Chaos in the Galaxy

N-body simulations of tidal streams formed by Galactic satellites on regular orbits (A & B), a weakly chaotic orbit (C), and a strongly chaotic (D). The orbits of the four satellites are quite similar in terms of eccentricity and apo/pericenters, but the resulting streams show (and amplify) the underlying chaos.

During Adrian Price-Whelan’s dissertation talk today at the winter meeting (AAS227) of the American Astronomical Society in Kissimmee, Florida,  I was reminded that I haven’t mentioned our publication here. Adrian went through a whole lot of effort and characterized regular and chaotic orbits in a typical galactic gravitational potential. Usually, orbits in such a potential can be broadly categorized into chaotic and non-chaotic orbits. Adrian looked at this distinction in terms of the streams that are formed by satellites on such orbits.

In the above figure the main conclusion is visualized: if a satellite is on a chaotic orbit, the stream will reflect that by showing signs of stream fanning (see Sarah Pearson’s paper). Adrian’s important new contribution is the following: if a satellite is on a weakly chaotic orbit, that is, when the timescale, on which the orbit of the satellite changes significantly, is larger than, say, the age of the Universe, then chaos shouldn’t matter for such a satellite – and neither for its stream. But simple numerical experiments beautifully demonstrate that this is not at all the case! Weak chaos is strongly reflected in tidal streams, and destroys the coherence of tidal streams on a much smaller timescale (see stream C in the above figure).

This is important for streams in our own Milky Way Galaxy! Since we live in a quite complex galaxy, in which large fractions of orbits should be at least weakly chaotic, this will have significant impact on the occurrence of thin and well-defined streams on the sky. On the other hand: if we see some thin and long tidal streams on the sky, we may be able to infer that these satellites must orbit in very regular regions of orbit space within the galactic potential! We will see what this implies for the two long, thin streams in the Galactic halo: Palomar 5 and GD-1.


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