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Date and Time of the Query: 2019-04-25 T15:46:57 PDT
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For refcode 2009ApJ...705..962C:
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2009ApJ...705..962C Hubble Space Telescope Survey of Interstellar High-velocity Si III Collins, Joseph A.; Shull, J. Michael; Giroux, Mark L. Abstract. 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_ ~ 5 x 10^11^ 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); 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_ ~ 10^18^-10^19^ cm^-2^ for gas of hydrogen density n_H_ ~ 0.1 cm^-3^ and 10% solar metallicity. With typical neutral fractions N_H I_/N_H_ ~ 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_sun_ and produce a mass infall rate of 1 M_sun_ yr^-1^ to the Galactic disk. Key words: Galaxy: halo, ISM: abundances, ISM: clouds, ultraviolet: general
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