A low density-wave spiral pattern speed, from the tracer separations (age gradient) across a spiral arm in the Milky Way

TL;DR

This paper measures the spiral pattern speed of the Milky Way's density wave using separations between star-forming regions and masers, finding a low pattern speed of 12 to 17 km/s/kpc.

Contribution

It introduces a novel method of estimating the spiral pattern speed through tracer separations and models of HII regions and masers.

Findings

Pattern speed near 12-17 km/s/kpc

Consistent measurements from optical and radio tracers

Supports a low-density wave model

Abstract

We observe the density wave angular pattern speed OMEGA-p to be near 12 to 17 km / s / kpc, by the separation between a typical optical HII region (from the spiral arm dust lane) and using a HII evolution time model to yield its relative speed, and independently by the separation between a typical radio maser (from the spiral arm dust lane) with a maser model.