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

TL;DR

This study uses Gaia DR2 data and an axisymmetric disc model to measure the Milky Way's local rotation speed and solar motions from Cepheids, assessing the impact of non-axisymmetry on these measurements.

Contribution

It provides new measurements of the local centrifugal speed and solar motions using Cepheids, and evaluates the robustness of axisymmetric models in non-axisymmetric galactic environments.

Findings

Measured local centrifugal speed as 236 km/s with 3 km/s uncertainty.

Found that axisymmetric models recover rotation speed well despite non-axisymmetry.

Identified potential systematic uncertainty of up to 6 km/s in Cepheid-based measurements.

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 $V_{\odot}=12.4\pm0.7$ km s$^{-1}$ and $U_{\odot}=7.7\pm0.9$ km s$^{-1}$, respectively. These results are obtained with strong priors on the solar radius, $R_0=8.2\pm0.1$ kpc, and Sun's angular rotation velocity, $\Omega_{\odot}=30.24\pm0.12$ 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$^{-1}$.