Wrinkles in Time. II. Stellar Age Trends in Kinematic Signatures from Transient Spiral Structure

Abstract

Spiral arms in the disks of galaxies like the Milky Way can generate kinematic signatures, which appear as ridges or wrinkles in action space. Such signatures have proven difficult to disentangle using kinematic measures alone. In this study, we investigate how including stellar age as an additional dimension for analysis may provide a novel insight into the physical characteristics, timescales, and nature of the progenitors of such perturbations, where these novel insights could contribute to our understanding of the history of spiral arms in the Milky Way. We used a suite of tracer particle simulations that modeled a variety of prescriptions for spiral arms to characterize observable trends. The Lindblad resonances of nonwinding spirals produce signature overdensities, or wrinkles, in a kinematic space that is typically associated with older stellar populations (high radial action). We find that these wrinkles are preferentially populated with stars that were initially in nearly circular orbits, kinematics that is generally correlated with younger stellar ages. It follows that the stellar age distribution of wrinkle populations could serve to place constraints on the past passage of a transient spiral pattern in the solar neighborhood. For example, our simulations suggest that a physically motivated spiral pattern could significantly populate a wrinkle with zero-age stars in orbits typically occupied by stars much older than the Sun.