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Although Blue Horizontal Branch (BHB) stars are commonly used to trace halosubstructure, the stars bluer than (g-r)<-0.3 are ignored due to the difficultyin determining their absolute magnitudes. The blue extention of the horizontalbranch (HBX) includes BHB tail stars and Extreme Horizontal Branch (EHB) stars.We present a method for identifying HBX stars in the field, using spectra andphotometry from the Sloan Digital Sky Survey Data Release 14 (SDSS DR14). Wederive an estimate for the absolute magnitudes of BHB tail and EHB stars as afunction of color, and use this relationship to calculate distances. Weidentify an overdensity of HBX stars that appears to trace the northern end ofthe Hercules-Aquila Cloud (HAC). We identify three stars that are likely partof a tidal stream, but this is not enough stars to explain the observedoverdensity. Combining SDSS data with Gaia DR2 proper motions allows us to showthat the orbits of the majority of the HBX stars in the overdensity are on higheccentricity orbits similar to those in the Virgo RadialMerger/Gaia-Enceladus/Gaia Sausage structure, and that the overdensity of higheccentricity orbits extends all the way to the Virgo Overdensity. We usestellar kinematics to separate the HBX stars into disk stars andhalo stars. Thehalo stars are primarily on highly eccentric (radial) orbits. The fraction ofHBX stars that are EHBs is highest in the disk population and lowest in the loweccentricity halo stars.

Mapping Milky Way Halo Substructure using Stars in the Extended Blue Tail of the Horizontal Branch