Galaxy Zoo: star-formation versus spiral arm number
  Spiral arms are common features in low-redshift disc galaxies, and are
prominent sites of star-formation and dust obscuration. However, spiral
structure can take many forms: from galaxies displaying two strong `grand
design' arms, to those with many `flocculent' arms. We investigate how these
different arm types are related to a galaxy's star-formation and gas properties
by making use of visual spiral arm number measurements from Galaxy Zoo 2. We
combine UV and mid-IR photometry from GALEX and WISE to measure the rates and
relative fractions of obscured and unobscured star formation in a sample of
low-redshift SDSS spirals. Total star formation rate has little dependence on
spiral arm multiplicity, but two-armed spirals convert their gas to stars more
efficiently. We find significant differences in the fraction of obscured
star-formation: an additional $\sim 10$ per cent of star-formation in two-armed
galaxies is identified via mid-IR dust emission, compared to that in many-armed
galaxies. The latter are also significantly offset below the IRX-$\beta$
relation for low-redshift star-forming galaxies. We present several
explanations for these differences versus arm number: variations in the spatial
distribution, sizes or clearing timescales of star-forming regions (i.e.,
molecular clouds), or contrasting recent star-formation histories.
