Flying far and fast: the distribution of distant hypervelocity star
  candidates from Gaia DR2 data
  Context. Hypervelocity stars move fast enough to leave the gravitational
field of their home galaxies and venture into intergalactic space. The most
extreme examples known have estimated speeds in excess of 1000 km/s. These can
be easily induced at the centres of galaxies via close encounters between
binary stars and supermassive black holes; however, a number of other
mechanisms operating elsewhere can produce them as well.
  Aims. Recent studies suggest that hypervelocity stars are ubiquitous in the
local Universe. In the Milky Way, the known hypervelocity stars are
anisotropically distributed, but it is unclear why. Here, we used Gaia Data
Release 2 (DR2) data to perform a systematic exploration aimed at confirming or
refuting these findings.
  Methods. We assume that the farther the candidate hypervelocity stars are,
the more likely they are to be unbound from the Galaxy. We used the statistical
analysis of both the spatial distribution and kinematics of these objects to
achieve our goals.
  Results. Focussing on nominal Galactocentric distances greater than 30 kpc,
which are the most distant candidates, we isolated a sample with speeds in
excess of 500 km/s that exhibits a certain degree of anisotropy but remains
compatible with possible systematic effects. We find that the effect of the
Eddington-Trumpler-Weaver bias is important in our case: over 80% of our
sources are probably located further away than implied by their parallaxes;
therefore, most of our velocity estimates are lower limits. If this bias is as
strong as suggested here, the contamination by disc stars may not affect our
overall conclusions.
  Conclusions. The subsample with the lowest uncertainties shows stronger, but
obviously systematic, anisotropies and includes a number of candidates of
possible extragalactic origin and young age with speeds of up to 2000 km/s.
