A Gravitational Double Scattering Mechanism for Generating High Velocity Objects

TL;DR

This paper introduces a novel double scattering gravitational mechanism during halo mergers that can accelerate particles to high velocities, potentially explaining observed high velocity objects like certain globular clusters.

Contribution

It analytically characterizes a new double scattering mechanism in halo mergers, linking it to high velocity particle ejections and providing a potential explanation for observed high velocity objects.

Findings

Mechanism can produce particles with velocities twice the virial velocity.

Analytical model relates energy kicks to particle positions in halos.

Simulation suggests particles can reach beyond 5 virial radii.

Abstract

We present a dynamical model describing how halo particles can receive a significant energy kick from the merger between their own host halo and a target halo. This is highly relevant for understanding the growth of cosmological halos, and could especially provide an explanation for some high velocity objects. The model we present includes a \emph{double scattering mechanism}, where a halo particle is given a significant energy kick by undergoing two subsequent gravitational deflections during the merger. The first deflection is by the potential of the target halo, whereas the second is by the potential of the particle's original host halo. The resultant energy kick arises because the two halos move relative to each other during the two deflections. To our knowledge, this mechanism has never been characterized in this context before. We derive analytically a halo particle's total kick energy, which is composed of energy from the double scattering mechanism and energy release from tidal fields, as a function of its position in its original host halo. In the case of a $1:10$ merger between two Hernquist halos, we estimate that the presented mechanisms can generate particles with a velocity $\sim 2$ times the virial velocity of the target halo measured at its virial sphere. This motivates us to suggest that the high velocity of the recently discovered globular cluster HVCG-1 \citep{2014ApJ...787L..11C} can be explained by a head-on halo merger. Finally, we illustrate the orbital evolution of particles outside the virial sphere of the target halo, by solving the equation of motion in an expanding universe. We find a 'sweet spot' around a scale factor of $0.3-0.5$ for ejecting particles into large orbits, which easily can reach beyond $\sim 5$ virial radii.