The Milky Way bar/bulge in proper motions: a 3D view from VIRAC and Gaia

Abstract

We have derived absolute proper motions of the entire Galactic bulge region from VIRAC and Gaia. We present these as both integrated on-sky maps and, after isolating standard candle red clump (RC) stars, as a function of distance using RC magnitude as a proxy. These data provide a new global, 3-dimensional view of the Milky Way barred bulge kinematics. We find a gradient in the mean longitudinal proper motion, <µ l >, between the different sides of the bar, which is sensitive to the bar pattern speed. The split RC has distinct proper motions and is colder than other stars at similar distance. The proper motion correlation map has a quadrupole pattern in all magnitude slices showing no evidence for a separate, more axisymmetric inner bulge component. The line-of-sight integrated kinematic maps show a high central velocity dispersion surrounded by a more asymmetric dispersion profile. σ µ l /σ µ b is smallest, ∼1.1, near the minor axis and reaches ∼1.4 near the disc plane. The integrated <µ b > pattern signals a superposition of bar rotation and internal streaming motion, with the near part shrinking in latitude and the far part expanding. To understand and interpret these remarkable data, we compare to a made-to-measure barred dynamical model, folding in the VIRAC selection function to construct mock maps. We find that our model of the barred bulge, with a pattern speed of 37.5 km s −1 kpc −1 , is able to reproduce all observed features impressively well. Dynamical models like this will be key to unlocking the full potential of these data.There is still an ongoing debate as to whether there exists a secondary classical bulge component in the central parts of the bulge (Shen et al. 2010;Rojas-Arriagada et al. 2017;Di Matteo et al. 2015;Barbuy et al. 2018). With modern stellar surveys, the MW bulge and bar can be studied at great depth, rapidly making the MW a prototypical system for understanding the formation and evolution of similar galaxies.A prominent feature of the barred bulge is the split red clump (RC) which was first reported by Nataf et al. (2010); McWilliam & Zoccali (2010) using OGLE-III photometry and 2MASS data respectively. They showed that this phenomenon occurs close to the MW minor axis at latitudes of |b| 5 • . From these analyses it was suggested that the split RC could be the result of a funnel shaped component in the bulge which is now commonly referred to as X-shaped.

New VIRAC proper motion maps show signature of galactic boxy/peanut bulge

“…The magnitude sliced analysis shows there are correlated proper motions at all magnitude intervals through the bulge (Clarke et al 2019). The maps shown in figure 1 and the further analysis presented in Clarke et al (2019) demonstrate the impressive agreement between the predictions of the fiducial model and the VIRAC data. We see no discrepancy that might be attributed to the presence of a secondary classical bulge component.…”

“…Here we focus on the comparison between the integrated kinematic maps. A more extended description of the work described here is given in Clarke et al (2019). The columns show, from left to right, the mean longitudinal and latitudinal proper motions, the corresponding dispersions, the dispersion ratio, and the proper motion correlation.…”

“…The mask at b ≈ 0 • covers regions of high extinction, AK > 1.0 mag, where the VIRAC data is less trustworthy. Adapted from Clarke et al (2019).…”

PopSyCLE: A New Population Synthesis Code for Compact Object Microlensing Events

“…However, this is nearly twice as fast as the most recent values reported in the literature, which range from around 36 to 44 km/s/kpc (Bovy et al 2019;Clarke et al 2019;Sanders et al 2019), determined using combinations of Gaia DR2, VVV, and APOGEE.…”

Dynamics of the Milky Way Bar/Bulge

“…With the recent availability of the VIRAC near-infrared proper motion catalogue (Smith et al 2018), and by cross matching with the Gaia DR2 catalogue, it has become possible to obtain absolute proper motion (PM) maps across the entire bulge except in the most crowded and extincted regions in the Galactic plane (Clarke et al 2019, Sanders et al 2019, see also talk by Sanders). After careful foreground subtraction, Clarke et al (2019) derived both both line-of-sight integrated, on-sky PM maps, and maps as a function of distance, using RC-magnitude as a proxy. Figure 1 compares the VIRAC PMs with previous data.…”