Young stars as tracers of a barred-spiral Milky Way
  Identifying the structure of our Galaxy has always been fraught with
difficulties, and while modern surveys continue to make progress building a map
of the Milky Way, there is still much to understand. The arm and bar features
are important drivers in shaping the interstellar medium, but their exact
nature and influence still require attention. We present results of smoothed
particle hydrodynamic simulations of gas in the Milky Way including star
formation, stellar feedback, and ISM cooling, when exposed to different arm and
bar features, with the aim of better understanding how well newly formed stars
trace out the underlying structure of the Galaxy. The bar is given a faster
pattern speed than the arms, resulting in a complex, time-dependent morphology
and star formation. Inter-arm branches and spurs are easily influenced by the
bar, especially in the two-armed spiral models where there is a wide region of
resonance overlap in the disc. As the bar over-takes the spiral arms it induces
small boosts in star formation and enhances spiral features, which occur at
regularly spaced beat-like intervals. The locations of star formation events
are similar to those seen in observational data, and do not show a perfect 1:1
correspondence with the underlying spiral potential, though arm tangencies are
generally well traced by young stars. Stellar velocity fields from the newly
formed stars are compared to data from Gaia DR2, showing that the spiral and
bar features can reproduce many of the non-axisymmetric features seen in the
data. A simple analytical model is used to show many of these feature are a
natural response of gas to rigidly rotating spiral and bar potentials.
