In Highly Ionized High-velocity Clouds

We present a Far-Ultraviolet Spectroscopic Explorer survey of highly ionized high-velocity clouds (HVCs) in 66 extragalactic sight lines with S/N1030 >8. We searched the spectra for high-velocity (100 < |vLSR| < 400km s ) O VI absorption and found a total of 63 absorbers, 16 with 21 cmemitting H I counterparts and 47 “highly ionized” absorbers without 21 cm emission. The highly ionized HVC population is characterized by 〈b(O VI)〉=38±10km s and 〈logNa(O VI)〉=13.83±0.36, with negative-velocity clouds generally found at l < 180 and positive-velocity clouds found at l > 180. 11 of these high-velocity O VI absorbers are positive-velocity wings (broad O VI features extending asymmetrically to velocities of up to 300km s). We find that 81% (30/37) of high-velocity O VI absorbers have clear accompanying C III absorption, and 76% (29/38) have accompanying H I absorption in the Lyman series. We present the first (O VI-selected) sample of C III and H I absorption line HVCs, and find 〈b(C III)〉=30±8km s, logNa(C III) ranges from 12.8 to >14.4, 〈b(H I)〉=22±5km s, and logNa(H I) ranges from 15.1 to >16.9. The lower average width of the high-velocity H I absorbers implies the H I lines arise in a separate, lower temperature phase than the O VI. The ratio Na(C III)/Na(O VI) is generally constant with velocity in high-velocity O VI absorbers, suggesting that C III resides in the same gas as the O VI. Collisional ionization equilibrium models with solar abundances can explain the O VI/C III ratios for temperatures near 1.7×10K; nonequilibrium models with the O VI “frozen-in” at lower temperatures are also possible. Photoionization models are not viable since they under-predict O VI by several orders of magnitude. The presence of associated C III and H I strongly suggests the high-velocity O VI absorbers are not formed in the hotter plasma that gives rise to O VII and O VIII X-ray absorption. We find that the shape of the O VI positive-velocity wing profiles is well reproduced by a radiatively cooling, vertical outflow moving with ballistic dynamics, with T0 = 10 6 K, n0 ≈ 2 × 10 −3 cm, and v0 ≈ 250km s . However, the outflow has to be patchy and out of equilibrium to explain the sky distribution and the simultaneous presence of O VI, C III, and H I. We found that a spherical outflow can produce high-velocity O VI components (as opposed to the wings), showing that the possible range of outflow model results is too broad to conclusively identify whether or not an outflow has left its signature in the data. An alternative model, supported by the similar multi-phase structure and similar O VI properties of highly ionized and 21 cm HVCs, is one where the highly ionized HVCs represent the low N(H I) tail of the HVC population, with the O VI formed at the interfaces around the embedded H I cores. Though we cannot rule out the possibility that some high-velocity O VI absorbers exist in the Local Group or beyond, we favor a Galactic origin. This is based on the recent evidence that both H I HVCs and the million-degree gas detected in X-ray absorption are Galactic phenomena. Since the highly ionized HVCs appear to trace the interface between these two Galactic phases, it follows that highly ionized HVCs are Galactic themselves. However, the non-detection of high-velocity O VI in halo star spectra implies that any Galactic high-velocity O VI exists at z-distances beyond a few kpc. Subject headings: Galaxy: halo – intergalactic medium – ISM: clouds – ultraviolet: ISM – ISM: kinematics and dynamics