Highly Ionized Envelopes of High Velocity Clouds

Abstract

We present recent results on highly ionized gas in Galactic High‐Velocity Clouds (HVCs), originally surveyed in O VI [1], In a new FUSE/HST survey of Si II/III/IV (Shull et al. 2009) toward 37 AGN, we detected Si III (λ 1206.500 Å) absorption with a sky coverage fraction 81±5% (61 HVCs along 30 of 37 high‐latitude sight lines). The Si III (λ 1206.500 Å) line is typically 4–5 times stronger than O VI (λ 1031.926 Å). The mean HVC colum density of perhaps 1019 cm−2 of low‐metallicity (0.1–0.2Z⊙) ionized gas in the low halo. Recent determinations of HVC distances allow us to estimate a total reservoir of ∼108M⊙. Estimates of infall velocities indicate an infall rate of around 1M⊙ yr−1, comparable to the replenishment rate for star formation in the disk. HVCs appear to be sheathed by intermediate‐temperature gas (104.0–104.5 K) detectable in Si III and Si IV, as well as hotter gas seen in O VI and other high ions. To prepare for HST observations of 10 HVC‐selected sight lines with the Cosmic Origins Spectrograph (COS), we compile FUSE/STIS spectra of these ions, plus Fe III, C III, C IV, and S IV. Better constraints on the physical properties of HVC envelopes and careful treatment of HVC kinematics and infall rates should come from high‐quality (S/N ≈30–40) COS data.