A Fundamental Plane of Spiral Structure in Disk Galaxies

TL;DR

This paper identifies a fundamental planar relationship linking spiral arm pitch angle, neutral hydrogen density, and bulge mass in disk galaxies, challenging existing theories and suggesting a new universal framework.

Contribution

It introduces a new fundamental plane relating spiral structure, gas density, and bulge mass, providing a key constraint for theories of spiral formation.

Findings

Pitch angle correlates with neutral hydrogen density.

Pitch angle correlates with bulge mass.

The three quantities form a fundamental plane.

Abstract

Spiral structure is the most distinctive feature of disk galaxies and yet debate persists about which theory of spiral structure is the correct one. Many versions of the density wave theory demand that the pitch angle is uniquely determined by the distribution of mass in the bulge and disk of the galaxy. We present evidence that the tangent of the pitch angle of logarithmic spiral arms in disk galaxies correlates strongly with the density of neutral atomic hydrogen in the disk and with the central stellar bulge mass of the galaxy. These three quantities, when plotted against each other, form a planar relationship which, we argue should be fundamental to our understanding of spiral structure in disk galaxies. We further argue that any successful theory of spiral structure must be able to explain this relationship.