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HUBBLE SPACE TELESCOPESURVEY OF INTERSTELLAR HIGH-VELOCITY Si III
- Source :
- The Astrophysical Journal. 705:962-977
- Publication Year :
- 2009
- Publisher :
- American Astronomical Society, 2009.
-
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 = 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 K to 10^4.5 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 to 10^19 cm^-2 for gas of hydrogen density n(H) = 0.1 cm^(-3) and 10% solar metallicity. With typical neutral fractions N(HI)/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 to the Galactic disk.<br />Comment: 23 pages, 6 figures, accepted for ApJ (Nov 10 issue, Vol 705)
- Subjects :
- Physics
FOS: Physical sciences
Astronomy and Astrophysics
Astrophysics::Cosmology and Extragalactic Astrophysics
Astrophysics
Photoionization
Cooling flow
Astrophysics - Astrophysics of Galaxies
Galactic halo
Space and Planetary Science
Astrophysics of Galaxies (astro-ph.GA)
Ionization
Disc
Equivalent width
Astrophysics::Galaxy Astrophysics
Space Telescope Imaging Spectrograph
Line (formation)
Subjects
Details
- ISSN :
- 15384357 and 0004637X
- Volume :
- 705
- Database :
- OpenAIRE
- Journal :
- The Astrophysical Journal
- Accession number :
- edsair.doi.dedup.....7c51dfc2fb693f0c90e147d960a15f22