Understanding the spiral structure of the Milky Way using the local kinematic groups

TL;DR

This study investigates how different models of the Milky Way's spiral arms influence local stellar kinematics, revealing that spiral arm parameters significantly affect the formation of kinematic groups and can help constrain the galaxy's spiral structure.

Contribution

It compares the effects of two spiral arm models, TWA and PERLAS, on stellar kinematics, providing insights into their influence on local velocity structures and the potential to refine MW spiral arm parameters.

Findings

Strong kinematic imprints near the Sun for pattern speeds around 17 km/s/kpc

Kinematic groups are sensitive to spiral arm phase and pattern speed

PERLAS model produces more substructure over a wider parameter range

Abstract

We study the spiral arm influence on the solar neighbourhood stellar kinematics. As the nature of the Milky Way (MW) spiral arms is not completely determined, we study two models: the Tight-Winding Approximation (TWA) model, which represents a local approximation, and a model with self-consistent material arms named PERLAS. This is a mass distribution with more abrupt gravitational forces. We perform test particle simulations after tuning the two models to the observational range for the MW spiral arm properties. We explore the effects of the arm properties and find that a significant region of the allowed parameter space favours the appearance of kinematic groups. The velocity distribution is mostly sensitive to the relative spiral arm phase and pattern speed. In all cases the arms induce strong kinematic imprints for pattern speeds around 17 km/s/kpc (close to the 4:1 inner resonance) but no substructure is induced close to corotation. The groups change significantly if one moves only ~0.6 kpc in galactocentric radius, but ~2 kpc in azimuth. The appearance time of each group is different, ranging from 0 to more than 1 Gyr. Recent spiral arms can produce strong kinematic structures. The stellar response to the two potential models is significantly different near the Sun, both in density and kinematics. The PERLAS model triggers more substructure for a larger range of pattern speed values. The kinematic groups can be used to reduce the current uncertainty about the MW spiral structure and to test whether this follows the TWA. However, groups such as the observed ones in the solar vicinity can be reproduced by different parameter combinations. Data from velocity distributions at larger distances are needed for a definitive constraint.