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

We study the driving of turbulence in star-forming 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 similar to 2 in all discs at all times. In haloes that evolve to a mass <= 10(12) M-circle dot by z = 0 ( <= 10(11.5) M-circle dot at z similar to 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 haloes with present-day mass >10(12)M(circle dot) make a transition from accretion to transport dominance at intermediate redshifts, z - 3, when their mass was >= 10(11.5) M-circle dot. The predicted relation between star formation rate and gas velocity dispersion is consistent with observations.

Automated summary

We studied the driving of turbulence in star-forming disc galaxies of different masses at different epochs using an analytic model. The main drivers of turbulence were found to be stellar feedback, inward transport due to disc instability, and clumpy accretion via streams. These three energy sources contributed roughly equally to the sustained turbulence.