A halo of trapped interstellar matter surrounding the solar system

TL;DR

This paper demonstrates that the solar system can trap interstellar particles into a halo via Galactic tidal forces, creating a density spike that impacts dark matter detection and the Oort Cloud's structure.

Contribution

It introduces a new mechanism for trapping interstellar matter around the solar system and provides theoretical and simulation evidence for the resulting density profile and potential observational effects.

Findings

A steady-state density spike scales as r^{-3/2}.

The trapped halo may contain around 10^7 objects like 'Oumuamua.

Estimated dark matter mass in the halo is about 10^{-13} solar masses.

Abstract

This paper shows that gravitating bodies travelling through the Galaxy can trap lighter interstellar particles that pass nearby with small relative velocities onto temporarily-bound orbits. The capture mechanism is driven by the Galactic tidal field, which can decelerate infalling objects to a degree where their binding energy becomes negative. Over time, trapped particles build a local overdensity -- or `halo'-- that reaches a steady state as the number of particles being captured equals that being tidally stripped. This paper uses classical stochastic techniques to calculate the capture rate and the phase-space distribution of particles trapped by a point-mass. In a steady state, bound particles generate a density enhancement that scales as $\delta(r)\sim r^{-3/2}$ (a.k.a `density spike') and follow a velocity dispersion profile $\sigma_h(r)\sim r^{-1/2}$. Collisionless $N$-body experiments show excellent agreement with these theoretical predictions within a distance range $r\gtrsim r_\epsilon$, where $r_\epsilon\simeq 0.8\,\exp[-V_\star^2/(2\sigma^2)]\,Gm_\star/\sigma^2$ is the thermal critical radius of a point-mass $m_\star$ moving with a speed $V_\star$ through a sea of particles with a velocity dispersion $\sigma$. Preliminary estimates that ignore collisions with planets and Galactic substructures suggest that the solar system may be surrounded by a halo that contains the order of $N^{\rm ISO}(<0.1\,{\rm pc})\sim 10^7$ energetically-bound 'Oumuamua-like objects, and a dark matter mass of $M^{\rm DM}(<0.1\,{\rm pc})\sim 10^{-13}M_\odot$. The presence of trapped interstellar matter in the solar system can affect current estimates on the size of the Oort Cloud, and leave a distinct signal in direct dark matter detection experiments.