Tidal Streams as a Probe of the Milky Way Assembly History

Abstract: The current Lambda Cold Dark Matter cosmology predicts galaxy growth by hierarchical merging, leading to galaxy substructure like tidal streams and dark matter subhalos devoid of stars. The goal of this thesis is to investigate whether tidal stream-subhalo interactions can leave observable gaps in the streams, and at which rate this might happen in a galaxy like the Milky Way. The stream-subhalo interactions are investigated by numerical modelling of a set of representative stream and subhalo orbits, and how often these get close enough to interact. The average flyby-rate in the distance range 20-100 kpc is found to be R = 0.148 (Gyr kpc)^-1 of stream length for subhalo masses between 10^6-10^9 Solar Masses, within the large error bars from observations of gaps in streams. Various impact parameters for a stream-subhalo encounter are modelled for a stream on an eccentric and inclined trajectory with 4000 particles. The subhalos are modelled as Plummer spheres with masses M = (10^7, 10^7.5, 10^8) Solar Masses and respective scale radii rs = (250, 395.3, 625) pc. It is found that the parameters that matter the most in damaging the stream are the mass of the subhalo, the relative flyby velocity and the distance of the flyby. It is also found that the angle of the impact matters less, as the angle are related to the relative velocities. Lastly, it is found that slow flybys with relative velocity less than 200 km/s, where the impulse approximation is not valid, happen often enough to merit more study, and also leave traces in the stream distinct from faster flybys. In conclusion, if subhalos exist and are as numerous as predicted from simulations, thin streams with width less than 1 kpc should encounter somewhere between 1-15 subhalos that can leave gaps in them.