Kinematic Structure in the Galactic Halo at the North Galactic Pole: RR Lyrae and BHB Stars show different kinematics

Radial velocities and proper motions (derived from the GSC-II database) are given for 38 RR Lyrae (RRL) stars and 79 blue horizontal branch (BHB) stars in a ∼ 200 deg area around the North Galactic Pole (NGP). Both heliocentric (UVW) and galactocentric (VR, Vφ, Vz) space motions are derived for these stars using a homogeneous distance scale consistent with (m−M)0=18.52 for the LMC. An analysis of the 26 RRL and 52 BHB stars whose height (Z) above the plane is less than 8 kpc, shows that this halo sample is not homogeneous. Our BHB sample (like that of Sirko et al. 2004b) has a zero galactic rotation (Vφ) and roughly isotropic velocity dispersions. The RRL sample shows a definite retrograde rotation (Vφ = −95±29 km s) and non-isotropic velocity dispersions. The combined BHB and RRL sample has a retrograde galactic rotation (V) that is similar to that found by Majewski (1992) for his sample of subdwarfs in SA 57. The velocity dispersion of the RRL stars that have a positive W motion is significantly smaller than the dispersion of those “streaming down” with a negative W. Also, the ratio of RRL to BHB stars is smaller for the sample that has positive W. Our halo sample occupies 10.4 cubic kpc at a mean height of 5 kpc above the Galactic plane. In this volume, one component (rich in RR Lyrae stars) shows retrograde rotation and the streaming motion that we associate with the accretion process. The other component (traced by the BHB stars) shows essentially no rotation and less evidence of streaming. These two components have HB morphologies that suggest that they may be the field star equivalents of the young and old halo globular clusters respectively. Clearly, it is very desirable to use more than one tracer in any kinematic analysis of the halo.