Galaxy Zoo: constraining the origin of spiral arms

TL;DR

This study tests the swing amplification theory for spiral arm formation in galaxies using observational data and models, finding it explains about 40% of unbarred spiral galaxies, while others are likely influenced by interactions.

Contribution

It provides observational evidence supporting swing amplification in a subset of spiral galaxies and introduces a halo contraction/expansion model revealing bimodality in spiral arm origins.

Findings

Approximately 40% of unbarred spirals are consistent with swing amplification.

Universal dark matter profiles cannot predict spiral arm numbers accurately.

Halo contraction/expansion models reveal bimodality in spiral galaxy populations.

Abstract

Since the discovery that the majority of low-redshift galaxies exhibit some level of spiral structure, a number of theories have been proposed as to why these patterns exist. A popular explanation is a process known as swing amplification, yet there is no observational evidence to prove that such a mechanism is at play. By using a number of measured properties of galaxies, and scaling relations where there are no direct measurements, we model samples of SDSS and S$^4$G spiral galaxies in terms of their relative halo, bulge and disc mass and size. Using these models, we test predictions of swing amplification theory with respect to directly measured spiral arm numbers from Galaxy Zoo 2. We find that neither a universal cored or cuspy inner dark matter profile can correctly predict observed numbers of arms in galaxies. However, by invoking a halo contraction/expansion model, a clear bimodality in the spiral galaxy population emerges. Approximately 40 per cent of unbarred spiral galaxies at $z \lesssim 0.1$ and $\mathrm{M_*} \gtrsim 10^{10} \mathrm{M_\odot}$ have spiral arms that can be modelled by swing amplification. This population display a significant correlation between predicted and observed spiral arm numbers, evidence that they are swing amplified modes. The remainder are dominated by two-arm systems for which the model predicts significantly higher arm numbers. These are likely driven by tidal interactions or other mechanisms.