Dawes Review 4: Spiral Structures in Disc Galaxies
  The majority of astrophysics involves the study of spiral galaxies, and stars
and planets within them, but how spiral arms in galaxies form and evolve is
still a fundamental problem. Major progress in this field was made primarily in
the 1960s, and early 1970s, but since then there has been no comprehensive
update on the state of the field. In this review, we discuss the progress in
theory, and in particular numerical calculations, which unlike in the 1960s and
1970s, are now commonplace, as well as recent observational developments. We
set out the current status for different scenarios for spiral arm formation,
the nature of the spiral arms they induce, and the consequences for gas
dynamics and star formation in different types of spiral galaxies. We argue
that, with possible the exception of barred galaxies, spiral arms are
transient, recurrent and initiated by swing amplified instabilities in the
disc. We suppose that unbarred m = 2 spiral patterns are induced by tidal
interactions, and slowly wind up over time. However the mechanism for
generating spiral structure does not appear to have significant consequences
for star formation in galaxies.
