Kinematic structure in the Galactic halo at the North Galactic Pole: RR Lyrae and BHB stars show different kinematics

Abstract Heliocentric (UVW) and galactocentric (V R VΦ V Z ) space motions were derived for 38 RR Lyrae (RRL) and 79 blue horizontal branch (BHB) stars in a 200-sq degree area near the North Galactic Pole (NGP). A kinematic analysis of the 26 RRL and 52 BHB stars whose height (Z) above the plane is < 8 kpc shows that the sample is not homogeneous. Our BHB sample shows zero galactic rotation and roughly isotropic velocity dispersions. whereas the RRL sample shows a definite retrograde rotation and non-isotropic velocity dispersions. The combined BHB and RRL sample shows a smaller retrograde rotation that is similar to that found by Majewski et al. (1996) for a sample of subdwarfs in SA 57 at the NGP. There are significantly more RRL with negative W-velocity (streaming down) than positive W-velocity, whereas the numbers of BHB stars are comparable. This indicates the presence near the NGP of an accreted halo component that is rich in RRL (probably Oosterhoff type I) stars. These results are presented in detail in a forthcoming paper (Kinman et al. 2007).