Substructure in the stellar halo near the Sun. II. Characterisation of
  independent structures
  In L\"ovdal et al, we presented a data-driven method for clustering in
Integrals of Motion space and applied it to a large sample of nearby halo stars
with 6D phase-space information. We identified a large number of clusters, many
of which could tentatively be merged into larger groups. Our goal is to
establish the reality of the clusters through a combined study of their stellar
populations to gain more insights into the accretion history of the Milky Way.
We develop a procedure that quantifies the similarity of clusters based on KS
tests using their metallicity distribution functions, and an isochrone fitting
method to determine their average age, which is also used to compare the
distribution of stars in the Colour-Absolute magnitude diagram. This allows us
to group clusters into substructures, and to compare substructures with one
another. The clusters identified are merged into 12 extended substructures,
while 8 small clusters remain as such. The large substructures include the
previously known Gaia-Enceladus, Helmi streams, Sequoia, and Thamnos 1 and 2.
We identify overdensities associated with the hot thick disc and hosting a
metal-poor population. Especially notable is our largest substructure which,
although peaking at the metallicity characteristic of the thick disk has a well
populated metal-poor component, and dynamics in-between hot thick disc and
halo. We identify additional debris in the region occupied by Sequoia with
distinct kinematics, likely remnants of three different accretion events with
progenitors of similar mass. We also identify different trends of [Mg/Fe] vs
[Fe/H] for the various substructures confirming our dissection of the nearby
halo. At least 20\% of the halo near the Sun is associated to substructures.
When comparing their global properties, we note that those substructures on
retrograde orbits are not only more metal-poor on average but also older.
