Star Streams in Triaxial Isochrone Potentials with Sub-Halos

TL;DR

This study models the evolution of tidal streams in a triaxial isochrone potential with sub-halos, revealing how sub-halos influence stream morphology, density variations, and velocity distributions, aiding dark matter substructure detection.

Contribution

It introduces a simplified yet accurate model for stream evolution in complex potentials with sub-halos, enabling detailed analysis of their effects on stream properties.

Findings

High eccentricity streams can disperse significantly, with only a few percent of material remaining dense.

Sub-halos broaden streams and displace their centerlines, depending on orbital parameters.

Density gaps and velocity offsets are detectable even with small sub-halo mass fractions (~0.2%).

Abstract

The velocity, position, and action variable evolution of a tidal stream drawn out of a star cluster in a triaxial isochrone potential containing a sub-halo population reproduces many of the orbital effects of more general cosmological halos but allows easy calculation of orbital actions. We employ a spherical shell code which we show accurately reproduces the results of a tree gravity code for a collisionless star cluster. Streams from clusters on high eccentricity orbits, $e\gtrsim 0.6$, can spread out so much that the amount of material at high enough surface density to stand out on the sky may be only a few percent of the stream's total mass. Low eccentricity streams remain more spatially coherent, but sub-halos both broaden the stream and displace the centerline with details depending on the orbits allowed within the potential. Overall, the majority of stream particles have changes in their total actions of only 1-2\%, leaving the mean stream relatively undisturbed. A halo with 1\% of the mass in sub-halos typically spreads the velocity distribution about a factor of two wider than would be expected for a smooth halo. Strong density variations, "gaps", along with mean velocity offsets, are clearly detected in low eccentricity streams for even a 0.2\% sub-halo mass fraction. Around one hundred velocity measurements per kiloparsec of stream will enable tests for the presence of a local sub-halo density as small as 0.2-0.5\% of the local mass density, with about 1\% predicted for 30 kiloparsec orbital radii streams.