Gas dynamics in whole galaxies: SPH
  I review the progress of SPH calculations for modelling galaxies, and
resolving gas dynamics on GMC scales. SPH calculations first investigated the
response of isothermal gas to a spiral potential, in the absence of self
gravity and magnetic fields. Surprisingly though, even these simple
calculations displayed substructure along the spiral arms. Numerical tests
indicate that this substructure is still present at high resolution (100
million particles, ~10 pc), and is independent of the initial particle
distribution. One interpretation of the formation of substructure is that
smaller clouds can agglomerate into more massive GMCs via dissipative
collisions. More recent calculations have investigated how other processes,
such as the thermodynamics of the ISM, and self gravity affect this simple
picture. Further research has focused on developing models with a more
realistic spiral structure, either by including stars, or incorporating a tidal
interaction.
