Galaxy Zoo and SpArcFiRe: Constraints on spiral arm formation mechanisms
  from spiral arm number and pitch angles
  In this paper we study the morphological properties of spiral galaxies,
including measurements of spiral arm number and pitch angle. Using Galaxy Zoo
2, a stellar mass-complete sample of 6,222 SDSS spiral galaxies is selected. We
use the machine vision algorithm SpArcFiRe to identify spiral arm features and
measure their associated geometries. A support vector machine classifier is
employed to identify reliable spiral features, with which we are able to
estimate pitch angles for half of our sample. We use these machine measurements
to calibrate visual estimates of arm tightness, and hence estimate pitch angles
for our entire sample. The properties of spiral arms are compared with respect
to various galaxy properties. The star formation properties of galaxies vary
significantly with arm number, but not pitch angle. We find that galaxies
hosting strong bars have spiral arms substantially ($4-6^\mathrm{o}$) looser
than unbarred galaxies. Accounting for this, spiral arms associated with
many-arm structures are looser (by 2$^\mathrm{o}$) than those in two-arm
galaxies. In contrast to this average trend, galaxies with greater
bulge-to-total stellar mass ratios display both fewer and looser spiral arms.
This effect is primarily driven by the galaxy disc, such that galaxies with
more massive discs contain more spiral arms with tighter pitch angles. This
implies that galaxy central mass concentration is not the dominant cause of
pitch angle and arm number variations between galaxies, which in turn suggests
that not all spiral arms are governed by classical density waves or modal
theories.
