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

Abstract

We report the results of an extensive FUSE study of high velocity OVI\nabsorption along 102 complete sight lines through the Galactic halo. The high\nvelocity OVI traces a variety of phenomena, including tidal interactions with\nthe Magellanic Clouds, accretion of gas, outflow from the Galactic disk,\nwarm/hot gas interactions in a highly extended Galactic corona, and\nintergalactic gas in the Local Group. We identify 85 high velocity OVI features\nat velocities of -500 < v(LSR) < +500 km/s along 59 of the 102 sight lines.\nApproximately 60% of the sky (and perhaps as much as 85%) is covered by high\nvelocity H+ associated with the high velocity OVI. Some of the OVI is\nassociated with known high velocity HI structures (e.g., the Magellanic Stream,\nComplexes A and C), while some OVI features have no counterpart in HI 21cm\nemission. The smaller dispersion in the OVI velocities in the GSR and LGSR\nreference frames compared to the LSR is necessary (but not conclusive) evidence\nthat some of the clouds are extragalactic. Most of the OVI cannot be produced\nby photoionization, even if the gas is irradiated by extragalactic background\nradiation. Collisions in hot gas are the primary OVI ionization mechanism. We\nfavor production of some of the OVI at the boundaries between warm clouds and a\nhighly extended [R > 70 kpc], hot [T > 10^6 K], low-density [n < 10^-4 cm^-3]\nGalactic corona or Local Group medium. A hot Galactic corona or Local Group\nmedium and the prevalence of high velocity OVI are consistent with predictions\nof galaxy formation scenarios. Distinguishing between the various phenomena\nproducing high velocity OVI will require continuing studies of the distances,\nkinematics, elemental abundances, and physical states of the different types of\nhigh velocity OVI features found in this study. (abbreviated)\n