The Sagittarius stream in Gaia eDR3 and the origin of the bifurcations
  The Sagittarius dwarf spheroidal (Sgr) is a dissolving galaxy being tidally
disrupted by the Milky Way (MW). Its stellar stream still poses serious
modelling challenges, which hinders our ability to use it effectively as a
probe of the MW gravitational potential. Our goal is to construct the best
possible sample of stars with which we can advance our understanding of the
Sgr-MW interaction, focusing on the characterisation of the bifurcations. We
improve on previous methods based on the use of the wavelet transform to
systematically search for the kinematic signature of the Sgr stream in the Gaia
data. We refine our selection via the use of a clustering algorithm on the
statistical properties of the colour-magnitude diagrams. Our final sample
contains > 700k candidate stars, 3x larger than previous samples. We have been
able to detect the bifurcation of the stream in both the northern and southern
hemispheres, requiring four branches to fully describe this system. We present
the detailed proper motion distribution of the trailing arm as a function of
Lambda showing the presence of a sharp edge (on the side of the small proper
motions) beyond which there are no Sgr stars. We also characterise the
correlation between kinematics and distance. The chemical analysis of our
sample shows a significant difference between the faint and bright branches. We
provide analytical descriptions for the proper motion trends as well as for the
sky distribution of the four branches of the stream. We interpret the
bifurcations as the misaligned overlap of the material stripped at the
antepenultimate pericentre (faint branches) with the stars ejected at the
penultimate pericentre (bright branch). The source of this misalignment is
still unknown but we argue that models with some internal rotation in the
progenitor, at least during the time of stripping of the stars in the faint
branches, are worth exploring.
