Ambiguity in defining the escape distance for local dark matter escape speed

Determine the physically appropriate finite distance a dark matter particle must travel from the Solar neighborhood to be considered as having “escaped” for the purpose of defining the local escape speed used to truncate the Milky Way’s dark matter speed distribution. Clarify this definition so that the local escape speed does not rely on the formal escape to infinity and can be applied consistently in analyses of Solar-region dark matter kinematics.

Background

In modeling local dark matter kinematics for direct detection, it is standard to truncate the speed distribution at an escape speed. While the formal escape speed to infinity can be computed from the gravitational potential, this choice is not necessarily the physically relevant criterion for whether a particle has ‘escaped’ the Solar region.

To address this ambiguity pragmatically, the paper defines the escape speed in simulations as the highest speed observed for dark matter particles within the Solar-ring region. However, the authors explicitly acknowledge that the physically correct distance a particle should travel to qualify as having escaped the Solar neighborhood is not clearly established, motivating a more precise definition.