Title: The Three Phases of Gap Growth in Tidal Streams

Abstract:
Gaps in tidal streams are a signature of the dark matter substructure expected from hierarchical 
structure formation. In recent work, we developed a comprehensive framework to study the 
growth of these gaps and discovered a rich set of dynamics. This framework is built on a toy 
model for streams on circular orbits around a host potential which are perturbed by the passage 
of a subhalo represented by a Plummer sphere. The effect of the perturbation can be solved 
analytically at first order for any spherically symmetric host potential. The main effect of the 
flyby is to kick particles in the stream towards the point of closest approach. Using this model 
we find that gaps have three distinct phases. During the first phase, there is a density 
enhancement at the point of closest approach. The kicks from the subhalo alter the period of 
each stream particle and after roughly an orbital period, the motion towards the point of closest 
approach reverses and a gap forms. The second phase is characterised by a gap which grows 
linearly in time. After a time on the order a Gyr for typical stream orbits around the Milky Way, 
the third and final phase begins when caustics form on the leading edge of the gap and the gap 
growth is proportional to t^(1/2). Although this analytical model is very simple, it agrees with the 
results of N-body simulations of stream disruptions on circular orbits which interact with 
substructure. In addition, the same qualitative results are seen for stream progenitors on 
arbitrary orbits. This model also correctly predicts the density in the both the trough of the gap 
and the peaks around the gap. In the final part of the talk, we will discuss the degeneracy which 
arises in constraining the properties of the substructure which created the gap. See 
http://arxiv.org/abs/1412.6035 for more details.