A Low-Mass Stellar-Debris Stream Associated with a Globular Cluster Pair
  in the Halo
  There are expected to be physical relationships between the globular clusters
(GCs) and stellar substructures in the Milky Way, not all of which have yet
been found. We search for such substructures from a combined halo sample of
SDSS blue horizontal-branch and SDSS+LAMOST RR Lyrae stars, cross-matched with
astrometric information from $Gaia$ DR2. This is a sample of old stars which
are also excellent tracers of structures, ideal for searching for ancient
relics in the outer stellar halo. By applying the neural-network-based method
StarGO to the full 4D dynamical space of our sample, we rediscover the
Sagittarius Stream, and find the debris of the $Gaia$-Enceladus-Sausage (GES)
and the Sequoia events in the outer halo, as well as their linkages with
several GCs. Most importantly, we find a new, low-mass, debris stream
associated with a pair of GCs (NGC 5024 and NGC 5053), which we dub LMS-1. This
stream has a very polar orbit, and occupies a region between 10 to 20 kpc from
the Galactic center. NGC 5024 (M53), the more-massive of the associated GC
pair, is very likely the nuclear star cluster of a now-disrupted dwarf galaxy
progenitor, based on the results from N-body simulations.
