Kinematics of symmetric Galactic longitudes to probe the spiral arms of the Milky Way with Gaia

TL;DR

This paper proposes a novel method using symmetric Galactic longitudes and Gaia data to detect and analyze the effects of spiral arms on stellar kinematics, enabling constraints on spiral structure properties.

Contribution

It introduces a new approach comparing symmetric longitudes' stellar kinematics without assuming axisymmetry, and assesses Gaia's capability to detect these signatures.

Findings

Differences in transverse velocity can exceed 10 km/s in certain regions.

Gaia can measure median transverse velocities with precision <1 km/s up to 4-6 kpc.

The method can distinguish between different spiral arm models and dynamics.

Abstract

We model the effects of the spiral arms of the Milky Way on the disk stellar kinematics in the Gaia observable space. We also estimate the Gaia capabilities of detecting the predicted signatures. We use both controlled orbital integrations in analytic potentials and self-consistent simulations. We introduce a new strategy, which consists of comparing the stellar kinematics of symmetric Galactic longitudes (+l and -l), in particular the median transverse velocity (from parallaxes and proper motions). This approach does not require the assumption of an axisymmetric model. The typical differences between the transverse velocity in symmetric longitudes in the models are of ~2 km/s, but can be larger than 10 km/s for certain longitudes and distances. The kinematic differences for +l and -l show trends that depend on the properties of spiral arms. Thus, this method can be used to quantify the importance of the effects of spiral arms on the orbits of stars in the different disk regions, and to constrain the location of the arms, main resonances and, thus, pattern speed. Moreover, the method allows us to test the dynamical nature of the spiral structure (e.g. grand design versus transient multiple arms). We estimate the number of stars of each spectral type that Gaia will observe in representative Galactic longitudes, their errors in distance and transverse velocity, and the error in computing the median velocity as a function of distance. We will be able to measure the median transverse velocity with Gaia data, with precision smaller than ~1 km/s up to distances of ~4-6 kpc for certain giant stars, and up to ~2-4 kpc and better kinematic precision (<= 0.5 km\s) for certain sub-giants and dwarfs. These are enough to measure the typical signatures seen in the models. We also show that a similar strategy can be used with line-of-sight velocities from other upcoming spectroscopic surveys.(Abridged)