Galaxy Zoo: comparing the demographics of spiral arm number and a new
  method for correcting redshift bias
  The majority of galaxies in the local Universe exhibit spiral structure with
a variety of forms. Many galaxies possess two prominent spiral arms, some have
more, while others display a many-armed flocculent appearance. Spiral arms are
associated with enhanced gas content and star-formation in the disks of
low-redshift galaxies, so are important in the understanding of star-formation
in the local universe. As both the visual appearance of spiral structure, and
the mechanisms responsible for it vary from galaxy to galaxy, a reliable method
for defining spiral samples with different visual morphologies is required. In
this paper, we develop a new debiasing method to reliably correct for
redshift-dependent bias in Galaxy Zoo 2, and release the new set of debiased
classifications. Using these, a luminosity-limited sample of ~18,000 Sloan
Digital Sky Survey spiral galaxies is defined, which are then further
sub-categorised by spiral arm number. In order to explore how different spiral
galaxies form, the demographics of spiral galaxies with different spiral arm
numbers are compared. It is found that whilst all spiral galaxies occupy
similar ranges of stellar mass and environment, many-armed galaxies display
much bluer colours than their two-armed counterparts. We conclude that
two-armed structure is ubiquitous in star-forming disks, whereas many-armed
spiral structure appears to be a short-lived phase, associated with more
recent, stochastic star-formation activity.
