ar X iv : a st ro - p h / 99 07 42 5 v 1 2 9 Ju l 1 99 9 Simple Models for Turbulent Self - Regulation in Galaxy Disks

Received ; accepted – 2 – ABSTRACT Supernova explosions, and winds and energetic photon fluxes from young star clusters drive outflows and supersonic turbulence in the interstellar medium in galaxy disks, and provide broad spectrum heating which generates a wide range of thermal phases in the gas. Star formation, the source of the energy inputs, is itself regulated by heating and phase exchanges in the gas. However, thermohydrodynamic self-regulation cannot be a strictly local process in the interstellar gas, since galaxy disks also have a nearly universal structure on large scales. We propose that turbulent heating, wave pressure and gas exchanges between different regions of disks play a dominant role in determining the preferred, quasi-equilibrium, self-similar states of gas disks on large-scales. In this paper we present simple families of analytic, thermohydrodynamic models for these global states, which include terms for turbulent pressure and Reynolds stresses. In these model disks star formation rates, phase balances, and hydrodynamic forces are all tightly coupled and balanced. The models have stratified radial flows, with the cold gas slowly flowing inward in the midplane of the disk, and with the warm/hot phases that surround the midplane flowing outward. The models suggest a number of results that are in accord with observation, as well as some novel predictions, including the following. 1) The large-scale gas density and thermal phase distributions in galaxy disks can be explained as the result of turbulent heating and spatial couplings. 2) The turbulent pressures and stresses that drive radial outflows in the warm gas above and below the disk midplane also allow a reduced circular velocity there. This effect was observed by Swaters, Sancisi & van der Hulst in NGC 891, a particularly turbulent edge-on disk. The models predict that the effect should be universal – 3 – in such disks. 3) Since dissipative processes generally depend on the square of the gas density, the heating and cooling balance in these models requires a star formation rate like that of the Schmidt Law. Conversely, they suggest that the Schmidt Law is the natural result of global thermohydrodynamical balance, and may not obtain in disks far from equilibrium.