Probing the Sagittarius stream with blue horizontal branch stars

Abstract

Abstract We present two-degree field spectroscopic observations of a sample of 96 A-type stars selected from the Sloan Digital Sky Survey Data Release 3 (SDSS DR3). Our aim is to identify blue horizontal branch (BHB) stars in order to measure the kinematic properties of the tidal tails of the Sagittarius dwarf spheroidal galaxy. We confine our attention to the 44 classifiable stars with spectra of signal-to-noise ratio >15 Å−1. Classification produces a sample of 29 BHB stars at distances of 5–47 kpc from the Sun. We split our sample into three bins based on their distance. We find 10 of the 12 stars at 14–25 kpc appear to have coherent, smoothly varying radial velocities which are plausibly associated with old debris in the Sagittarius tidal stream. Further observations along the orbit and at greater distances are required to trace the full extent of this structure on the sky. Three of our BHB stars in the direction of the globular cluster Palomar (Pal) 5 appear to be in an overdensity but are in the foreground of Pal 5. More observations are required around this overdensity to establish any relation to Pal 5 and/or the Sgr stream. We emphasize observations of BHB stars have unlimited potential for providing accurate velocity and distance information in old distant halo streams and globular clusters alike. The next-generation multi-object spectrographs provide an excellent opportunity to accurately trace the full extent of such structures.