Star formation in galaxies: the role of spiral arms
  Studying star formation in spiral arms tells us not only about the evolution
of star formation, and molecular clouds, but can also tell us about the nature
of spiral structure in galaxies. I will address both these topics using the
results of recent simulations and observations. Galactic scale simulations are
beginning to examine in detail the evolution of GMCs as they form in spiral
arms, and then disperse by stellar feedback or shear. The overall timescale for
this process appears comparable to the crossing time of the GMCs, a few Myrs
for $10^5$ M$_{\odot}$ clouds, 20 Myr or so for more massive GMCs. Both
simulations and observations show that the massive clouds are found in the
spiral arms, likely as a result of cloud-cloud collisions. Simulations
including stars should also tell us about the stellar age distribution in GMCs,
and across spiral arms. More generally, recent work on spiral galaxies suggests
that the dynamics of gas flows in spiral arms are different in longlived and
transient spiral arms, resulting in different age patterns in the stars. Such
results could be used to help establish the main driver of spiral structure in
the Milky Way (Toomre instabilities, the bar, or nearby companion galaxies) in
conjunction with future surveys.
