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

Abstract

We study the spiral arm influence on the solar neighbourhood stellar\nkinematics. As the nature of the Milky Way (MW) spiral arms is not completely\ndetermined, we study two models: the Tight-Winding Approximation (TWA) model,\nwhich represents a local approximation, and a model with self-consistent\nmaterial arms named PERLAS. This is a mass distribution with more abrupt\ngravitational forces. We perform test particle simulations after tuning the two\nmodels to the observational range for the MW spiral arm properties. We explore\nthe effects of the arm properties and find that a significant region of the\nallowed parameter space favours the appearance of kinematic groups. The\nvelocity distribution is mostly sensitive to the relative spiral arm phase and\npattern speed. In all cases the arms induce strong kinematic imprints for\npattern speeds around 17 km/s/kpc (close to the 4:1 inner resonance) but no\nsubstructure is induced close to corotation. The groups change significantly if\none moves only ~0.6 kpc in galactocentric radius, but ~2 kpc in azimuth. The\nappearance time of each group is different, ranging from 0 to more than 1 Gyr.\nRecent spiral arms can produce strong kinematic structures. The stellar\nresponse to the two potential models is significantly different near the Sun,\nboth in density and kinematics. The PERLAS model triggers more substructure for\na larger range of pattern speed values. The kinematic groups can be used to\nreduce the current uncertainty about the MW spiral structure and to test\nwhether this follows the TWA. However, groups such as the observed ones in the\nsolar vicinity can be reproduced by different parameter combinations. Data from\nvelocity distributions at larger distances are needed for a definitive\nconstraint.\n