HUBBLE SPACE TELESCOPE SURVEY OF INTERSTELLAR HIGH-VELOCITY Si III

Journal Article · Sun Nov 01 04:00:00 EST 2009 · Astrophysical Journal

DOI:https://doi.org/10.1088/0004-637X/705/1/962· OSTI ID:21378308

Collins, Joseph A; Shull, J Michael ^[1]

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1. University of Colorado, CASA, Department of Astrophysical and Planetary Sciences, Campus Box 389, Boulder, CO 80309 (United States)

We describe an ultraviolet spectroscopic survey of interstellar high-velocity cloud (HVC) absorption in the strong lambda1206.500 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 lambda1031.926, it provides a sensitive probe of ionized gas down to column densities N {sub SiIII} approx 5 x 10{sup 11} cm{sup -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 {sub 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 halo gas at temperatures 10{sup 4.0-4.5} K indicative of a cooling flow. Photoionization models suggest that typical Si III absorbers with 12.5 < log N {sub SiIII} < 13.5 have total hydrogen column densities N {sub H} approx 10{sup 18}-10{sup 19} cm{sup -2} for gas of hydrogen density n {sub H} approx 0.1 cm{sup -3} and 10% solar metallicity. With typical neutral fractions N {sub HI}/N {sub H} approx 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 {sub H} >= 1 cm{sup -3}, the corresponding neutral hydrogen could be visible in deep observations. This reservoir of ionized gas may contain 10{sup 8} M{sub sun} and produce a mass infall rate of 1 M{sub sun} yr{sup -1} to the Galactic disk.

• OSTI ID: 21378308
• Journal Information: Astrophysical Journal, Journal Name: Astrophysical Journal Journal Issue: 1 Vol. 705; ISSN ASJOAB; ISSN 0004-637X
• Country of Publication: United States
• Language: English

Related Subjects

79 ASTRONOMY AND ASTROPHYSICS
ABSORPTION
ABUNDANCE
DENSITY
ELECTROMAGNETIC RADIATION
ELEMENTS
GALAXIES
HYDROGEN
IONIZATION
MASS
NONMETALS
PHOTOIONIZATION
PHYSICAL PROPERTIES
RADIATIONS
SORPTION
TELESCOPES
ULTRAVIOLET RADIATION
VELOCITY