Quantifying Kinematic Substructure in the Milky Way's Stellar Halo

TL;DR

This study analyzes over 4000 BHB stars in the Milky Way's halo to detect and quantify substructure, providing evidence for the hierarchical assembly of the stellar halo through position-velocity correlations.

Contribution

It introduces a novel statistical method using the cumulative close pair distribution to identify substructure in stellar halo data and compares observations with simulations.

Findings

Significant position-velocity substructure detected among BHB stars.

Observed substructure levels are comparable to those in simulated halos.

Stronger substructure observed beyond 20 kpc from the Galactic center.

Abstract

We present and analyze the positions, distances, and radial velocities for over 4000 blue horizontal-branch (BHB) stars in the Milky Way's halo, drawn from SDSS DR8. We search for position-velocity substructure in these data, a signature of the hierarchical assembly of the stellar halo. Using a cumulative "close pair distribution" (CPD) as a statistic in the 4-dimensional space of sky position, distance, and velocity, we quantify the presence of position-velocity substructure at high statistical significance among the BHB stars: pairs of BHB stars that are close in position on the sky tend to have more similar distances and radial velocities compared to a random sampling of these overall distributions. We make analogous mock-observations of 11 numerical halo formation simulations, in which the stellar halo is entirely composed of disrupted satellite debris, and find a level of substructure comparable to that seen in the actually observed BHB star sample. This result quantitatively confirms the hierarchical build-up of the stellar halo through a signature in phase (position-velocity) space. In detail, the structure present in the BHB stars is somewhat less prominent than that seen in most simulated halos, quite possibly because BHB stars represent an older sub-population. BHB stars located beyond 20 kpc from the Galactic center exhibit stronger substructure than at $\rm r_{gc} < 20$ kpc.