Gravitational Sling-Shot Mechanism

•Basic Idea:- In few body system you can exchange energy between particles.

Gravitational Sling-Shot Mechanism

• You can speed particles up to high energies.• Positive energy comes from increasing binding energy.

Gravitational Sling-Shot Mechanism

• Speeding up probes in the solar system.• Gains of order 10 km/sec per passage

Exchange energy with planets Example: Cassini’s trip to Saturn

Gravitational Sling-Shot Mechanism• Gravitational wave sources are build this way!• GRBs are very likely collisions between NSs.• Few-body interactions could (is) be future to probe fundamental physics!

Before:

After:

Dynamical Mechanism:• SN/stars with no host and hypervelocity stars.• http://arxiv.org/abs/1102.0007 ‘Cosmology with Hypervelocity Stars’- Avi Loeb.

• Can we do something similar but with the current observed field?• How is the tracers created?

Galaxy dominate Cosmology dominates

This work: Galaxy Mergers• We consider dark matter mergers - a highly non-linear feature.• Particles are kicked out by an effect similar to the 3-body sling-shot.

Merger:

Ejection: Reduced a highly non-linear problem down to a simple physical mechanism!

A Few Motivations• Whole community (try to) calculate DM steady state: here we show part of the

particles are distributed according to simple sling-shot effect. It’s a great and funny mechanical problem!

• Recent work by e.g. Beehzori, Wechsler, Loeb describe fraction of unbound particles in halos. They don’t include any dynamical arguments or history of the ejected particles.

• Direct DM experiments can be very sensitive to the high energy part of the DM distribution.

• Observations of hyper velocity stars/gas/galaxies.

• Observations: - Mapping the halo by stacking – use BG sources e.g. QSO and absorption lines – Stellar evolution and ejection age etc. – Outskirts of clusters can hold enormous information! Don’t restrict yourself to the virial sphere!

MOVIE!Movie: DM halo merger

Ejected or Trapped?Ejected particles: Passes the center when the potential is declining.

Trapped particles:Cant escape!

Orbits are analytically known for some profiles.

A Few Fundamental Questions:• How far do they travel?• What is their distribution today?• What is the ‘halo-horizon’?• What are the dynamical signatures in phase-space?

Time -> z=0

(pos,vel)

Particles in an Expanding Universe

Total acceleration:

Halo mass:

Expansion

BG: attracting BG: repelling

Position and velocity at z=0

What is the most likely regime? Depends on cosmology!!

Ejection Velocity and Mass Rate

Mass ejection rate: Ejection velocity:

Merger rate (Fakhouri et al. 2010):

Mass, Ejection Age and Distance• Inverse age-distance relation compared to virialized part of the halo.

• Looking into the outer parts is looking back in time – ‘cosmic fossils’.

• Can be mapped out using background sources, e.g., QSOs (working on that).

• Slingshot mechanism only way to reach such distances!

Phase Space Distribution

• Depends on:- Cosmology.- accretion history.

• Distribution:- Is distributed in anotherpart of phase spacecompared to usual distributionsSuch as: infall, virialized matter, caustics etc.

Conclusions

• Classical slingshot mechanism ejects particles into large orbits where cosmology takes over.

• Funny mechanics problem that can explain the distribution of high energy particles - no need for any fancy statistical mechanics. Large part of the particles distribute according to this mechanism!

• New dynamical component and tracer of the field that can be studied around galaxy clusters.

• Could motivate observers to look for ‘host-less’ galaxies, gas etc.

• If map out in detail – reveals formation history and the interplay between BG and host halo gravitational field.

•Most of all: a highly complex system can be reduced to a simple physical mechanism that plays a role on all scales in our universe! A fun problem!