Are high velocity peaks in the Milky Way bulge due to the bar?

Abstract

Recently the commissioning APOGEE observations of the Galactic bulge reported that a significant fraction of stars ($\sim 10$) are in a cold (${\sigma }_{\mathrm{V}}\approx 30$ km/s) high velocity peak (Galactocentric radial velocity $\approx 200$ km/s). These stars are speculated to reflect the stellar orbits in the Galactic bar. In this study, we use two $N$-body models of the Milky Way-like disk galaxy with different bar strengths to critically examine this possibility. The general trends of the Galactocentric radial velocity distribution in observations and simulations are similar, but neither our models nor the BRAVA data reveal a statistically significant cold high velocity peak. A Monte Carlo test further suggests that it is possible for a spurious high velocity peak to appear if there are only a limited number of stars observed. Thus, the reported cold high velocity peak, even if it is real, is unlikely due to stars on the bar-supporting orbits. Our models do predict an excess of stars with high radial velocity, but not in a distinct peak. In the distance--velocity diagram, the high velocity particles in different fields exist at a similar distance $\sim 8.5\pm 1$ kpc away from the Sun. This result may be explained with geometric intersections between the line-of-sight and the particle orbits; high velocity stars naturally exist approximately at the tangent point, without constituting a distinct peak. We further demonstrate that even without the presence of a bar structure, particle motions in an axisymmetric disk can also exhibit an excess of high velocity stars.

Accepted to publish in ApJL. Extra discussion and new Figure 2 were added to discuss a possible false peak due to small number statistics. Main conclusions remain unchanged

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• A Kiloparsec-Scale Nuclear Stellar Disk in the Milky Way as a Possible Explanation of the High Velocity Peaks in the Galactic Bulge
• Resonant Orbits and the High Velocity Peaks Towards the Bulge
• Mapping the Three-Dimensional "X-Shaped Structure" in Models of the Galactic Bulge
• Theoretical Models of the Galactic Bulge
• Predicted Stellar Kinematics of a Kiloparsec-Scale Nuclear Disc (or Ring) in the Milky Way
• Signatures of the Galactic bar on stellar kinematics unveiled by APOGEE
• Understanding the velocity distribution of the Galactic Bulge with APOGEE and Gaia
• Chemical abundances and ages of the bulge stars in APOGEE high-velocity peaks
• A new look at the kinematics of the bulge from an N-body model
• Spectroscopic Observations of Obscured Populations in the Inner Galaxy: 2MASS-GC02, Terzan 4, and the 200 km/s stellar peak
• Models of Bars with Exponential Density Profiles