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. Recent
results using the \apg{} survey find significant fluctuations in velocity for
stars in the midplane ($|z|<$0.25 kpc) out to 5 kpc, suggesting that the
dynamical influence of non-axisymmetric features i.e., the Milky Way's bar,
spiral arms and merger events extends out to the Solar neighborhood. Their
measured power spectrum has a characteristic amplitude of 11 \kms{} on a scale
of 2.5 kpc. The existence of such large-scale streaming motions has important
implications for determining the Sun's motion about the Galactic Centre. Using
Red Clump stars from \glh{} and \apg{}, we map the line-of-sight velocities
around the Sun (d$<$5 kpc), and $|z|<$1.25 kpc from the midplane. By
subtracting a smooth axisymmetric model for the velocity field, we study the
residual fluctuations and compare our findings with mock survey generated by
\glx{}. We find negligible large-scale fluctuations away from the plane. In the
mid-plane, we reproduce the earlier \apg{} power spectrum but with 20\% smaller
amplitude (9.3 \kms{}) after taking into account a few systematics (e.g.,
volume completeness). Using a flexible axisymmetric model the power-amplitude
is further reduced to 6.3 \kms{}. Additionally, our simulations show that, in
the plane, distances are underestimated for high-mass Red Clump stars which can
lead to spurious power-amplitude of about 5.2 \kms{}. Taking this into account,
we estimate the amplitude of real fluctuations to be $<$4.6 \kms{}, about a
factor of three less than the \apg{} result.
