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