Kinematics of the Galactic Halo Using Blue Horizontal Branch Stars Selected from the Sloan Digital Sky Survey
Abstract
A sample of ∼ 1000 Galactic halo blue horizontal branch (BHB) stars is drawn from the Sloan Digital Sky Survey (SDSS) spectroscopic database. Preliminary cuts in (u*-g^*,g^*-r^*) distinguish candidate BHBs, which have a strong Balmer jump, from other halo stars. Surface-gravity indicators based on Balmer lines are further used to reject main sequence stars and blue stragglers: the D0.2 method (linewidths at 80% of the continuum) and the scalewidth-shape method (Clewley et al. 2002). After all three cuts, contamination by non-BHB stars is estimated to be less than 10% among stars brighter than g*=18. For fainter stars the linewidth analyses are unreliable, so a more stringent (u*-g^*,g^*-r^*) cut is devised which, by comparison with the spectroscopically purified bright sample, is estimated to produce a contamination fraction of ∼ 25% for 18< g* <20. Radial velocities and photometric distances are derived for the surviving stars. A maximum-likelihood method is used to determine the velocity ellipsoid of the halo. We find the velocity ellipsoid to be nearly isotropic with (σ r,σθ,σφ) = (99 +/- 3, 94 +/- 14, 118 +/- 18) km s-1 or (σR, σφ, σz) = (99 +/- 8, 116 +/- 18, 99 +/- 6) km s-1 in spherical and cylindrical coordinates, respectively. When the halo rotation and solar velocity are allowed to be free parameters, we also find v{halo} = 7 +/- 18 km s-1 and (U, V, W) = (13 +/- 8, 228 +/- 13, -9 +/- 5) km s-1. The halo parameters are robust with respect to variations in the errors assumed: photometric distance errors up to 0.5 mag, and velocity errors up to 30 {km s-1}. Monte Carlo simulations validate the maximum likelihood technique. The conclusion that the radial component is as small as ∼ 100 {km s-1} is firm but contrasts with many previous studies based on halo stars in the solar neighborhood.
- Publication:
-
AAS/Division of Dynamical Astronomy Meeting #34
- Pub Date:
- August 2003
- Bibcode:
- 2003DDA....34.0303S