The innate origin of radial and vertical gradients in a simulated galaxy
  disc
  We examine the origin of radial and vertical gradients in the age/metallicity
of the stellar component of a galaxy disc formed in the APOSTLE cosmological
hydrody- namical simulations. Some of these gradients resemble those in the
Milky Way, where they have sometimes been interpreted as due to internal
evolution, such as scattering off giant molecular clouds, radial migration
driven by spiral patterns, or orbital reso- nances with a bar. Secular
processes play a minor role in the simulated galaxy, which lacks strong spiral
or bar patterns, and where such gradients arise as a result of the gradual
enrichment of a gaseous disc that is born thick but thins as it turns into
stars and settles into centrifugal equilibrium. The settling is controlled by
the feedback of young stars; which links the star formation, enrichment, and
equilibration timescales, inducing radial and vertical gradients in the gaseous
disc and its descendent stars. The kinematics of coeval stars evolve little
after birth and provide a faithful snapshot of the gaseous disc structure at
the time of their formation. In this interpretation, the age-velocity
dispersion relation would reflect the gradual thinning of the disc rather than
the importance of secular orbit scattering; the outward flaring of stars would
result from the gas disc flare rather than from radial migration; and vertical
gradients would arise because the gas disc gradually thinned as it enriched.
Such radial and vertical trends might just reflect the evolving properties of
the parent gaseous disc, and are not necessarily the result of secular
evolutionary processes.
