Galactic Corona or Local Group Intergalactic Medium?
  Cosmological hydrodynamic simulations predict that the low redshift universe comprises a web of warm-hot intergalactic gas and galaxies, with groups of galaxies and clusters forming at dense knots in these filaments. Our own Galaxy being no exception is also expected to be surrounded by the warm-hot intergalactic medium, filling the Local Group. Some theoretical models also predict the existence of a hot Galactic corona. With X-ray and FUV observations of extragalactic sources, we can probe the warm-hot gas through absorption lines of highly ionized elements. Indeed, Chandra, XMM and FUSE observations have detected z=0 absorption lines toward many sightlines. The debate that has emerged is over the interpretation of these observations: are the z=0 absorption systems from the halo of our Galaxy or from the extended Local Group environment? This has important implications for our understanding of the mass of the Local Group, the physical conditions in the intergalactic medium, the structure of the Galaxy and galaxy formation in general. We will present the current status of the debate and discuss our ongoing observing program aimed at understanding the z=0 absorption systems, with an emphasis on the high quality Chandra spectra of the Mrk 421 and Mrk 279 sightlines. 
Rik J. Williams (Ohio State), Smita Mathur (Ohio State), Fabrizio   Nicastro (CfA)