Bipolar hypershell models of the extended hot interstellar medium in spiral galaxies

We simulated tile million degree interstellar medium and its soft X-ray ima ges in the disk and halo of spiral galaxies using the bipolar hypershell (B HS) model. In this model dumbbell- or hourglass-shaped expanding shells of several kpc radii are produced by a sudden energy release in tile central r egion. We then applied our model to a mini-sample of starburst galaxies see n under different inclinations, namely the nearly edge-on galaxies NGC 253, NGC 3079 and M 82, the highly inclined galaxies NGC 4258 and NGC 1808 as w ell as the nearly face-on galaxy M 83. For ail galaxies, our results reprod uce the X-ray characteristics observed in the 0.1-2.4 keV ROSAT Energy band : the bipolar hypershell morphology the spectral energy distribution of the diffuse disk and halo emission as well as absorption gaps in the diffuse X -ray emission caused by a shadowing of soft X-rays due to cold intervening gas in the disks of the galaxies. In general, the required total energy for the starburst is estimated to be of the order of 10(55) ergs, correspondin g to the overall kinetic energy generated in similar to 10(4) type-II super nova explosions. The expansion velocity of the shells is estimated to be si milar to 200 km s(-1): which is necessary to heat the gas to similar to0.2 keV (2.3 million K), and the age to be of the order of 3 10(7) years. In th e case of the very nearby, nearly edge-on galaxy NGC 253 all characteristic s of the BHS model can be studied with high spatial resolution. Using the p roperty that the shell morphology is sensitive to the ambient density distr ibution, we propose using soft X-ray data to probe the gas distributions in the disk, halo and intergalactic space in general. The application of our model to images at higher spatial and spectral resolution, as provided by C handra and XMM, will help us to further disentangle the ISM density distrib utions and will lead to a better understanding of the disk halo interface.