Beads on a String and Spurs in Galactic Disks
  The organization of the interstellar medium in disk galaxies obeys the large
scale dynamics set by kpc-size structures. Improving our knowledge of how the
dense, molecular gas is distributed in a disk is an important step in our
understanding of star formation at galactic scale. Using a recently published
simulation of a Milky Way-like galaxy, we explore the formation and dynamical
organization of the star forming gas in a proto-typical disk. Along spiral
arms, we report the formation of regularly spaced clouds, called beads on a
string and spurs. The former form through gravitational instabilities while the
later originate from Kelvin-Helmholtz instabilities. We propose that the
co-existence of both structures in the same galaxy can be explained by a
different role of the disk dynamics, depending on the location within the disk.
In particular, we highlight the impact of the pitch angle of the spiral arm in
the development of either type of structure.
