Galaxy Zoo: Unwinding the Winding Problem - Observations of Spiral Bulge
  Prominence and Arm Pitch Angles Suggest Local Spiral Galaxies are Winding
  We use classifications provided by citizen scientists though Galaxy Zoo to
investigate the correlation between bulge size and arm winding in spiral
galaxies. Whilst the traditional spiral sequence is based on a combination of
both measures, and is supposed to favour arm winding where disagreement exists,
we demonstrate that, in modern usage, the spiral classifications Sa-Sd are
predominantly based on bulge size, with no reference to spiral arms.
Furthermore, in a volume limited sample of galaxies with both automated and
visual measures of bulge prominence and spiral arm tightness, there is at best
a weak correlation between the two. Galaxies with small bulges have a wide
range of arm winding, while those with larger bulges favour tighter arms. This
observation, interpreted as revealing a variable winding speed as a function of
bulge size, may be providing evidence that the majority of spiral arms are not
static density waves, but rather wind-up over time. This suggests the "winding
problem" could be solved by the constant reforming of spiral arms, rather than
needing a static density wave. We further observe that galaxies exhibiting
strong bars tend have more loosely wound arms at a given bulge size than
unbarred spirals. This observations suggests that the presence of a bar may
slow the winding speed of spirals, and may also drive other processes (such as
density waves) which generate spiral arms. It is remarkable that after over 170
years of observations of spiral arms in galaxies our understanding of them
remains incomplete.
