On the Two-Phase Structure of Protogalactic Clouds

In the gaseous envelope of protogalaxies, thermal instability leads to the formation of a population of cool fragments that are confined by the pressure of a residual hot background medium. In order to remain in a quasi-hydrostatic equilibrium, the residual gas evolves at approximately the virial temperature of the dark matter halo. Its density is determined by the requirements of thermal equilibrium. The hot gas is heated by compression and shock dissipation. The heating is balanced by direct energy loss due to bremsstrahlung emission and by conductive losses into the cool clouds, which are efficient radiators. The cool fragments are photoionized and heated by the extragalactic UV background and nearby massive stars. Several processes interact to determine the size distribution of the cool fragments. The smallest are evaporated due to conductive heat transfer from the hot gas. All fragments are subject to disruption due to hydrodynamic instabilities. The fragments also gain mass as a result of collisions and mergers and of condensation from the hot gas due to conduction. The size distribution of the fragments in turn determines the rate and efficiency of star formation during the early phase of galactic evolution. We have performed one-dimensional hydrodynamic simulations of the evolution of the hot and cool gas. The cool clouds are assumed to follow a power-law size distribution, and fall into the galactic potential, subject to drag from the hot gas. The relative amounts of the hot and cool gas are determined by the processes discussed above, and star formation occurs at a rate sufficient to maintain the cool clouds at 104 K. We present density distributions for the two phases and also for the stars for several cases, parameterized by the circular speeds of the potentials. Under some conditions, primarily low densities of the hot gas, conduction is more efficient than radiative processes at cooling the hot gas, limiting the X-ray radiation from the halo gas.