Beads on a String and Spurs in Galactic Disks

TL;DR

This study uses galaxy simulations to analyze how dense gas structures like beads on a string and spurs form and organize within galactic disks, influenced by spiral arm dynamics and instabilities.

Contribution

It identifies the formation mechanisms of beads on a string and spurs in galactic disks and links their occurrence to spiral arm pitch angles and local disk dynamics.

Findings

Beads on a string form via gravitational instabilities.

Spurs originate from Kelvin-Helmholtz instabilities.

The coexistence of both structures depends on disk location and spiral arm pitch angle.

Abstract

The organization of the interstellar medium in disk galaxies obeys the large scale dynamics set by kpc-size structures. Improving our knowledge of how the dense, molecular gas is distributed in a disk is an important step in our understanding of star formation at galactic scale. Using a recently published simulation of a Milky Way-like galaxy, we explore the formation and dynamical organization of the star forming gas in a proto-typical disk. Along spiral arms, we report the formation of regularly spaced clouds, called beads on a string and spurs. The former form through gravitational instabilities while the later originate from Kelvin-Helmholtz instabilities. We propose that the co-existence of both structures in the same galaxy can be explained by a different role of the disk dynamics, depending on the location within the disk. In particular, we highlight the impact of the pitch angle of the spiral arm in the development of either type of structure.