Recent X - ray Observations of Disk Galaxies : Tracing the Dynamic Interstellar

I review recent results from our deep ROSAT and Chandra observations of two galaxies, M101 and NGC 4631, in fields of exceptionally low Galactic extinction. Large amounts of X-ray-emitting gas are detected in these galaxies. Such gas is produced primarily in massive star forming regions and have an average characteristic temperature of a few times 10 6 K. Cooler gas (∼ 10 6 K) is found typically outside galactic disks and may represent outflows from blown-out superbubbles. Propagation of star formation, driven by the expansion of hot gas, appears to be operating in giant HII complexes. A substantial fraction of photo-evaporated gas in such complexes may be mass-loaded into hot gas, which explains their large X-ray luminosities. These processes likely play an important role in determining the global properties of the interstellar medium, especially the disk/halo interaction.