Nearby kinematic wiggles from LEGUE

Extract

In its first two observing seasons, the LEGUE (LAMOST Experiment for Galactic Understanding and Exploration; Deng et al., Zhao et al. 2012) survey has obtained ~1.7 million science-quality spectra. We apply corrections to the PPMXL proper motions (PMs; Roeser et al. 2010) as a function of position, as determined from the measured PMs of extragalactic objects discovered in LAMOST spectra (see Fig. 1, left and center panels). LAMOST radial velocities and corrected PMs are used to derive 3D space velocities for ~480,000 F-stars (assuming M_V=4 to derive distances). The right panel of Fig. 1 shows the radial component of Galactic cylindrical velocities (V_R) for stars between 7.8<R_GC<9.8 kpc (with R_⊙,GC=7.8 kpc) as a function of height (Z) and angle (θ) from the Galactic X-axis. Each dot represents the average position of stars in a 200x200 pc box, color-coded by the mean V_R of those stars. Assuming circularrotation, V_R should be zero. This is true on average for θ>0° (3rd Galactic quadrant), but not for θ<0°. The velocities are also asymmetric across the Galactic plane for θ<0° (2nd quadrant), with most positions 〈 V_R 〉> 0 above the disk (radially outward), and 〈 V_R 〉 < 0 below the disk. Similar structure to this apparent “shearing” motion has been seen in RAVE (e.g., Williams et al. 2013; Siebert et al. 2012), and SDSS (Widrow et al. 2012).