Probability of forming gaps in the GD-1 stream by close encounters of
  globular clusters
  One of the most intriguing properties of the GD-1 stellar stream is the
existence of three gaps. If these gaps were formed by close encounters with
dark matter subhalos, the GD-1 stream opens an exciting window through which we
can see the size, mass, and velocity distributions of the dark matter subhalos
in the Milky Way. However, in order to use the GD-1 stream as a probe of the
dark matter substructure, we need to disprove that these gaps are not due to
the perturbations from baryonic components of the Milky Way. Here we ran a
large number of test particle simulations to investigate the probability that
each of the known globular clusters (GCs) can form a GD-1-like gap, by using
the kinematical data of the GD-1 stream and GCs from Gaia EDR3 and by fully
taking into account the observational uncertainty. We found that the
probability that all of the three gaps were formed by GCs is as low as
$1.2\times10^{-5}$ and the expected number of gaps formed by GCs is only
$0.057$ in our fiducial model. Our result highly disfavors a scenario in which
GCs form the gaps. Given that other baryonic perturbers (e.g., giant molecular
clouds) are even less likely to form a gap in the retrograde-moving GD-1
stream, we conclude that at least one of the gaps in the GD-1 stream was formed
by dark matter subhalos if the gaps were formed by flyby perturbations.
