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

TL;DR

This study develops a method to identify and analyze blue tail horizontal branch stars in the Milky Way halo, revealing their association with known substructures and their orbital characteristics using SDSS and Gaia data.

Contribution

Introduces a new technique for identifying HBX stars and estimates their absolute magnitudes, enabling detailed mapping of halo substructure and stellar kinematics.

Findings

Overdensity of HBX stars traces the Hercules-Aquila Cloud.

Most HBX halo stars are on high eccentricity orbits similar to Gaia-Enceladus.

EHB fraction is highest in disk stars and lowest in low-eccentricity halo stars.

Abstract

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.