Tracing Milky Way substructure with an RR Lyrae hierarchical clustering
  forest
  RR Lyrae variable stars have long been reliable standard candles used to
discern structure in the Local Group. With this in mind, we present a routine
to identify groupings containing a statistically significant number of RR Lyrae
variables in the Milky Way environment. RR Lyrae variable groupings, or
substructures, with potential Galactic archaeology applications are found using
a forest of agglomerative, hierarchical clustering trees, whose leaves are
Milky Way RR Lyrae variables. Each grouping is validated by ensuring that the
internal RR Lyrae variable proper motions are sufficiently correlated.
Photometric information was collected from the Gaia second data release and
proper motions from the (early) third data release. After applying this routine
to the catalogue of 91234 variables, we are able to report sixteen unique RR
Lyrae substructures with physical sizes of less than 1 kpc. Five of these
substructures are in close proximity to Milky Way globular clusters with
previously known tidal tails and/or a potential connection to Galactic merger
events. One candidate substructure is in the neighbourhood of the Large
Magellanic Cloud but is more distant (and older) than known satellites of the
dwarf galaxy. Our study ends with a discussion of ways in which future surveys
could be applied to the discovery of Milky Way stellar streams.
