Exploring Halo Substructure with Giant Stars: Substructure in the Local Halo as Seen in the Grid Giant Star Survey Including Extended Tidal Debris from Omega Centauri

TL;DR

This study reveals significant kinematic substructure in the nearby Galactic halo, identifying a tidal debris stream from Omega Centauri through velocity, positional, and chemical signatures in giant stars.

Contribution

It provides the first observational evidence linking a specific stellar stream to Omega Centauri using combined kinematic and chemical data.

Findings

Identification of a retrograde stellar stream matching Omega Centauri's properties.

Chemical signatures confirm the stream's origin from Omega Centauri.

Omega Centauri debris constitutes most retrograde stars in the southern Galactic halo.

Abstract

We present the latitude-normalized radial velocity (vb) distribution of 3318 subsolar metallicity, V<13.5 stars from the Grid Giant Star Survey (GGSS) in Southern Hemisphere fields. The sample includes giants mostly within ~5 kpc from the Galactic disks and halo. The nearby halo is found to (1) exhibit significant kinematical substructure, and (2) be prominently represented by several velocity coherent structures, including a very retrograde "cloud" of stars at l~285 deg and extended, retrograde "streams" visible as relatively tight l-vb sequences. One sequence in the fourth Galactic quadrant lies within the l-vb space expected to contain tidal debris from the "star cluster" wCentauri. Not only does wCen lie precisely in this l-vb sequence, but the positions and vb of member stars match those of N-body simulations of tidally disrupting dwarf galaxies on orbits ending with wCen's current position and space motion. But the ultimate proof that we have very likely found extended parts of the wCen tidal stream comes from echelle spectroscopy of a subsample of the stars that reveals a very particular chemical abundance signature known to occur only in wCen. The newly discovered wCen debris accounts for almost all fourth Galactic quadrant retrograde stars in the southern GGSS, which suggests wCen is a dominant contributor of retrograde giant stars in the inner Galaxy.