Spiral arm crossings inferred from ridges in Gaia stellar velocity distributions

TL;DR

This paper uses Gaia data to identify stellar velocity ridges that indicate recent crossings of spiral arms, revealing multiple spiral features and suggesting a flocculent outer Galaxy.

Contribution

It introduces a method to infer recent spiral arm crossings from local velocity distribution ridges in Gaia data, highlighting multiple spiral structures.

Findings

Multiple ridges suggest multiple spiral arms.

Velocity ridges indicate recent arm crossings.

Outer Galaxy is likely flocculent, not grand design.

Abstract

The solar neighborhood contains disc stars that have recently crossed spiral arms in the Galaxy. We propose that boundaries in local velocity distributions separate stars that have recently crossed or been perturbed by a particular arm from those that haven't. Ridges in the stellar velocity distributions constructed from the second Gaia data release trace orbits that could have touched nearby spiral arms at apocentre or pericentre. The multiple ridges and arcs seen in local velocity distributions are consistent with the presence of multiple spiral features and different pattern speeds and imply that the outer Galaxy is flocculent rather than grand design.