Distribution and Kinematics of O VI in the Galactic Halo

By B. Savage, K. Sembach, B. Wakker, P. Richter, M. Meade, E. Jenkins, J. Michael Shull, H. Moos, G. Sonneborn. 2002

171 citations recorded by the source corpus.

Far-Ultraviolet Spectroscopic Explorer (FUSE) spectra of 100 extragalactic objects and two distant halo stars are analyzed to obtain measures of O VI λλ1031.93, 1037.62 absorption along paths through the Milky Way thick disk/halo. Strong O VI absorption over the velocity range from -100 to 100 km s-1 reveals a widespread but highly irregular distribution of O VI, implying the existence of substantial amounts of hot gas with T ∼ 3 × 105 K in the Milky Way thick disk/halo. The integrated column density, log [N(O VI) cm-2], ranges from 13.85 to 14.78 with an average value of 14.38 and a standard deviation of 0.18. Large irregularities in the gas distribution are found to be similar over angular scales extending from <1° to 180°, implying a considerable amount of small- and large-scale structure in the absorbing gas. The overall distribution of O VI is not well described by a symmetrical plane-parallel layer of patchy O VI absorption. The simplest departure from such a model that provides a reasonable fit to the observations is a plane-parallel patchy absorbing layer with an average O VI midplane density of n0(O VI) = 1.7 × 10-8 cm-3, a scale height of ∼2.3 kpc, and a ∼0.25 dex excess of O VI in the northern Galactic polar region. The distribution of O VI over the sky is poorly correlated with other tracers of gas in the halo, including low- and intermediate-velocity H I, Hα emission from the warm ionized gas at ∼104 K, and hot X-ray-emitting gas at ∼106 K. The O VI has an average velocity dispersion, b ≈ 60 km s-1, and standard deviation of 15 km s-1. Thermal broadening alone cannot explain the large observed profile widths. The average O VI absorption velocities toward high-latitude objects (|b| > 45°) range from -46 to 82 km s-1, with a high-latitude sample average of 0 km s-1 and a standard deviation of 21 km s-1. High positive velocity O VI absorbing wings extending from ∼100 to ∼250 km s-1 observed along 21 lines of sight may be tracing the flow of O VI into the halo. A combination of models involving the radiative cooling of hot fountain gas, the cooling of supernova bubbles in the halo, and the turbulent mixing of warm and hot halo gases is required to explain the presence of O VI and other highly ionized atoms found in the halo. The preferential venting of hot gas from local bubbles and superbubbles into the northern Galactic polar region may explain the enhancement of O VI in the north. If a fountain flow dominates, a mass flow rate of approximately 1.4 M⊙ yr-1 of cooling hot gas to each side of the Galactic plane with an average density of 10-3 cm-3 is required to explain the average value of log [N(O VI) sin |b|] observed in the southern Galactic hemisphere. Such a flow rate is comparable to that estimated for the Galactic intermediate-velocity clouds.

Subfields: Astronomy and Astrophysics, Atmospheric Science

View Paper: https://www.semanticscholar.org/paper/cb8cd0d88bc56b040f876937a7e9be5cbe354611

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In Astronomy and Astrophysics

1. J. Xavier Prochaska
2. E. Jenkins
3. B. Savage
4. D. Cox
5. K. Sembach
6. T. Tripp
7. P. Richter
8. J. Michael Shull

In Atmospheric Science

1. E. Jenkins

Citations in the corpus

• Highly Ionized High-Velocity Gas in the Vicinity of the Galaxy (2002, methodology): “The other two papers include a catalog with basic measurements and illustrations of all of the O vi profiles observed in the survey (Wakker et al. 2003) and a companion study of the O vi absorption associated with the thick disk/halo of the Milky Way (Savage et al. 2003).”
• PROBING THE INTERGALACTIC MEDIUM/GALAXY CONNECTION. V. ON THE ORIGIN OF Lyα AND O vi ABSORPTION AT z < 0.2 (2011, background): “Lastly, studies of O VI absorption toward extragalactic sources reveal a high covering fraction from an inferred halo of collisionally ionized gas at T ≈ 105K (Savage & de Boer 1979; Savage et al. 2003; Sembach et al. 2006).”
• Multiphase High-Velocity Clouds toward HE 0226–4110 and PG 0953+414 (2005, background): “Such a trend may be related to the general enhancement of O vi in the northern hemisphere of the Galactic halo (Savage et al. 2003) and to the opening of the Local Bubble in this direction (Welsh et al. 1999).”
• The warm ionized medium in spiral galaxies (2009, background): “Observations of O+5 and detections of other ions in high states of ionization (Savage et al., 2000; Sembach and Savage, 1992; Sembach et al., 2003) show that interfaces between hot (∼ 106 K) gas and cooler (.”
• THE THREE-PHASE INTERSTELLAR MEDIUM REVISITED (2005, background): “Attempts to measure the column density perpendicular to the plane have been made by Savage et al. (2003), who find the OVI to be very patchy with values ranging from 0.6 × 1014 to 3 × 1014 cm−2; the larger values are prevalent in the north.”
• A High-Resolution Survey of Low-Redshift QSO Absorption Lines: Statistics and Physical Conditions of O VI Absorbers (2007, background): “Many of the QSOs in our sample have also been observed with FUSE for various purposes (e.g., Savage et al. 2003; Sembach et al. 2003).”
• On the Continuing Formation of the Andromeda Galaxy: Detection of H I Clouds in the M31 Halo (2003, background): “The primary source of such coronal gas may actually be the action of a large-scale “galactic fountain” (Shapiro & Field 1976; Bregman 1980; Savage et al. 2003) in the recent or distant past.”