The pattern speed of the Milky Way bar/bulge from VIRAC and Gaia

Clarke, J. P., Gerhard, O. 2022, MNRAS, 512, 2171

Figure 1: Outline of the probability contours when comparing the observational data to the model grid. The panels on top and to the right show the marginalised, 1D distributions, which are used to compute constraints on the individual parameters.

Figure 1: Outline of the probability contours when comparing the observational data to the model grid. The panels on top and to the right show the marginalised, 1D distributions, which are used to compute constraints on the individual parameters.

The Galactic bar and bulge dominates the mass distribution in the centre of the Milky Way’s disk. It has been known for a long time that the bar rotates but the precise speed is still hotly debated. The reason this is important is that the rotation rate not only affects the kinematics in the bar/bulge region but is also responsible for resonance effects in the disk near the sun, and is a likely culprit for some of the deformities observed in stellar streams. We measure the bar pattern speed, and the tangential velocity of the sun, by comparing proper motion data in the Galactic bulge to a series of models which have been constructed with known pattern speeds. Using a careful statistical analysis, the results of which are shown in Figure 1, we determine the pattern speed to be approximately 33 km/s/kpc which is slower than many previous estimates. We then use this value, and measurements of the rotation curve of the Milky Way, to compute the radii at which the resonances from the bar should manifest. We find a corotation radius (the radius at which a circular orbit has the same orbital frequency as the bar structure) at about 7 kpc from the galactic centre. This work is important because many previous estimates have studied resonant structures in the disk, thereby leveraging local observations to constrain the bar pattern speed. Our approach differs in that we use measurements of the kinematics of individual stars within the bar itself to constrain this important parameter.

Figure 2: Plot outlining the calculation of the resonance radii in the disk. The horizontal shaded region shows our measurement of the pattern speed and we highlight the regions in which it intersects with the frequency curves which indicate the location of, from left to right, the corotation radius, the higher order m=4 outer Lindblad resonance, and the outer Lindblad resonance.

Figure 2: Plot outlining the calculation of the resonance radii in the disk. The horizontal shaded region shows our measurement of the pattern speed and we highlight the regions in which it intersects with the frequency curves which indicate the location of, from left to right, the corotation radius, the higher order m=4 outer Lindblad resonance, and the outer Lindblad resonance.

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