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| Paper: |
Surveying the Highly Ionized HVCs with FUSE and HST |
| Volume: |
348, Astrophysics in the Far Ultraviolet: Five Years of Discovery with FUSE |
| Page: |
388 |
| Authors: |
Collins, J.A.; Shull, J.M.; Giroux, M.L. |
| Abstract: |
The highly ionized high-velocity clouds detected in O VI present a conundrum since they do not correlate with known Galactic halo HVCs mapped in H I 21-cm emission. Since their distances are unknown, it is not clear whether these objects trace low-column-density Galactic halo clouds or cosmological structures within the Local Group. In this contribution, we present the results of a FUSE and HST survey of twelve sight lines containing highly ionized HVCs. Using photoionization and collisional ionization models of the observed ionization pattern, we investigate the physical conditions of these objects. We measure absorption in these clouds over a range of ionization states, from neutral gas in O I to highly ionized gas in O VI. We find that the majority of the population (9 of the 12 cases studied) is characterized by a photoionized component traced by the low ions C II and Si II. These detections are consistent with the highly ionized HVCs tracing multiphase halo objects similar to the 21-cm HVCs, albeit at low total-hydrogen column density. |
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