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

We report the results of a FUSE study of high-velocity O VI absorption along complete sight lines through the Galactic halo in directions toward 100 extragalactic objects and two halo stars. The high-velocity O VI traces a variety of phenomena, including tidal interactions with the Magellanic Clouds, accretion of gas, outflowing material from the Galactic disk, warm/hot gas interactions in a highly extended Galactic corona, and intergalactic gas in the Local Group. We identify 84 high-velocity O VI features at ≥3 σ confidence at velocities of -500 < vLSR < +500 km s-1. The 84 O VI features have velocity centroids ranging from -372 ≲ LSR ≲ -90 km s-1 to +93 ≲ LSR ≲ +385 km s-1, line widths b ∼ 16-72 km s-1 with an average of 〈b〉 = 40 ± 13 km s-1, and an average O VI column density 〈log N〉 = 13.95 ± 0.34 with a median value of 13.97. Values of b greater than the 17.6 km s-1 thermal width expected for O VI at T ∼ 3 × 105 K indicate that additional nonthermal broadening mechanisms are common. The O VI λ1031. 926 absorption is detected at ≥3 σ confidence along 59 of the 102 sight lines surveyed. The high-velocity O VI detections indicate that ∼60% of the sky (and perhaps as much as ∼85%, depending on data quality considerations) is covered by high-velocity H+ associated with the O VI. We find that N(H+) ≳ 1018 cm-2 if the high-velocity hot gas has a metallicity similar to that of the Magellanic Stream; this detection rate is considerably higher than that of high-velocity warm H I traced through its 21 cm emission at a comparable column density level. Some of the high-velocity O VI is associated with known H I structures (the Magellanic Stream, Complex A, Complex C, the Outer Spiral Arm, and several discrete H I HVCs). Some of the high-velocity O VI features have no counterpart in H I 21 cm emission, including discrete absorption features and positive velocity absorption wings extending from ∼100 to ∼300 km s-1 that blend with lower velocity absorption produced by the Galactic thick disk/halo. The discrete features may typify clouds located in the Local Group, while the O VI absorption wings may be tidal debris or material expelled from the Galactic disk. Most of the O VI features have velocities incompatible with those of the Galactic halo, even if the halo has decoupled from the underlying Galactic disk. The reduction in the dispersion about the mean of the high-velocity O VI centroids when the velocities are converted from the LSR to the GSR and LGSR reference frames is necessary (but not conclusive) evidence that some of the clouds are located outside the Galaxy. Most of the O VI cannot be produced by photoionization, even if the gas is irradiated by extragalactic ultraviolet background radiation. Several observational quantities indicate that collisions in hot gas are the primary ionization mechanism responsible for the production of the O VI. These include the ratios of O VI column densities to those of other highly ionized species (C IV, N V) and the strong correlation between N(O VI) and O VI line width. Consideration of the possible sources of collisional ionization favors production of some of the O VI at the boundaries between cool/warm clouds of gas and a highly extended (R ≳ 70 kpc), hot (T > 106 K), low-density (n ≲ 10-4-10-5 cm-3) Galactic corona or Local Group medium. The existence of a hot, highly extended Galactic corona or Local Group medium and the prevalence of high-velocity O VI are consistent with predictions of current galaxy formation scenarios. Distinguishing between the various phenomena producing high-velocity O VI in and near the Galaxy will require continuing studies of the distances, kinematics, elemental abundances, and physical states of the different types of high-velocity O VI found in this study. Descriptions of galaxy evolution will need to account for the highly ionized gas, and future X-ray studies of hot gas in the Local Group will need to consider carefully the relationship of the X-ray absorption/emission to the complex high-velocity absorption observed in O VI.