3D Asymmetrical motions of the Galactic outer disc with LAMOST K giant stars

Abstract

We present a three dimensional velocity analysis of Milky Way disk kinematics using LAMOST K giant stars and the GPS1 proper motion catalogue. We find that Galactic disk stars near the anticenter direction (in the range of Galactocentric distance between R = 8 and 13 kpc and vertical position between Z = −2 and 2 kpc) exhibit asymmetrical motions in the Galactocentric radial, azimuthal, and vertical components. Radial motions are not zero, thus departing from circularity in the orbits; they increase outwards within R 12 kpc, show some oscillation in the northern (0 < Z < 2 kpc) stars, and have north-south asymmetry in the region corresponding to a well-known nearby northern structure in the velocity field. There is a clear vertical gradient in azimuthal velocity, and also an asymmetry that shifts from a larger azimuthal velocity above the plane near the solar radius to faster rotation below the plane at radii of 11-12 kpc. Stars both above and below the plane at R 9 kpc exhibit net upward vertical motions. We discuss some possible mechanisms that might create the asymmetrical motions, such as external perturbations due to dwarf galaxy minor mergers or dark matter sub-halos, warp dynamics, internal processes due to spiral arms or the Galactic bar, and (most likely) a combination of some or all of these components.

Gaia-DR2 extended kinematical maps

“…9) at X > 10 kpc and Z < 0 than at Z > 0, but not in other azimuths. This is also similar to the results in Wang et al (2018) only for R < 13 kpc. We think that the special feature of the anticenter with respect to this asymmetry is real, but it has nothing to do with systematic effects towards that line of sight as we could not identify any source of systematic error in the data.…”

“…13 (left) of G18 reveals some asymmetry at the maximum reached, R ≈ 13 kpc. A decrease of the azimuthal velocity with |Z| was also previously known, although with less-precise data and studying a lower R range than here (e.g., Bond et al 2010;Bobylev & Bajkova 2010;Williams et al 2013;López-Corredoira 2014;Wang et al 2018;G18). The extremely low value given by López-Corredoira (2014) of V φ = 82 ± 5(stat.)…”

“…This same behavior has already been noted for regions with R 13 kpc (Siebert et al 2011;Williams et al 2013;Wang et al 2018;Carrillo et al 2018;G18), and there are even works that reached R ≈ 16 kpc (López-Corredoira & González-Fernández 2016;Tian et al 2017).…”

“…Data of this figure are available at the CDS. existence for R < 16 kpc, similarly to Wang et al (2018) and Kawata et al (2018) for R = 11−12 kpc. Furthermore, careful examination of Fig.…”

“…The chemical abundances of the thin and thick disks were also widely explored (e.g., Rong et al 2001;Ak et al 2007;Casagrande et al 2011;Haywood et al 2013;Hayden et al 2015) showing the different gradients of metallicities and alphaenhancement. However, the kinematics of the disk is not so well known, and we have only very recently begun to gather relevant information away from the solar neighbourhood; see for example, Bond et al (2010), Siebert et al (2011), Williams et al (2013), Carrillo et al (2018), Wang et al (2018) and the are available at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/621/A48 and at http://www.iac.es/galeria/martinlc/codes/ GaiaDR2extkin/ set of three papers by one of the current authors dedicated to the azimuthal, vertical, and radial Galactocentric components, respectively: López-Corredoira (2014), , López-Corredoira & González-Fernández (2016).…”