Dynamics of stars around spiral arms in an N-body/SPH simulated
  barred-spiral galaxy
  We run N-body smoothed particle hydrodynamics (SPH) simulations of a Milky
Way sized galaxy. The code takes into account hydrodynamics, self-gravity, star
formation, supernova and stellar wind feedback, radiative cooling and metal
enrichment. The simulated galaxy is a barred-spiral galaxy consisting of a
stellar and gas disc, enveloped in a static dark matter halo. Similar to what
is found in our pure N-body simulation of a non-barred galaxy in Grand et. al.
(2012), we find that the spiral arms are transient features whose pattern
speeds decrease with radius, in such a way that the pattern speed is similar to
the rotation of star particles. Compared to the non-barred case, we find that
the spiral arm pattern speed is slightly faster than the rotation speed of star
particles: the bar appears to boost the pattern speed ahead of the rotational
velocity. We trace particle motion around the spiral arms at different radii,
and demonstrate that there are star particles that are drawn towards and join
the arm from behind (in front of) the arm and migrate toward the outer (inner)
regions of the disc until the arm disappears as a result of their transient
nature. We see this migration over the entire radial range analysed, which is a
consequence of the spiral arm rotating at similar speeds to star particles at
all radii, which is inconsistent with the prediction of classical density wave
theory. The bar does not prevent this systematic radial migration, which is
shown to largely preserve circular orbits. We also demonstrate that there is no
significant offset of different star forming tracers across the spiral arm,
which is also inconsistent with the prediction of classical density wave
theory.
