Dependence of Spiral Arms Pitch Angle on Wavelength as a Test of Density Wave Theory

TL;DR

This study tests density wave theory by examining how spiral arm pitch angles vary across different wavelengths, finding that younger stars trace tighter arms, consistent with theoretical predictions.

Contribution

It provides empirical evidence that spiral arm pitch angles decrease in bluer wavelengths, supporting density wave theory's predictions about star formation and age gradients.

Findings

Pitch angle decreases from red to blue bands.

Younger stars trace tighter spiral arms.

Pitch angle offset increases with overall pitch angle.

Abstract

Large-scale galactic shocks, predicted by density wave theory, trigger star formation (SF-arms) downstream from the potential of the oldest stars (P-arms), resulting in a color jump from red to blue across spiral arms in the direction of rotation, while aging of these newly formed young stars induces the opposite but coexisting classic age gradient further downstream from the SF-arms. As the techniques for measuring pitch angle are intensity-weighted, they trace both the SF-arms and P-arms and are not sensitive to the classic age gradient. Consequently, the measured pitch angle of spiral arms should be systematically smaller in bluer bandpasses compared to redder bandpasses. We test these predictions using a comprehensive sample of high-quality optical ($BVRI$) images of bright, nearby spiral galaxies acquired as part of the Carnegie-Irvine Galaxy Survey, supplemented by $Spitzer$ 3.6 $\mu m$ data to probe evolved stars and $GALEX$ ultraviolet images to trace recent star formation. We apply one-dimensional and two-dimensional techniques to measure the pitch angle of spiral arms, paying close attention to adopt consistent procedures across the different bandpasses to minimize error and systematic bias. We find that the pitch angle of spiral arms decreases mildly but statistically significantly from the reddest to the bluest bandpass, demonstrating conclusively that young stars trace tighter spiral arms than old stars. Furthermore, the correlation between the pitch angle of blue and red bandpasses is non-linear, such that the absolute value of pitch angle offset increases with increasing pitch angle. Both effects can be naturally explained in the context of the density wave theory for spiral structure.