Spatial and velocity offsets of Galactic masers from the centers of spiral arms

TL;DR

This study investigates the spatial and velocity offsets of galactic masers relative to spiral arms, revealing that masers are typically located near the inner edge of arms and providing insights into the Milky Way's structure and dynamics.

Contribution

It introduces a method to compare maser locations with a recent 4-arm spiral model, offering new constraints on the Galaxy's spiral structure and pattern speed.

Findings

Most masers are near the inner edge of spiral arms.

Masers are offset inward by approximately 0.34 kpc from the model arms.

The median velocity offset is around 10 km/s.

Abstract

Some galactic theories of spiral arms predict an offset between different tracers of star formation. Our goal here is to find such an offset between the observed location of radio masers and the location of the arms, using a recent 4-arm model fitted to the CO 1-0 gas. Our method here is to compare a recent global 4-arm spiral model (as fitted to the arm tangents in the observed broad CO 1-0 gas) with the recent results for the trigonometric distances of radio masers, for the main arms (Cygnus-Norma, Perseus, Sagittarius-Carina, Scutum, and Norma). Our results indicate that most radio masers are near the inner arm edge (toward the Galactic Center) of spiral arms. These masers are offsets from the model arm (where the broad CO 1-0 molecular region resides) by 0.34 (plus or minus 0.06) kpc inward. In radial velocity space, the median offset between masers and the CO-fitted model is around 10 (plus or minus 1) km/s. Based on the masers being observed here to be radially inward of the broad CO gas in the Cygnus arm at 15 kpc along the Galactic Meridian, the corotation radius of the Milky Way disk is more than 15 kpc distant from the Galactic Centre and the density-wave angular pattern speed is less than 15 km/s/kpc. Arm pitch angle should be measured using many arm tracers, and located on both side of the Galactic Meridian, to ensure better precision and avoid a bias pertinent to a single tracer.