Gaps in globular cluster streams: giant molecular clouds can cause them
  too
  As a result of their internal dynamical coherence, thin stellar streams
formed by disrupting globular clusters (GCs) can act as detectors of dark
matter (DM) substructure in the Galactic halo. Perturbations induced by close
flybys amplify into detectable density gaps, providing a probe both of the
abundance and of the masses of DM subhaloes. Here, we use N-body simulations to
show that the Galactic population of giant molecular clouds (GMCs) can also
produce gaps (and clumps) in GC streams, and so may confuse the detection of DM
subhaloes. We explore the cases of streams analogous to the observed Palomar 5
and GD1 systems, quantifying the expected incidence of structure caused by GMC
perturbations. Deep observations should detect such disturbances regardless of
the substructure content of the Milky Way's halo. Detailed modelling will be
needed to demonstrate that any detected gaps or clumps were produced by DM
subhaloes rather than by molecular clouds.
