HUBBLE SPACE TELESCOPE SURVEY OF INTERSTELLAR HIGH-VELOCITY Si III

We describe an ultraviolet spectroscopic survey of interstellar high-velocity cloud (HVC) absorption in the strong lambda 1206.500 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 lambda 1031.926, it provides a sensitive probe of ionized gas down to column densities N(Si) (III) approximate to 5 x 1011 cm(-2) at Si III equivalent width 10 m angstrom. We detect high-velocity Si III over 91% +/- 4% of the sky (53 of 58 sight lines); 59% of the HVCs show negative local standard of rest velocities. The mean HVC column density per sight line is < log N(Si) (III)> = 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 halo gas at temperatures 10(4.0-4.5) K indicative of a cooling flow. Photoionization models suggest that typical Si III absorbers with 12.5 < log N(Si III) < 13.5 have total hydrogen column densities N(H) approximate to 10(18)-10(19) cm(-2) for gas of hydrogen density n(H) approximate to 0.1 cm(-3) and 10% solar metallicity. With typical neutral fractions N(HI)/N(H) approximate to 0.01, these HVCs may elude even long-duration 21 cm observations at Arecibo, the EVLA, and other radio facilities. However, if Si III is associated with higher density gas, n(H) >= 1 cm(-3), the corresponding neutral hydrogen could be visible in deep observations. This reservoir of ionized gas may contain 10(8) M(circle dot) and produce a mass infall rate of 1 M(circle dot) yr(-1) to the Galactic disk.