GLOBAL-MODELS OF THE GALACTIC INTERSTELLAR-MEDIUM - COMPARISON TO X-RAY AND H-I OBSERVATIONS

In a previous paper, we calculated numerical hydrodynamic models of th e interstellar medium in the Galaxy, which suggested that hot gas (T g reater than or equal to 3 x 10(5) K) has a filling factor near 50% in the midplane, and that it is separated by cooler material (see recent paper by Rosen & Bregman). Here we extend the work to examine the X-ra y emission characteristics of the best model and calculate a variety o f observable measures for comparison with the observed soft X-ray back ground. For five observer locations in the disk (three hot bubbles, a cooler bubble, and a neutral gas region), we calculate the X-ray inten sities, spectra, and hardness ratios in 0.16-0.28 keV and 0.53-0.87 ke V bands as the Galactic latitude and cool gas column are varied. We co mpare these to strip scans of observational data, N-HI from Dickey & L ockman, and C band and M(1) band X-ray data from the Wisconsin surveys (by McCammon et al.). The calculated neutral hydrogen column density distribution has a broad range and a median value that is typically 2. 5-6 times smaller than the mean value (seen from a location in the dis k or perpendicular to the disk). This difference between the mean and median N-HI offers a natural explanation for the observed difference ( a factor of 3) between the average N-HI at the solar circle and the lo cal value deduced from high-latitude observations. The observed distri bution of N-HI is similar to that seen from one of the simulated bubbl es, with the important exception that the minimum hydrogen column in t he model is too low. The low minimum hydrogen column is a common resul t of the models and indicates that neutral gas is too easily compresse d into small structures. The models suggest that the X-ray emission in the 0.16-0.28 keV band is dominated by hot gas within 0.1-0.5 kpc, wh ile in the 0.53-0.87 keV band nearly all emission originates within 2 kpc of the observer and often much closer. The model X-ray emission ge nerally hardens toward the plane, for observers in bubbles. Also, ther e are clear examples of anticorrelations between H I and X-ray emissio n as well as correlations between H I and X-rays, which are caused by an increased emission measure as a shock enters a cool gas region. Sta tistically, anticorrelations are slightly more common than correlation s. X-ray spectra are calculated from the models, and these reveal that for observations to have strong diagnostic power in probing the hot I SM, a spectral resolution of E/Delta E > 30 is required. The X-ray obs ervations reveal shortcomings in the models in that the angular distri butions of the model X-ray intensities and the hardness ratios vary fa r more than the observations in either energy band. Also, the model X- ray intensities are typically fainter than observed. Some of these sho rtcomings might have been alleviated had we chosen a more uniform and denser hot bubble, but we suggest that this problem, as well as the lo w H I values, might be solved by including magnetic fields in future s imulations.