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Dynamical Signatures of Dark Matter Substructures on Tidal Streams

Wayne Ngan

Doctor of Philosophy 2015
Graduate Department of Astronomy and Astrophysics, University of Toronto

Despite the success of the LamdaCDM model at describing the universe on large scales, the model's predictions are at odds with observations on sub-galactic scales. In particular, the model predicts that a Milky-Way-sized dark matter halo should contain substructures in the form of low mass subhalos. However, there has yet to be convincing evidence that these subhalos exist.

One promising method to probe the abundance of subhalos is by their dynamical effects on tidal streams, which are elongated remnants of tidally disrupted stellar systems as they orbit around the host galaxy. Previous theoretical studies have shown that subhalos can induce longitudinal density variations (or "gaps") along streams. Although gaps have indeed been observed in some streams, it remains unclear whether those gaps correspond to those expected from subhalos. Previous simulations used idealized models, which did not account for realistic effects that may also induce gaps along streams. In this thesis we investigate a number of these effects.

In the first chapter, we simulate streams with self-gravity, in which we successfully model the gaps caused by epicyclic overdensities, intrinsic to the orbit of a stream, rather than subhalo perturbations. We show that the two kinds of gaps are distinguishable in their length distributions.

In the second chapter, we simulate self-gravitating streams inside a realistic halo that is constructed from a high-resolution cosmological simulation. We find that streams with similar orbital apo- and pericenters can have very different morphologies which cannot be modelled in idealized halos.

In the third chapter, we simulate a collection of self-gravitating streams inside the same realistic halo in order to quantify the difference between using an idealized halo and a realistic halo. We show that the shape of the halo has a large effect on the dispersal of streams. Nevertheless, we find that many of those streams still remain cold and thin. This suggests that upcoming stellar surveys are likely to discover more streams that are suitable for probing the abundance of subhalos.

Reproduced with permission. library@astro.utoronto.ca
September 24, 2015