Mapping the Galactic Halo with blue horizontal branch stars from the 2dF quasar redshift survey

TL;DR

This study maps the Galactic halo using blue horizontal branch stars from the 2Qz survey, revealing its extent, density profile, and substructure, and suggesting it is largely spherical with ongoing accretion events.

Contribution

It provides the first detailed 4D map of the Galactic halo using BHB stars, showing its extent, density profile, and evidence of substructure and recent accretion.

Findings

Halo extends to at least 100 kpc from the Galactic center.

Halo density follows a power-law with index ~-2.5, indicating sphericity.

At least 20% of halo stars are in substructures, which are dynamically young.

Abstract

We use 666 blue horizontal branch (BHB) stars from the 2Qz redshift survey to map the Galactic halo in four dimensions (position, distance and velocity). We find that the halo extends to at least 100 kpc in Galactocentric distance, and obeys a single power-law density profile of index ~-2.5 in two different directions separated by 150 degrees on the sky. This suggests that the halo is spherical. Our map shows no large kinematically coherent structures (streams, clouds or plumes) and appears homogeneous. However, we find that at least 20% of the stars in the halo reside in substructures and that these substructures are dynamically young. The velocity dispersion profile of the halo appears to increase towards large radii while the stellar velocity distribution is non Gaussian beyond 60 kpc. We argue that the outer halo consists of a multitude of low luminosity overlapping tidal streams from recently accreted objects.