The physics of gas phase metallicity gradients in galaxies
  We present a new model for the evolution of gas phase metallicity gradients
in galaxies from first principles. We show that metallicity gradients depend on
four ratios that collectively describe the metal equilibration timescale,
production, transport, consumption, and loss. Our model finds that most galaxy
metallicity gradients are in equilibrium at all redshifts. When normalized by
metal diffusion, metallicity gradients are governed by the competition between
radial advection, metal production, and accretion of metal-poor gas from the
cosmic web. The model naturally explains the varying gradients measured in
local spirals, local dwarfs, and high-redshift star-forming galaxies. We use
the model to study the cosmic evolution of gradients across redshift, showing
that the gradient in Milky Way-like galaxies has steepened over time, in good
agreement with both observations and simulations. We also predict the evolution
of metallicity gradients with redshift in galaxy samples constructed using both
matched stellar masses and matched abundances. Our model shows that massive
galaxies transition from the advection-dominated to the accretion-dominated
regime from high to low redshifts, which mirrors the transition from
gravity-driven to star formation feedback-driven turbulence. Lastly, we show
that gradients in local ultraluminous infrared galaxies (major mergers) and
inverted gradients seen both in the local and high-redshift galaxies may not be
in equilibrium. In subsequent papers in this series, we show that the model
also explains the observed relationship between galaxy mass and metallicity
gradients, and between metallicity gradients and galaxy kinematics.
