Highly Ionized Plasma in the Halo of a Luminous Spiral Galaxy Near z = 0.225

Type Journal Article
Names Anand Narayanan, Blair D. Savage, Bart P. Wakker
Publication The Astrophysical Journal
Volume 712
Issue 2
Pages 1443-1460
Date April 1, 2010
URL http://adsabs.harvard.edu/abs/2010ApJ...712.1443N
Library Catalog NASA ADS
Abstract We present analyses of the physical conditions in the z(\O\,{\sc vi}) = 0.22496 and z(\O\,{\sc vi}) = 0.22638 multiphase absorption systems detected in the ultraviolet Hubble Space Telescope/STIS and FUSE spectra of the quasar H 1821+643 (mV = 14.2, zem = 0.297). Both absorbers are likely associated with the extended halo of a ~2L* B Sbc-Sc galaxy situated at a projected distance of ~116 h -1 71 kpc from the sight line. The z = 0.22496 absorber is detected in C II, C III, C IV, O III, O VI, Si II, Si III, and H I (Ly α-Lyθ) at >3σ significance. The components of Si III and Si II are narrow with implied temperatures of T <~ 3 × 104 K. The low and intermediate ions in this absorber are consistent with an origin in a T ~ 104 K photoionized gas with [Si/H] and [C/H] of ~-0.6 dex. In contrast, the broader O VI absorption is likely produced in collisionally ionized plasma under nonequilibrium conditions. The z(\O\,{\sc vi}) = 0.22638 system has broad Ly α (BLA) and C III absorption offset by v = -53 km s-1 from O VI. The H I and C III line widths for the BLA imply T = 1.1 × 105 K. For non-equilibrium cooling we obtain [C/H] ~-1.5 dex and N(H) = 3.2 × 1018 cm-2 in the BLA. The O VI, offset from the BLA with no detected H I or C III, is likely collisionally ionized at T ~ 3 × 105 K. From the observed multiphase properties and the proximity to a luminous galaxy, we propose that the z = 0.22496 absorber is an extragalactic analog of a highly ionized Galactic HVC, in which the O VI is produced in transition temperature plasma (T ~ 105 K) at the interface layers between the warm (T < 5 × 104 K) HVC gas phase and the hot (T >~ 106 K) coronal halo of the galaxy. The z = 0.22638 O VI-BLA absorber could be tracing a cooling condensing fragment in the nearby galaxy's hot gaseous halo. Based on observations with the NASA/ESA Hubble Space Telescope, obtained at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS 05-26555, and the NASA-CNES/ESA Far Ultraviolet Spectroscopic Explorer mission, operated by the Johns Hopkins University, supported by NASA contract NAS 05-32985.
Tags Galaxies: Halos, intergalactic medium, quasars: absorption lines, quasars: individual: H 1821+643, ultraviolet: general
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