GLOBAL-MODELS OF THE INTERSTELLAR-MEDIUM IN DISK GALAXIES

We have computed six two-dimensional simulations with two cospatial, i nteracting fluids that represent the stars and gas in a disk galaxy. T he two fluids interact through star formation, mass loss, stellar wind s, and supernovae, and the gas includes an optically thin cooling func tion. Our previous simulations have been able to produce the multiphas e nature and the overall topology of the cold and warm interstellar me dium (ISM) and here we extend the previous work by allowing for millio n-degree gas and an impulsive form of heating that represents supernov ae. The six simulations differ in the form and rates of energy injecti on; for simulations with heating only from stellar winds and from both supernovae and stellar winds, we have run three simulations with low, moderate and high energy injection rates (with a factor of 4 differen ce between adjacent rates), where the moderate energy injection rate c orresponds to that of the Galaxy. The three values of Lj in our simula tions correspond to 2 (low), 8 (moderate), and 32 (high) x 10(41) ergs s(-1) for a disk of 16 kpc and are equivalent to supernova rates of 0 .0075, 0.03, and 0.12 yr(-1), respectively (assuming E(SN) = 6 x 10(50 ) ergs, and a ratio of energy released from supernovae to that from st ellar winds of 3:1). We use a grid that is 2 kpc across and +/-15 kpc in the vertical direction and impose a constant gravitational potentia l along this direction. Our simulations create a three-phase medium wi th filaments of dense, cold and warm gas surrounding bubbles of hot ga s, which are usually hundreds of parsecs across and can exceed 1 kpc i n size. This filamentary topology is very similar to that inferred for our Galaxy and others, based largely on H I observations. The evoluti on of the cold, dense filaments is dominated by a loss of identity fro m filament-filament collisions, although in higher energy injection ca ses a Rayleigh-Taylor instability can cause a filament to fragment. We calculate central densities, scale heights for the density, and filli ng factors of the three phases of gas, and demonstrate that the vertic al density distribution of each phase is usually best fit with more th an one component. The calculated central densities, scale heights, and filling factors for each gas phase reproduce the observational values made for the Galaxy in the moderate energy injection rate simulations . Also, the computed median pressure and pressure scale height best re produce the Galactic values in the moderate energy injection rate simu lations. Velocity information for the cold (or neutral) gas is analyze d at one or two times in each simulation. The occasional multicomponen t nature of H I emission profiles and the holes of H I in position-vel ocity plots occur in the simulations, although we fail to recreate qua ntitatively the amount and velocities of high-velocity gas that are ob served in the Galaxy. Specifically, only the highest energy injection rate cases have significant amounts of neutral gas (more than 0.25% of the mass of cold gas) at \upsilon\ greater than or equal to 50 km s(- 1), and the maximum velocities are smaller than those observed in H I( greater than or similar to 100 km s(-1)). We attribute these shortcomi ngs to numerical viscosity in the simulations. Further analysis of the cold gas velocities reveals an anticorrelation between the column den sity of cold gas and its velocity dispersion for a galaxy viewed face- on. Also, one calculation of the net mass flux of each gaseous phase a s a function of height demonstrates that most of the hot gas is rising , while cooler gas is falling, which is consistent with the galactic f ountain model. Varying the rate of energy injection has a large effect on the nature of the ISM, especially the extent of the cold gas, so w e can constrain to a relatively narrow range the energy injection rate within the Galaxy. Our results suggest that neither the low-ES simula tions nor the high-E case can reproduce the distribution (i.e., centra l densities, scale heights, and filling factors) of the multiphase ISM in the Galaxy, although the high-E simulations are more effective at reproducing the high-velocity H I observed in the Galaxy.