Re-examining the dynamics of the Milky Way with new distance estimates

Abstract: The use of reliable distance estimates is key to derive accurate three-dimensional velocities of celestial objects. Systematic overestimation and large uncertainties in the distances will thus provide misleading information of an object's kinematics. In turn, astronomers will get a flawed understanding of dynamical properties of substructures in the Milky Way. In this paper, we employ the improved distance estimates of observations common to the TGAS and RAVE catalogues, made available in McMillan et al. (2018), to re-investigate claims of three previous papers on Milky Way dynamics where lower quality distance estimates have been used. We opt for a general approach where we use analysis in the form of statistical resampling as well as a velocity model of Gaussian nature adopted from Kordopatis et al. (2013) to evaluate the validity of the claims. Further, we use at least two different subsets of our data with varying quality in each investigation to gain a sense of how lower quality data points affect the observed dynamical phenomena. We find that there are results in each paper that can be attributed to the use of poor distance estimates. Firstly, we find that there are no indications of the existence of the dearth of zero angular momentum stars in the Milky Way, which was proposed in Hunt et al. (2016). Secondly, we find that the excess of retrograde halo orbits in the Milky Way found in Helmi et al. (2017) is a product of the systematic overestimation of distances. Our results indicate that the fraction of retrograde halo orbits is substantially lower than they have claimed. Finally, we find that the contraction-like behaviour of the Galactic disc detected in Williams et al. (2013) can be connected to systematic overestimation and/or large uncertainties in their distance estimates.