NEW RESULTS CONCERNING THE GALACTIC FOUNTAIN

We have calculated the time-dependent, nonequilibrium thermal and ionization history of hot, optically thin, radiatively cooling gas in a one-dimensional, planar, steady-state flow model of the galactic fountain. Consistent with previous work, we find that the calculation of such flow which neglects photoionization cannot simultaneously reproduce the observed C IV, Si IV, and N V column densities through galactic halo gas. We show, however, that when photoionization is taken into account, a fountain flow can match the observations of both UV absorption and emission lines for a range of ionizing fluxes which depends on the characteristic size of the cooling regions within the flow. We determine what levels of external photoionizing flux, such as that contributed by galactic starlight, supernova remnants, and the metagalactic radiation background, can produce agreement with observational results. Remarkably, we find that the ionizing radiation emitted by the cooling gas itself is sufficient to cause the flow to match the observed column densities and line emission. Such a match implies a fountain flow with initial hydrogen density of approximately 2 x 10(-2) cm-3, outflow velocity of approximately 100 km s-1, and initial size for the cooling regions of approximately 1 kpc or greater.