The Pattern Speed of the Milky Way Bar/Bulge from VIRAC & Gaia

TL;DR

This study measures the Milky Way's bar pattern speed using combined VIRAC and Gaia data, providing new constraints on the galaxy's dynamical structure and supporting the 'long-slow' bar model.

Contribution

It introduces a comprehensive method to determine the bar pattern speed by comparing detailed proper motion data with dynamical models, accounting for various systematic uncertainties.

Findings

Bar pattern speed measured as 33.29 ± 1.81 km/s/kpc.

Derived corotation and resonance radii consistent with the 'long-slow' bar scenario.

Quantified systematic effects influencing the pattern speed measurement.

Abstract

We compare distance resolved, absolute proper motions in the Milky Way bar/bulge region to a grid of made-to-measure dynamical models with well defined pattern speeds. The data are obtained by combining the relative VVV Infrared Astrometric Catalog v1 proper motions with the Gaia DR2 absolute reference frame. We undertake a comprehensive analysis of the various errors in our comparison, from both the data and the models, and allow for additional, unknown, contributions by using an outlier-tolerant likelihood function to evaluate the best fitting model. We quantify systematic effects such as the region of data included in the comparison, the possible overlap from spiral arms, and the choice of synthetic luminosity function and bar angle used to predict the data from the models. Resulting variations in the best-fit parameters are included in their final errors. We thus measure the bar pattern speed to be ${\Omega}_b=33.29\pm1.81 \mathrm{km \, s^{-1} \, kpc^{-1}}$ and the azimuthal solar velocity to be $V_{\phi,\,\odot} = 251.31 \pm 1.95 \mathrm{km \, s^{-1}}$. These values, when combined with recent measurements of the Galactic rotation curve, yield the distance of corotation, 6.5 < $R_{CR}$ [kpc] < 7.5, the outer Lindblad resonance (OLR), 10.7 < $R_{OLR}$ [kpc] < 12.4, and the higher order, m = 4, OLR, 8.7 < $R_{{OLR}_4}$ [kpc] < 10.0. The measured pattern speed provides strong evidence for the "long-slow" bar scenario.