Far Ultraviolet Spectroscopic Explorer Observations of O VI in High-Velocity Clouds
Abstract
We have used moderate-resolution (FWHM ~ 25 km s-1) spectra of active galactic nuclei and QSOs observed by the Far Ultraviolet Spectroscopic Explorer to make the first definitive measurements of absorption by hot gas in high-velocity clouds (HVCs) at large distances from the Galactic plane. Seven of the 11 sight lines studied exhibit high-velocity (VLSR>100 km s-1) O VI λ1031.93 absorption with logN(O VI)~13.79-14.62. High-velocity O VI absorption is detected in the distant gas of H I HVC complex C, the Magellanic Stream, several HVCs believed to be in the Local Group, and the outer Galaxy. The fraction of O VI in HVCs along the seven sight lines containing high-velocity O VI averages ~30%, with a full range of ~10%-70%. The O VI detections imply that hot (T~3×105 K), collisionally ionized gas is an important constituent of the HVCs since O VI is difficult to produce by photoionization unless the path lengths over which the absorption occurs are very large (>100 kpc). The association of O VI with H I HVCs in many cases suggests that the O VI may be produced at interfaces or mixing layers between the H I clouds and hot, low-density gas in the Galactic corona or Local Group. Alternatively, the O VI may originate within cooling regions of hot gas clouds as they are accreted onto the Galaxy.
- Publication:
-
The Astrophysical Journal
- Pub Date:
- July 2000
- DOI:
- 10.1086/312785
- Bibcode:
- 2000ApJ...538L..31S
- Keywords:
-
- Galaxy: Halo;
- Galaxies: Intergalactic Medium;
- ISM: Clouds;
- ISM: Evolution;
- ISM: Kinematics and Dynamics;
- Ultraviolet: ISM