The GALAH survey: Velocity fluctuations in the Milky Way using Red Clump giants

Shourya Khanna, Sanjib Sharma, Joss Bland-Hawthorn, Michael Hayden, David M. Nataf, Yuan Sen Ting, Janez Kos, Sarah Martell, Tomaž Zwitter, Gayandhi De Silva, Martin Asplund, Sven Buder, Ly Duong, Jane Lin, Jeffrey D. Simpson, Borja Anguiano, Jonathan Horner, Prajwal R. Kafle, Geraint F. Lewis, Thomas NordlanderRosemary F.G. Wyse, Robert A. Wittenmyer, Daniel B. Zucker

Research output: Contribution to journalArticlepeer-review

7 Citations (Scopus)


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 APOGEE survey find significant fluctuations in velocity for stars in the mid-plane (|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 neighbourhood. Their measured power spectrum has a characteristic amplitude of 11 km s−1 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 GALAH and APOGEE, we map the line-of-sight velocities around the Sun (d < 5 kpc), and |z| <1.25 kpc from the mid-plane. By subtracting a smooth axisymmetric model for the velocity field, we study the residual fluctuations and compare our findings with mock survey generated by GALAXIA. We find negligible large-scale fluctuations away from the plane. In the mid-plane, we reproduce the earlier APOGEE power spectrum but with 20 per cent smaller amplitude (9.3 km s−1) 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 km s−1. 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 km s−1. Taking this into account, we estimate the amplitude of real fluctuations to be <4.6 km s−1, about a factor of three less than the APOGEE result.

Original languageEnglish
Pages (from-to)4215-4232
Number of pages18
JournalMonthly Notices of the Royal Astronomical Society
Issue number3
Publication statusPublished - 21 Jan 2019


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