Supergiant Shells and Hot Gas in NGC 4449
Supergiant shells are the largest interstellar structures in galaxies. They are outlined by long H II filaments enclosing an inner space containing very little warm or cold gas. In the LMC, two supergiant shells have been detected in X-rays indicating the existence of hot gas: LMC2 (Wang & Helfand 1992) and LMC 4 (Bomans et al. 1994). It is not yet clear whether the hot gas in a galactic halo is supplied by such supergiant shells.