A cosmic sling-shot mechanism Johan 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. ‘Cosmology with Hypervelocity Stars’- Avi Loeb. Can we do something similar but with the current observed field? How is the tracers created? Galaxy dominateCosmology 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: attractingBG: 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 another part of phase space compared to usual distributions Such 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!