New Signatures of the Milky Way Formation in the Local Halo and Inner Halo Streamers in the Era of Gaia (1508.03736v1)

Abstract: We explore the vicinity of the Milky Way through the use of spectro-photometric data from the Sloan Digital Sky Survey and high-quality proper motions derived from multi-epoch positions extracted from the Guide Star Catalogue II database. In order to identify and characterise streams as relics of the Milky Way formation, we start with classifying, select, and study $2417$ subdwarfs with $\rm{[Fe/H] < -1.5}$ up to $3$ kpc away from the Sun as tracers of the local halo system. Then, through phase-space analysis, we find statistical evidence of five discrete kinematic overdensities among $67$ of the fastest-moving stars, and compare them to high-resolution N-body simulations of the interaction between a Milky-Way like galaxy and orbiting dwarf galaxies with four representative cases of merging histories. The observed overdensities can be interpreted as fossil substructures consisting of streamers torn from their progenitors, such progenitors appear to be satellites on prograde and retrograde orbits on different inclinations. In particular, of the five detected overdensities, two appear to be associated, yelding twenty-one additional main-sequence members, with the stream of Helmi et al. (1999) that our analysis confirms on a high inclination prograde orbit. The three newly identified kinematic groups could be associated with the retrograde streams detected by Dinescu (2002) and Kepley et al. (2007), whatever their origin, the progenitor(s) would be on retrograde orbit(s) and inclination(s) within the range $10^{\circ} \div 60^{\circ}$. Finally, we use our simulations to investigate the impact of observational errors and compare the current picture to the promising prospect of highly improved data expected from the Gaia mission.