[3.03] Kinematics of the Galactic Halo Using Blue Horizontal Branch Stars Selected from the Sloan Digital Sky Survey

34th Meeting of the AAS Division on Dynamical Astronomy, May 2003
3 Galactic Dynamics
Oral, Monday, May 5, 2003, 1:15-3:55pm,

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[3.03] Student Stipend Recipient: Kinematics of the Galactic Halo Using Blue Horizontal Branch Stars Selected from the Sloan Digital Sky Survey

E. Sirko, G. R. Knapp, J. Goodman, D. J. Schlegel, E. J. Knerr (Princeton University Observatory)

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 D_0.2 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 (\sigma_r,\sigma_\theta,\sigma_\phi) = (99 ±3, 94 ±14, 118 ±18) \,\rm{km\,s^-1} or (\sigma_R,\sigma_\phi,\sigma_z) = (99 ±8, 116 ±18, 99 ±6) \,\rm{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_\rm{halo} = 7 ±18 \,\rm{km\,s^-1} and (U, V, W) = (13 ±8, 228 ±13, -9 ±5) \,\rm{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 \,\rm{km\,s^-1}. Monte Carlo simulations validate the maximum likelihood technique. The conclusion that the radial component is as small as ~100 \,\rm{km\,s^-1} is firm but contrasts with many previous studies based on halo stars in the solar neighborhood.

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