Distant field blue horizontal branch stars - III. Identification of a probable outer halo stream at Galactocentric distance r = 70kpc

Abstract

We present FOcal Reducer/low dispersion Spectrograph-1 spectra (from the European Southern Observatory's Very Large Telescope) of a sample of 34 faint 20.0 < g* < 21.1 A-type stars selected from the Sloan Digital Sky Survey Early Data Release, with the goal of measuring the velocity dispersion of blue horizontal branch (BHB) stars in the remote Galactic halo, R ∼ 80 kpc. We show that colour selection with 1.08 < u* - g*< 1.40 and -0.2 < g* - r* < -0.04 minimizes contamination of the sample by less luminous blue stragglers. In classifying the stars we confine our attention to the 20 stars with spectra of signal-to-noise ratio >15 Å-1. Classification produces a sample of eight BHB stars at distances 65-102 kpc from the Sun (mean 80 kpc), which represents the most distant sample of Galactic stars with measured radial velocities. The dispersion of the measured radial component of the velocity with respect to the centre of the Galaxy is 58 ± 15 km s-1. This value is anomalously low in comparison with measured values for stars at smaller distances, as well as for satellites at similar distances. Seeking an explanation for the low measured velocity dispersion, further analysis reveals that six of the eight remote BHB stars are plausibly associated with a single orbit. Three previously known outer halo carbon stars also appear to belong to this stream. The velocity dispersion of all nine stars relative to the orbit is only 15 ± 4km s-1. Further observations along the orbit are required to trace the full extent of this structure on the sky.