Offset between stellar spiral arms and gas arms of the Milky Way

TL;DR

This study investigates the spatial relationship between stellar and gaseous spiral arms in the Milky Way, finding that gas and old stars are often offset, supporting the quasi-stationary density wave theory.

Contribution

It provides a detailed analysis of arm tangencies across multiple gas components and old stars, revealing consistent offsets that inform spiral arm formation models.

Findings

Gas and old star density peaks are offset by 1.3°-5.8° in certain arms.

No significant offset found among different gas tracers.

Offsets are comparable to spiral arm widths, supporting density wave theory.

Abstract

Spiral arms shown by different components may not be spatially coincident, which can constrain formation mechanisms of spiral structure in a galaxy. We reassess the spiral arm tangency directions in the Milky Way through identifying the bump features in the longitude plots of survey data for infrared stars, radio recombination lines (RRLs), star formation sites, CO, high density regions in clouds, and HI. The bump peaks are taken as indications for arm tangencies, which are close to the real density peaks near the spiral arm tangency point but often have $\sim$ 1$^\circ$ offset to the interior of spiral arms. The arm tangencies identified from the longitudes plots for RRLs, HII regions, methanol masers, CO, high density gas regions, and HI gas appear nearly the same Galactic longitude, and therefore there is no obvious offset for spiral arms traced by different gas components. However, we find obvious displacements of 1.3$^\circ-$ 5.8$^\circ$ between gaseous bump peaks from the directions of the maximum density of old stars near the tangencies of the Scutum-Centaurus Arm, the northern part of the Near 3 kpc Arm, and maybe also the Sagittarius Arm. The offsets between the density peaks of gas and old stars for spiral arms are comparable with the arm widths, which is consistent with expectations for quasi-stationary density wave in our Galaxy.