Statistical Properties of Blue Horizontal Branch Stars in the Spheroid: Detection of a Moving Group approximately 50 kpc from the Sun

TL;DR

This paper reports the discovery of a new moving group of Blue Horizontal Branch stars at 50 kpc from the Sun, with distinct metallicity and velocity properties, highlighting the importance of spectroscopic surveys in understanding the Galaxy's merger history.

Contribution

The study identifies a new distant moving group of BHB stars and introduces a technique for assessing the significance of stellar clumps in multidimensional data.

Findings

Discovered a new moving group at 50 kpc with specific velocity and metallicity.

The moving group is more metal-poor than the general stellar spheroid.

Outer halo BHB stars have a mean metallicity of [Fe/H] = -2.0 with wide scatter.

Abstract

A new moving group comprising at least four Blue Horizontal Branch (BHB) stars is identified at (l,b) = (65 deg, 48 deg). The horizontal branch at g0=18.9 magnitude implies a distance of 50 kpc from the Sun. The heliocentric radial velocity is RV = -157 +/- 4 km/s, corresponding to V(gsr) = -10 km/s; the dispersion in line-of-sight velocity is consistent with the instrumental errors for these stars. The mean metallicity of the moving group is [Fe/H] approximately -2.4, which is significantly more metal poor than the stellar spheroid. We estimate that the BHB stars in the outer halo have a mean metallicity of [Fe/H] = -2.0, with a wide scatter and a distribution that does not change much as a function of distance from the Sun. We explore the systematics of SDSS DR7 surface gravity metallicity determinations for faint BHB stars, and present a technique for estimating the significance of clumps discovered in multidimensional data. This moving group cannot be distinguished in density, and highlights the need to collect many more spectra of Galactic stars to unravel the merger history of the Galaxy.