On the Dynamics and Evolution of Gravitational Instability-Dominated Disks
by Krumholz, Mark R. and Burkert, Andreas
14 pages, 3 figures, emulateapj format, submitted to ApJ
We present a first-principles derivation of the evolution equations describing a thin axisymmetric disk of gas and stars with an arbitrary rotation curve that is kept in a state of marginal gravitational instability and energy equilibrium due to the balance between energy released by accretion and energy lost due to decay of turbulence. Unlike previous analyses of this problem, our results do not depend on an assumed model for the rate of mass and angular momentum transport due to gravitational instability, or on an order-of-magnitude energy equilibrium argument. Instead, we self-consistently determine the position- and time-dependent transport rates from the fluid dynamical equations. We show that there is a steady-state configuration for disks dominated by gravitational instability, and for disks in this state we analytically determine the velocity dispersion, surface density, and rates of mass and angular momentum transport as a function of the gas mass fraction, the rotation curve, and the rate of external accretion onto the disk edge. We show that disks that are initially out of steady state will evolve into it on timescales comparable to the orbital period if the accretion rate is high. Finally, we discuss the implications of these results for the structure of disks in a broad range of environments, including high redshift galaxies, the outer gaseous disks of local galaxies, and accretion disks around protostars.