The GALAH Survey: Velocity fluctuations in the Milky Way using red clump giants

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 find significant fluctuations in velocity for stars in the midplane (|z|< 0.25 kpc) and out to 5 kpc, which suggests 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 on a scale of ~ 2.5 kpc. The existence of large streaming motions on these scales has important implications for determining the Sun's motion about the Galactic Centre. Using red clump stars from the GALAH 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. By subtracting a smooth axisymmetric model for the velocity field, we study the residual velocity fluctuations and compare our findings with mock survey generated by Galaxia based on an axisymmetric, steady state model. We find negligible large-scale fluctuations away from the plane. In the mid-plane, we reproduce the earlier APOGEE power spectrum results but with 20\% smaller amplitude (9.5 km/s) after taking a few systematic effects into account (e.g. volume completeness). The amplitude power is further reduced to 6.7 km/s 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 5.5 km/s. Taking this into account, we estimate the amplitude of real fluctuations to be less than 4.2 km/s, about a factor of three less than the previous result from APOGEE.