2004ApJ...605..216C -
Astrophys. J., 605, 216-229 (2004/April-2)
Highly ionized high-velocity clouds toward PKS 2155-304 and Markarian 509.
COLLINS J.A., SHULL J.M. and GIROUX M.L.
Abstract (from CDS):
To gain insight into four highly ionized high-velocity clouds (HVCs) discovered by Sembach et al., we have analyzed data from the Hubble Space Telescope (HST) and Far Ultraviolet Spectroscopic Explorer (FUSE) for the PKS 2155-304 and Mrk 509 sight lines. We measure strong absorption in O VI and column densities of multiple ionization stages of silicon (Si II, III, and IV) and carbon (C II, III, and IV). We interpret this ionization pattern as a multiphase medium that contains both collisionally ionized and photoionized gas. Toward PKS 2155-304, for HVCs at -140 and -270 km/s, respectively, we measure logN(OVI)=13.80±0.03 and logN(OVI)=13.56±0.06; from Lyman series absorption, we find logN(HI)=16.37+0.22–0.14 and 15.23+0.38–0.22. The presence of high-velocity O VI spread over a broad (100 km/s) profile, together with large amounts of low-ionization species, is difficult to reconcile with the low densities, ne~5x10–6/cm3, in the collisional/photoionization models of Nicastro et al., although the HVCs show a similar relation in N(Si IV)/N(C IV) versus N(C II)/N(C IV) to that of high-z intergalactic clouds. Our results suggest that the high-velocity O VI in these absorbers does not necessarily trace the warm-hot intergalactic medium but instead may trace HVCs with low total hydrogen column density. We propose that the broad high-velocity O VI absorption arises from shock ionization, at bow shock interfaces produced from infalling clumps of gas with velocity shear. The similar ratios of high ions for HVC Complex C and these highly ionized HVCs suggest a common production mechanism in the Galactic halo.
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Journal keyword(s):
Galaxy: Halo - ISM: Abundances - ISM: Clouds - Galaxies: Quasars: Absorption Lines - Ultraviolet: ISM
Simbad objects:
7
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