VELOCITY FLUCTUATIONS IN THE MILKY WAY USING RED CLUMP GIANTS
Context: If the Galaxy is axisymmetric and in dynamical equilibrium, we expect negligible fluctuations in the residual line-of-sight velocity field. However, non-axisymmetric structures like a bar, spiral arms and merger events can generate velocity fluctuations. Recent results using the APOGEE survey claim significant fluctuations in velocity for stars in the mid plane (|z|<0.25 kpc) out to 5 kpc and suggest that the dynamical influence of the Milky Way's bar extends out to the Solar neighborhood. Their measured power spectrum has a characteristic amplitude of 11 km s-1 on a physical scale of ~ 2.5 kpc. The existence of large-scale streaming motions on these scales has important implications for determining the Sun's motion about the Galactic Centre.
Aims: Using red clump stars from GALAH (Galactic Archaeology with HERMES) and APOGEE surveys, we map the line-of-sight velocity field around the Sun out to distances of 5 kpc and up to 1.25 kpc from the Galactic Plane. Methods: By subtracting a smooth axisymmetric model for the velocity field, we study the residual velocity fluctuations and compare our findings with synthetic survey generated by GALAXIA based on an axisymmetric, steady state model.
Results: We find negligible large-scale fluctuations away from the plane. In the mid-plane, we reproduce the APOGEE power spectrum but with 20% smaller amplitude (9.1 km s-1) after taking into account systematic effects (e.g. window function). The amplitude power is further reduced to 6.9 km s-1 if a flexible axisymmetric model is used. Additionally, our mock simulations show that, in the plane, the distances are underestimated for high mass red-clump stars and this can lead to spurious power with amplitude of about 4.7 km s-1. Taking this into account, we estimate the amplitude due to real fluctuations to be less than 5.1 km s-1, about a factor of two below what is claimed by APOGEE.
