Characteristics of Spiral Arms in Late-type Galaxies
  We have measured the positions of large numbers of H II regions in four
nearly face-on, late-type, spiral galaxies: NGC 628 (M 74), NGC 1232, NGC 3184
and NGC 5194 (M 51). Fitting log-periodic spiral models to segments of each arm
yields local estimates of spiral pitch angle and arm width. While pitch angles
vary considerably along individual arms, among arms within a galaxy, and among
galaxies, we find no systematic trend with galactocentric distance. We estimate
the widths of the arm segments from the scatter in the distances of the H II
regions from the spiral model. All major arms in these galaxies show spiral arm
width increasing with distance from the galactic center, similar to the trend
seen in the Milky Way. However, in the outer-most parts of the galaxies, where
massive star formation declines, some arms reverse this trend and narrow. We
find that spiral arms often appear to be composed of segments of ~5 kpc length,
which join to form kinks and abrupt changes in pitch angle and arm width; these
characteristics are consistent with properties seen in the large N-body
simulations of D'Onghia, Vogelsberger and Hernquist (2013) and others.
