http://arxiv.org/abs/1401.2996
Recently the commissioning APOGEE observations of the Galactic bulge reported that a significant fraction of stars ($\sim10\%$) are in a cold ($\sigma_{\rm V} \approx 30$ 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. Thus, the reported cold high velocity peak, even if it is real, is unlikely due to stars on the bar-supporting orbits. Nevertheless, we do find that there is 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 $\sim8.5 \pm 1$ kpc away from the Sun, while at both smaller and larger distances the maximum radial velocity speed decreases. 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. However, these stars do not constitute 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.
Wed, 15 Jan 14
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