Hubble Space Telescope Survey of Interstellar High-velocity Si III
Abstract
We describe an ultraviolet spectroscopic survey of interstellar high-velocity cloud (HVC) absorption in the strong λ1206.500 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 λ1031.926, it provides a sensitive probe of ionized gas down to column densities N Si III ≈ 5 × 1011 cm-2 at Si III equivalent width 10 mÅ. 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 langlog N Si III rang = 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 104.0-4.5 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 ≈ 1018-1019 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 108 M sun and produce a mass infall rate of 1 M sun yr-1 to the Galactic disk.
- Publication:
-
The Astrophysical Journal
- Pub Date:
- November 2009
- DOI:
- 10.1088/0004-637X/705/1/962
- arXiv:
- arXiv:0909.4900
- Bibcode:
- 2009ApJ...705..962C
- Keywords:
-
- Galaxy: halo;
- ISM: abundances;
- ISM: clouds;
- ultraviolet: general;
- Astrophysics - Astrophysics of Galaxies
- E-Print:
- 23 pages, 6 figures, accepted for ApJ (Nov 10 issue, Vol 705)