The Kinematics of the Outer Galactic Disk from A and F Stars

Abstract

The kinematics of stars in the outer Galactic disk is poorly known. In addition to constraining the Galactic potential, knowledge of the motions of stars throughout the disk facilitates studies of non-axisymmetric motion, such as streaming motions due to spiral arms. Understanding the kinematics of the disk in full, including the rotation law, will help to map out its structure, and set constraints on its formation and evolution.

I explore the kinematics of A/F stars in two outer-disk pencil beams. I show that A stars in particular are useful probes of outer Galactic disk kinematics. The use of these relatively early-type stars is an alternative to the ISM gas tracers and clump giants that have been commonly used in previous studies. A/F stars offer the advantage of reduced kinematic scatter and asymmetric drift as compared to the older clump giants, and can provide a much denser sampling in the outer disk than ISM gas tracers. They are bright so can be detected out to great distances, and large samples can be efficiently selected from the IPHAS r-i, r-Hα plane.

The sightlines were chosen to sample the strong shear in Galactic rotation (ℓ= 118˚), and as a control (ℓ= 178˚). Radial velocities (RVs) and extinction-corrected spectro-photometric distances are computed for the sample of >1300 A/F stars with the aid of MCMC parameter fitting. The rotation law measured at ℓ=118˚ using the RV data is sharply rising out from RG ~11 kpc, going against expectations of a flat or slowly rising law.

Gaia DR2 astrometry, released in April 2018, provided proper motions for the sample. On combining these with the measured RVs and spectro-photometric distances, full space motions are obtained. The Galactocentric radial, azimuthal, and vertical velocity trends are constructed without any prior assumption about the velocity field. The measured rotation curve incorporating proper motion data is flat at ℓ = 178˚, but remains sharply rising at ℓ = 118˚, albeit less so than that determined using only RV data.

I consider the detailed form of the observed Galactocentric trends with specific perturbers in mind, finding no clear interpretation in terms of perturbation from the central bar or spiral arms. The variation of observed trends with longitude and distance is a reflection of the complex dynamics in our Galaxy.

The methods developed in this work and the use of A stars as tracers will be used in the future for fuller exploitation via spectroscopy on forthcoming massively multiplexed wide-field spectrographs. In particular, the PTMCMC method presented will be used to analyse WEAVE survey products. Future use of these young stellar tracers will help us to understand how our Galaxy is, and came to be, how it is now.