Galaxy Zoo: constraining the origin of spiral arms
  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.
