Highly Ionized High‐Velocity Gas in the Vicinity of the Galaxy

We report the results of an extensive FUSE study of high velocity OVIabsorption along 102 complete sight lines through the Galactic halo. The highvelocity OVI traces a variety of phenomena, including tidal interactions withthe Magellanic Clouds, accretion of gas, outflow from the Galactic disk,warm/hot gas interactions in a highly extended Galactic corona, andintergalactic gas in the Local Group. We identify 85 high velocity OVI featuresat velocities of -500 < v(LSR) < +500 km/s along 59 of the 102 sight lines.Approximately 60% of the sky (and perhaps as much as 85%) is covered by highvelocity H+ associated with the high velocity OVI. Some of the OVI isassociated with known high velocity HI structures (e.g., the Magellanic Stream,Complexes A and C), while some OVI features have no counterpart in HI 21cmemission. The smaller dispersion in the OVI velocities in the GSR and LGSRreference frames compared to the LSR is necessary (but not conclusive) evidencethat some of the clouds are extragalactic. Most of the OVI cannot be producedby photoionization, even if the gas is irradiated by extragalactic backgroundradiation. Collisions in hot gas are the primary OVI ionization mechanism. Wefavor production of some of the OVI at the boundaries between warm clouds and ahighly extended [R > 70 kpc], hot [T > 10^6 K], low-density [n < 10^-4 cm^-3]Galactic corona or Local Group medium. A hot Galactic corona or Local Groupmedium and the prevalence of high velocity OVI are consistent with predictionsof galaxy formation scenarios. Distinguishing between the various phenomenaproducing high velocity OVI will require continuing studies of the distances,kinematics, elemental abundances, and physical states of the different types ofhigh velocity OVI features found in this study. (abbreviated)Comment: 78 pages of text/tables + 31 figures, AASTeX preprint format. All figures are in PNG format due to astro-ph space restrictions. Bound copies of manuscript and two accompanying articles are available upon request. Submitted to ApJ