Galactic Rotation from Cepheids with Gaia DR2 and Effects of Non-Axisymmetry

Abstract

We apply a simple axisymmetric disc model to 218 Galactic Cepheids whose accurate measurements of the distance and velocities are obtained by cross-matching an existing Cepheids catalogue with the Gaia DR2 data. Our model fit determines the "local centrifugal speed", $V_{\mathrm{c}}$ $-$ defined as the rotation speed required to balance the local radial gravitational force $-$ at the Sun's location to be $V_c(R_0)=236\pm 3$ km s${}^{-1}$ and the Sun's azimuthal and radial peculiar motions to be $V_{\odot }=12.4\pm 0.7$ km s${}^{-1}$ and $U_{\odot }=7.7\pm 0.9$ km s${}^{-1}$, respectively. These results are obtained with strong priors on the solar radius, $R_0=8.2\pm 0.1$ kpc, and Sun's angular rotation velocity, $\hat{I}{\text{c◯}}_{\odot }=30.24\pm 0.12$ km s${}^{-1}$ kpc${}^{-1}$. We also applied the axisymmetric model to mock data from an N-body/hydrodynamic simulation of a Milky Way-like galaxy with a bar and spiral arms. We find that our axisymmetric model fit to the young stars recovers the local centrifugal speed reasonably well, even in the face of significant non-axisymmetry. However, the local centrifugal speed determined from our Cepheid sample could suffer from systematic uncertainty as large as 6 km s${}^{-1}$.

13 pages, 8 figures, 2 tables, accepted for publication in MNRAS