Tidal Tails of Star Clusters

TL;DR

This study uses N-body simulations to analyze the formation and behavior of overdensities in star cluster tidal tails, revealing their universal occurrence and dependence on orbital parameters, without implying dark matter substructures.

Contribution

It provides a comprehensive analysis of tidal tail substructures across various orbits, offering new approximations and clarifying their origins without dark matter implications.

Findings

Overdensities form in all tidal tails given sufficient time.

Distance of overdensities depends on cluster mass and tidal strength for circular orbits.

Eccentric orbits cause periodic oscillations in overdensity positions.

Abstract

Based on recent findings of a formation mechanism of substructure in tidal tails by Kuepper, Macleod & Heggie (2008) we investigate a more comprehensive set of N-body models of star clusters on orbits about a Milky-Way-like potential. We find that the predicted epicyclic overdensities arise in any tidal tail no matter which orbit the cluster follows as long as the cluster lives long enough for the overdensities to build up. The distance of the overdensities along the tidal tail from the cluster centre depends for circular orbits only on the mass of the cluster and the strength of the tidal field, and therefore decreases monotonically with time, while for eccentric orbits the orbital motion influences the distance, causing a periodic compression and stretching of the tails and making the distance oscillate with time. We provide an approximation for estimating the distance of the overdensities in this case. We describe an additional type of overdensity which arises in extended tidal tails of clusters on eccentric orbits, when the acceleration of the tidal field on the stellar stream is no longer homogeneous. Moreover, we conclude that a pericentre passage or a disk shock is not the direct origin of an overdensity within a tidal tail. Escape due to such tidal perturbations does not take place immediately after the perturbation but is rather delayed and spread over the orbit of the cluster. All observable overdensities are therefore of the mentioned two types. In particular, we note that substructured tidal tails do not imply the existence of dark-matter sub-structures in the haloes of galaxies.