Investigating the Origins of Spiral Structure in Disk Galaxies through a Multiwavelength Study

TL;DR

This study measures spiral arm pitch angles across multiple wavelengths in disk galaxies, finding consistent angles in star-forming regions and tighter spirals in optical/infrared, supporting density-wave theory predictions.

Contribution

It provides the first comprehensive multiwavelength measurement of spiral pitch angles, confirming the downstream placement of young stars relative to the density wave.

Findings

Pitch angles are consistent across star-forming wavelengths.

Optical and infrared images show tighter spirals than star-forming wavelengths.

Results support the density-wave theory with downstream star formation.

Abstract

The density-wave theory of spiral structure proposes that star formation occurs in or near a spiral-shaped region of higher density that rotates rigidly within the galactic disk at a fixed pattern speed. In most interpretations of this theory, newborn stars move downstream of this position as they come into view, forming a downstream spiral which is tighter, with a smaller pitch angle than that of the density wave itself. Rival theories, including theories which see spiral arms as essentially transient structures, may demand that pitch angle should not depend on wavelength. We measure the pitch angle of a large sample of galaxies at several wavelengths associated with star formation or very young stars (8.0 {\mu}m, H-{\alpha} line and 151 nm in the far-UV) and show that they all have the same pitch angle, which is larger than the pitch angle measured for the same galaxies at optical and near-infrared wavelengths. Our measurements in the B band and at 3.6 {\mu}m have unambiguously tighter spirals than the starforming wavelengths. In addition, we have measured in the u-band, which seems to fall midway between these two extremes. Thus, our results are consistent with a region of enhanced stellar light situated downstream of a starforming region.