Metallicity Gradients in Disks: Do Galaxies Form Inside-Out?
  We examine radial and vertical metallicity gradients using a suite of disk
galaxy simulations, supplemented with two classic chemical evolution
approaches. We determine the rate of change of gradient and reconcile
differences between extant models and observations within the `inside-out' disk
growth paradigm. A sample of 25 disks is used, consisting of 19 from our RaDES
(Ramses Disk Environment Study) sample, realised with the adaptive mesh
refinement code RAMSES. Four disks are selected from the MUGS (McMaster
Unbiased Galaxy Simulations) sample, generated with the smoothed particle
hydrodynamics (SPH) code GASOLINE, alongside disks from Rahimi et al. (GCD+)
and Kobayashi & Nakasato (GRAPE-SPH). Two chemical evolution models of
inside-out disk growth were employed to contrast the temporal evolution of
their radial gradients with those of the simulations. We find that systematic
differences exist between the predicted evolution of radial abundance gradients
in the RaDES and chemical evolution models, compared with the MUGS sample;
specifically, the MUGS simulations are systematically steeper at high-redshift,
and present much more rapid evolution in their gradients. We find that the
majority of the models predict radial gradients today which are consistent with
those observed in late-type disks, but they evolve to this self-similarity in
different fashions, despite each adhering to classical `inside-out' growth. We
find that radial dependence of the efficiency with which stars form as a
function of time drives the differences seen in the gradients; systematic
differences in the sub-grid physics between the various codes are responsible
for setting these gradients. Recent, albeit limited, data at redshift z=1.5 are
consistent with the steeper gradients seen in our SPH sample, suggesting a
modest revision of the classical chemical evolution models may be required.
