Hubble Space Telescope Survey of Interstellar High-Velocity Si III
Joseph A. Collins (1), Michael Shull (1), and Mark L. Giroux (2) ((1), University of Colorado, (2) East Tennessee State University)

TL;DR
This study uses Hubble's UV spectroscopy to survey high-velocity ionized gas in the Galactic halo via Si III absorption, revealing extensive ionized structures that may significantly contribute to galactic mass inflow.
Contribution
It provides the first large-scale, sensitive survey of high-velocity Si III absorption, highlighting the prevalence and properties of ionized high-velocity clouds in the Galactic halo.
Findings
High-velocity Si III detected in 91% of sky coverage.
Ionized gas may contain up to 10^8 solar masses.
Potentially contributes 1 solar mass per year to Galactic inflow.
Abstract
We describe an ultraviolet spectroscopic survey of interstellar high-velocity cloud (HVC) absorption in the strong 1206.500 Angstrom line of Si III using the Space Telescope Imaging Spectrograph aboard the Hubble Space Telescope. Because the Si III line is 4-5 times stronger than O VI 1031.926, it provides a sensitive probe of ionized gas down to column densities N(Si III) = 5x10^11 cm^-2 at Si III equivalent width 10 mA. We detect high-velocity Si III over (91+/-4)% of the sky (53 of 58 sight lines), and 59% of the HVCs show negative LSR velocities. Per sight line, the mean HVC column density is <log N(SiIII)> = 13.19 +/- 0.45, while the mean for all 90 velocity components is 12.92 +/- 0.46. Lower limits due to Si III line saturation are included in this average, so the actual mean/median values are even higher. The Si III appears to trace an extensive ionized component of Galactic…
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