The evolution of turbulent galactic discs: gravitational instability, feedback and accretion

Abstract: We study the driving of turbulence in star-froming disc galaxies of different masses at different epochs, using an analytic "bathtub" model. The disc of gas and stars is assumed to be in marginal Toomre instability. Turbulence is assumed to be sustained via an energy balance between its dissipation and three simultaneous energy sources. These are stellar feedback, inward transport due to disc instability and clumpy accretion via streams. The transport rate is computed with two different formalisms, with similar results. To achieve the energy balance, the disc self-regulates either the mass fraction in clumps or the turbulent viscous torque parameter. In this version of the model, the efficiency by which the stream kinetic energy is converted into turbulence is a free parameter, $\xi_a$. We find that the contributions of the three energy sources are in the same ball park, within a factor of $\sim!2$ in all discs at all times. In haloes that evolve to a mass $\leq 10^{12}\,\Msun$ by $z=0$ ($\leq 10^{11.5}\,\Msun$ at $z!\sim!2$), feedback is the main driver throughout their lifetimes. Above this mass, the main driver is either transport or accretion for very low or very high values of $\xi_a$, respectively. For an assumed $\xi_a(t)$ that declines in time, galaxies in halos with present-day mass $>!10^{12}$ M$_\odot$ make a transition from accretion to transport dominance at intermediate redshifts, $z! \sim!3$, when their mass was $\geq!10^{11.5}\,\Msun$. The predicted relation between star-formation rate and gas velocity dispersion is consistent with observations.