On feathers, bifurcations and shells: the dynamics of tidal streams across the mass scale

TL;DR

This paper provides an organic framework for understanding the dynamics and morphology of collisionless tidal streams across different mass scales, highlighting key dichotomies and mechanisms influencing their structure and substructure.

Contribution

It introduces a novel set of dimensionless inequalities that define regimes of tidal stream behavior and analyzes the mechanisms behind their internal coherence and bifurcations.

Findings

Steeper host density profiles accelerate stream growth.

Overdensities correlate with minima in streaming velocity.

Conditions for internal coherence and bifurcations are characterized.

Abstract

I present an organic description of the regimes of collisionless tidal streams and define the orderings between the physical quantities that shape their morphology. Three fundamental dichotomies are identified in the form of dimensionless inequalities. These govern i) the speed of the stream's growth, ii) its internal coherence, iii) its thickness or opening angles. The mechanisms that regulate such main properties are analysed. The slope of the host's density profile influences the speed of the stream's growth, in both length and width, as steeper profiles enhance differential streaming. Internal coherence is the requirement for the appearance of substructure in tidal debris, and I concentrate on the `feathering' typical of GC streams. Overdensities are associated with minima in the relative streaming velocity of the stream members. For streams with high circularity, these are caused by the epicyclic oscillations of stars; however, for highly non-circular progenitor's orbits, substructure is caused by the oscillating differences in energy and actions with which material is shed at different orbital phases of the progenitor. This modulation results in different streaming speeds: the streakline of material shed between two successive apocentric passages is folded along its length, pulled at its centre by the faster streaming of particles released near pericenter, which are therefore more widely scattered. When the stream is coherent enough, this mechanism is potentially capable of generating a bimodal profile in the density distributions of the longer wraps of more massive progenitors, which I dub `bifurcations'. The conditions for internal coherence are explored and I comment on the cases of Palomar 5, Willman 1, the Anticenter and Sagittarius' streams. Analytical methods are accompanied by numerical experiments, performed using a purposely built generative model, also presented here.