The Reduced Proper Motion selected halo: methods and description of the
  catalogue
  The Gaia mission has provided the largest ever astrometric chart of the Milky
Way. Using it to map the Galactic halo is helpful for disentangling its merger
history. The identification of halo stars in Gaia DR2 with reliable distance
estimates requires special methods because such stars are typically farther
away and scarce. We apply the reduced proper motion (RPM) method to identify
halo main sequence stars on the basis of Gaia photometry and proper motions.
Using the colour-absolute-magnitude relation for this type of stars, we
calculate photometric distances. Our selection results in a set of $\sim10^7$
tentative main sequence halo stars with typical distance uncertainties of $7\%$
and with median velocity errors of 20 km/s. The median distance of our sample
is $\sim 4.4$ kpc, with the faintest stars located at $\sim 16$ kpc. The
spatial distribution of the stars in our sample is centrally concentrated.
Visual inspection of the mean velocities of stars on the sky reveals
large-scale patterns as well as clear imprints of the GD-1 stream and tentative
hints of the Jhelum and Leiptr streams. Incompleteness and selection effects
limit our ability to interpret the patterns reliably as well as to identify new
substructures. We define a pseudo-velocity space by setting to zero the
line-of-sight velocities of our sample stars. In this space, we recover several
known structures such as the footprint of Gaia-Enceladus (i.e. the
Gaia-Sausage) as well as the Helmi streams and some other retrograde
substructures (Sequoia, Thamnos). We show that the two-point velocity
correlation function reveals significant clustering on scales smaller than 100
km/s, of similar amplitude as found for the 6D Gaia halo sample. This
clustering indicates the presence of nearby streams that are predominantly
phase-mixed.
