Mapping Milky Way Halo Substructure using Stars in the Extended Blue
  Tail of the Horizontal Branch
  Although Blue Horizontal Branch (BHB) stars are commonly used to trace halo
substructure, the stars bluer than (g-r)<-0.3 are ignored due to the difficulty
in determining their absolute magnitudes. The blue extention of the horizontal
branch (HBX) includes BHB tail stars and Extreme Horizontal Branch (EHB) stars.
We present a method for identifying HBX stars in the field, using spectra and
photometry from the Sloan Digital Sky Survey Data Release 14 (SDSS DR14). We
derive an estimate for the absolute magnitudes of BHB tail and EHB stars as a
function of color, and use this relationship to calculate distances. We
identify an overdensity of HBX stars that appears to trace the northern end of
the Hercules-Aquila Cloud (HAC). We identify three stars that are likely part
of a tidal stream, but this is not enough stars to explain the observed
overdensity. Combining SDSS data with Gaia DR2 proper motions allows us to show
that the orbits of the majority of the HBX stars in the overdensity are on high
eccentricity orbits similar to those in the Virgo Radial
Merger/Gaia-Enceladus/Gaia Sausage structure, and that the overdensity of high
eccentricity orbits extends all the way to the Virgo Overdensity. We use
stellar kinematics to separate the HBX stars into disk stars andhalo stars. The
halo stars are primarily on highly eccentric (radial) orbits. The fraction of
HBX stars that are EHBs is highest in the disk population and lowest in the low
eccentricity halo stars.
