Highly Ionized High‐Velocity Gas in the Vicinity of the Galaxy

Abstract

We report the results of a FUSE study of high-velocity O vi absorption along complete sight lines through the Galactic halo in directions toward 100 extragalactic objects and two halo stars. The high-velocity O vi traces a variety of phenomena, including tidal interactions with the Magellanic Clouds, accretion of gas, outflowing material from the Galactic disk, warm/hot gas interactions in a highly extended Galactic corona, and intergalactic gas in the Local Group. We identify 84 high-velocity O vi features at !3 confidence at velocities of À500 < v LSR < þ500 km s À1 . The 84 O vi features have velocity centroids ranging from À372d" v v LSR d À 90 km s À1 to þ93d" v v LSR d þ 385 km s À1 , line widths b $ 16 72 km s À1 with an average of hbi ¼ 40 AE 13 km s À1 , and an average O vi column density hlog Ni ¼ 13:95 AE 0:34 with a median value of 13.97. Values of b greater than the 17.6 km s À1 thermal width expected for O vi at T $ 3 Â 10 5 K indicate that additional nonthermal broadening mechanisms are common. The O vi 1031.926 absorption is detected at !3 confidence along 59 of the 102 sight lines surveyed. The high-velocity O vi detections indicate that $60% of the sky (and perhaps as much as $85%, depending on data quality considerations) is covered by high-velocity H + associated with the O vi. We find that NðH þ Þe10 18 cm À2 if the high-velocity hot gas has a metallicity similar to that of the Magellanic Stream; this detection rate is considerably higher than that of high-velocity warm H i traced through its 21 cm emission at a comparable column density level. Some of the high-velocity O vi is associated with known H i structures (the Magellanic Stream, Complex A, Complex C, the Outer Spiral Arm, and several discrete H i HVCs). Some of the high-velocity O vi features have no counterpart in H i 21 cm emission, including discrete absorption features and positive velocity absorption wings extending from $100 to $300 km s À1 that blend with lower velocity absorption produced by the Galactic thick disk/halo. The discrete features may typify clouds located in the Local Group, while the O vi absorption wings may be tidal debris or material expelled from the Galactic disk. Most of the O vi features have velocities incompatible with those of the Galactic halo, even if the halo has decoupled from the underlying Galactic disk. The reduction in the dispersion about the mean of the high-velocity O vi centroids when the velocities are converted from the LSR to the GSR and LGSR reference frames is necessary (but not conclusive) evidence that some of the clouds are located outside the Galaxy. Most of the O vi cannot be produced by photoionization, even if the gas is irradiated by extragalactic ultraviolet background radiation. Several observational quantities indicate that collisions in hot gas are the primary ionization mechanism responsible for the production of the O vi. These include the ratios of O vi column densities to those of other highly ionized species (C iv, N v) and the strong correlation between N(O vi) and O ...

The Rotation of the Hot Gas Around the Milky Way

“…The O VI lines toward background objects reveal Galactic and highvelocity absorbers (Nicastro et al 2003;Savage et al 2003;Sembach et al 2003). The Galactic absorbers (also seen toward stars) are consistent with an exponential disk of scale height 1-4 kpc Bowen et al 2008), whereas the high-velocity absorbers may come from the halo or outside the Galaxy.…”

Absorption-Line Systems and Galaxies in Front of the Second-brightest Quasar, PHL 1811

“…If we assume that these lines are near the linear portion of the curve of growth and adopt the combined f-value 0.0554 (D. C. Morton 2000, private communication 8 ), we arrive at log NðO iÞ ¼ 14:07, with an error in the linear value that scales in proportion to the listed uncertainty in equivalent width. Unfortunately, we cannot do the same for the N i 953.65 feature, which appears at 1030.83 Å , because there is a good chance that this feature is actually Galactic O vi at v ¼ À320 km s À1 (Sembach et al 2003). Other N i lines well removed from Galactic O vi are weaker and should be below our detection limit.…”

High‐resolution absorption spectroscopy of the circumgalactic medium of the Milky Way

“…Based on UV observations of five-times ionized oxygen (O VI) with the Far Ultraviolet Spectroscopic Explorer (FUSE), Sembach et al (2003) have shown that the velocity distribution of the highly-ionized circumgalactic gas component of the Milky Way extends to substantially higher radial velocities, with many O VI absorbers located in the velocity range |v LSR | = 200 − 400 km s −1 . The (on average) higher radial velocities of circumgalactic O VI compared to Ca II and Si II most likely indicate that the highlyionized gas phase in the CGM is spatially more extended than the neutral phase.…”

“…The resonance lines of atomic and molecular species located in the UV serve as tracer of cold, warm, and hot gas in a temperature range of T ≈ 30 − 300, 000 K. For collisionally ionized gas, O VI, with an ionization potential of creation of E = 114 eV and its two strong transitions at 1031.9 and 1037.6Å, is the best ion in the UV to study the warm-hot CGM around the Milky Way at T ≈ 300, 000 K. UV observations carried out with FUSE have demonstrated that high-velocity O VI absorption is present in many directions of the sky with a large covering fractions of f c ≈ 0.6 for column densities log N (O VI) ≥ 13.4 (Sembach et al 2003). The O VI absorption possibly indicates transitiontemperature gas in the interfaces between cooler gas clouds and a surrounding hot coronal gas (see also Fox 2011).…”

“…e-mail: prichter@astro.physik.uni-potsdam.de presence of a large number of diagnostic lines of low, intermediate, and high metal ions in the ultraviolet (UV) and at optical wavelengths. High-resolution absorption spectra of quasi-stellar objects (QSOs) and other extragalactic point sources therefore provide a wealth of information on the physical and chemical properties of the different gas phases in the CGM of the Milky Way and other galaxies (e.g., Wakker et al 1999;Sembach et al 2003;Richter et al 2001aRichter et al ,2001bRichter et al ,2009Richter et al ,2011Tripp et al 2003;Fox et al 2005Collins et al 2009;Shull et al 2009;Prochaska et al 2011;Tumlinson et al 2011a;Lehner et al 2013;Keeney et al 2013;Stocke et al 2013).…”