We simulated the million degree interstellar medium and its soft X-ray images in the disk and halo of spiral galaxies using the bipolar hypershell ( BHS ) model .
In this model dumbbell- or hourglass-shaped expanding shells of several kpc radii are produced by a sudden energy release in the central region .
We then applied our model to a mini-sample of starburst galaxies seen under different inclinations , namely the nearly edge-on galaxies NGC 253 , NGC 3079 and M 82 , the highly inclined galaxy NGC 4258 as well as the nearly face-on galaxies NGC 1808 and M 83 .
For all galaxies , our results reproduce 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 , corresponding to the overall kinetic energy generated in \sim 10 ^ { 4 } type-II supernova explosions .
The expansion velocity of the shells is estimated to be \sim 200 km s ^ { -1 } , which is necessary to heat the gas to \sim 0.2 keV ( 2.3 million K ) , and the age to be of the order of 3 \times 10 ^ { 7 } years .
In the case of the very nearby , nearly edge-on galaxy NGC 253 all characteristics of the BHS model can be studied with high spatial resolution .

Using the property that the shell morphology is sensitive to the ambient density distribution , 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 Chandra and XMM , will help us to further disentangle the ISM density distributions and will lead to a better understanding of the disk halo interface .

Keywords galaxies : general – galaxies : individual : NGC 253 , NGC 1808 , NGC 3079 , NGC 4258 , M 82 ( NGC 3034 ) , the Milky Way – galaxies : evolution – galaxies : ISM – galaxies : starburst – galaxies : spiral – galaxies : structure – galaxies : X-rays – X-rays : general – X-rays : galaxies