A Unified Model for Galactic Discs: Star Formation, Turbulence Driving,
  and Mass Transport
  We introduce a new model for the structure and evolution of the gas in
galactic discs. In the model the gas is in vertical pressure and energy
balance. Star formation feedback injects energy and momentum, and
non-axisymmetric torques prevent the gas from becoming more than marginally
gravitationally unstable. From these assumptions we derive the relationship
between galaxies' bulk properties (gas surface density, stellar content, and
rotation curve) and their star formation rates, gas velocity dispersions, and
rates of radial inflow. We show that the turbulence in discs can be powered
primarily by star formation feedback, radial transport, or a combination of the
two. In contrast to models that omit either radial transport or star formation
feedback, the predictions of this model yield excellent agreement with a wide
range of observations, including the star formation law measured in both
spatially resolved and unresolved data, the correlation between galaxies' star
formation rates and velocity dispersions, and observed rates of radial inflow.
The agreement holds across a wide range of galaxy mass and type, from local
dwarfs to extreme starbursts to high-redshifts discs. We apply the model to
galaxies on the star-forming main sequence, and show that it predicts a
transition from mostly gravity-driven turbulence at high redshift to star
formation-driven turbulence at low redshift. This transition, and the changes
in mass transport rates that it produces, naturally explain why galaxy bulges
tend to form at high redshift and discs at lower redshift, and why galaxies
tend to quench inside-out.
