Kinematic footprint of the Milky Way spiral arms in Gaia EDR3

Abstract

The Milky Way spiral arms are well established from star counts as well as from the locus of molecular clouds and other young objects, however, they have only recently started to be studied from a kinematics point of view. Using the kinematics of thin disc stars in Gaia EDR3 around the extended solar neighbourhood, we create x-y projections coloured by the radial, residual rotational, and vertical Galactocentric velocities (U, ∆V, W ). The maps are rich in substructures and reveal the perturbed state of the Galactic disc. We find that local differences between rotational velocity and circular velocity, ∆V , display at least five large-scale kinematic spirals; two of them closely follow the locus of the Sagittarius-Carina and Perseus spiral arms, with pitch angles of 9.12 • and 7.76 • , and vertical thickness of ∼ 400 pc and ∼ 600 pc, respectively. Another kinematic spiral is located behind the Perseus arm and appears as a distortion in rotation velocities left by this massive arm but with no known counterpart in gas/stars overdensity. A weaker signal close to the Sun's position is present in our three velocity maps, and appears to be associated with the Local arm. Our analysis of the stellar velocities in the Galactic disc shows kinematic differences between arms and inter-arms, that are in favour of Milky Way spiral arms that do not corotate with the disc. Moreover, we show that the kinematic spirals are clumpy and flocculent, revealing the underlying nature of the Milky Way spiral arms.

The SEDIGISM survey: The influence of spiral arms on the molecular gas distribution of the inner Milky Way

“…For example, Castro-Ginard et al (2021) using the most updated catalogue of open clusters measured different pattern speeds for different spiral arms, favouring a flocculent structure with transient arms that co-rotate with the disc. Alternatively, Martinez-Medina et al (2021), studying the kinematics of selected stars in the Gaia EDR3 sample, found instead that stars do not always co-rotate with the disc: objects in the spiral arms tend to rotate more slowly than objects in the inter-arm region.…”