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

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

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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 σ confidence at velocities of -500 < vLSR < +500 km s-1. The 84 O VI features have velocity centroids ranging from -372 ≲ LSR ≲ -90 km s-1 to +93 ≲ LSR ≲ +385 km s-1, line widths b ∼ 16-72 km s-1 with an average of 〈b〉 = 40 ± 13 km s-1, and an average O VI column density 〈log N〉 = 13.95 ± 0.34 with a median value of 13.97. Values of b greater than the 17.6 km s-1 thermal width expected for O VI at T ∼ 3 × 105 K indicate that additional nonthermal broadening mechanisms are common. The O VI λ1031.926 absorption is detected at ≥3 σ 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 N(H+) ≳ 1018 cm-2 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-1 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 VI line width. Consideration of the possible sources of collisional ionization favors production of some of the O VI at the boundaries between cool/warm clouds of gas and a highly extended (R ≳ 70 kpc), hot (T > 106 K), low-density (n ≲ 10-4-10-5 cm-3) Galactic corona or Local Group medium. The existence of a hot, highly extended Galactic corona or Local Group medium and the prevalence of high-velocity O VI are consistent with predictions of current galaxy formation scenarios. Distinguishing between the various phenomena producing high-velocity O VI in and near the Galaxy will require continuing studies of the distances, kinematics, elemental abundances, and physical states of the different types of high-velocity O VI found in this study. Descriptions of galaxy evolution will need to account for the highly ionized gas, and future X-ray studies of hot gas in the Local Group will need to consider carefully the relationship of the X-ray absorption/emission to the complex high-velocity absorption observed in O VI.

Subfields: Astronomy and Astrophysics

View Paper: https://www.semanticscholar.org/paper/642b30115bd05e69df35549bfdf6beb9bda3472c

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Citations in the corpus

THE STRUCTURE OF THE MILKY WAY'S HOT GAS HALO (2013, background): “The detection of hot gas at zero redshift by Chandra, XMM-Newton, and FUSE implies there exists a reservoir of gas that potentially contains a significant amount of baryonic mass in the Milky Way.”
The Origin of the Magellanic Stream and Its Leading Arm (2007, result): “This rough estimate of the density of the hot MW halo gas is consistent with most previous estimates (e.g. Stanimirović et al. 2002; Sembach et al. 2003).”
Multiphase High-Velocity Clouds toward HE 0226–4110 and PG 0953+414 (2005, background): “A recent survey of extragalactic sight lines with the Far Ultraviolet Spectroscopic Explorer (FUSE ) satellite (Sembach et al. 2003, hereafter S03) has found that high-velocity O vi absorption is detected at >3 significance 84 times in a survey of 102 sight lines.”
Distribution and Kinematics of O VI in the Galactic Halo (2002, background): “High-velocity O vi absorption with jvj > 100 km s 1 is seen along 60% of the sight lines and is considered by Sembach et al. (2003).”
OBSERVATIONS OF STARBURST GALAXIES WITH FAR-ULTRAVIOLET SPECTROGRAPHIC EXPLORER: GALACTIC FEEDBACK IN THE LOCAL UNIVERSE (2009, methodology): “For O viλ1032 , we use the apparent optical-depth method as described in Sembach et al. (2003).”
CHARACTERIZING THE CIRCUMGALACTIC MEDIUM OF NEARBY GALAXIES WITH HST/COS AND HST/STIS ABSORPTION-LINE SPECTROSCOPY (2012, background): “This is consistent with Lehner & Howk’s recent result, which places many (but not all) Si III HVCs within a few kpc of the galactic disk.”
The Parkes HI survey of the Magellanic System (2004, background): “Sembach et al. (2003) detected O absorption associated with the Magellanic Stream.”
On the Continuing Formation of the Andromeda Galaxy: Detection of H I Clouds in the M31 Halo (2003, background): “Associated ionized gas could perhaps be detected via deep Fabry-Perot imaging (Bland-Hawthorn & Maloney 1999; Tufte et al. 2002) or absorption line spectroscopy of background quasars (Murphy et al. 2000; Tripp et al. 2003; Sembach et al. 2003).”
The gravitational and hydrodynamical interaction between the Large Magellanic Cloud and the Galaxy (2004, background): “…detection of O VI and O VII absorption lines associated with H I structures [Magellanic Stream, some high-velocity clouds, Outer Spiral Arm, Complex A, Complex C, according to Sembach et al. (2003)], its density structure is still uncertain, particularly at large distances from the Galactic Centre.”
THE COS-HALOS SURVEY: RATIONALE, DESIGN, AND A CENSUS OF CIRCUMGALACTIC NEUTRAL HYDROGEN (2013, background): “…“multiphase” picture draws support, and COSHalos is also motivated by, the widespread detection of ionized gas and colder clouds within the hot medium surrounding the Milky Way, as traced by the population of high-velocity clouds (HVCs; Sembach et al. 2003; Fox et al. 2006; Collins et al. 2003).”
A Catalog of Neighboring Galaxies (2004, background): “It is likely that the remaining half of the local b spreads as warm gas in the intergalactic medium (Sembach et al. 2003; Braun & Thilker 2003).”
The Low-z Intergalactic Medium. III. H I and Metal Absorbers at z < 0.4 (2007, background): “These ions have been studied toward many Galactic high-velocity clouds (Sembach & Savage 1994; Indebetouw & Shull 2004; Sembach et al. 2003; Collins et al. 2003, 2007).”
The Search for the Missing Baryons at Low Redshift (2007, background): “Lower estimates to the density are given by Sembach et al. (2003) who argues that some of the OVI absorption detected by FUSE occurs at the boundaries between clouds (T ∼< 10 4 K) and the 106 K medium (with n ∼< 10 −4-10−5 cm−3).”
Simultaneous ram pressure and tidal stripping; how dwarf spheroidals lost their gas (2005, background): “The hot gas is in hydrostatic equilibrium within the halo potential and its temperature is ∼106 K at 50 kpc, consistent with O VI, O VII and X-ray absorption measurements in the Galactic halo (Sembach et al. 2003).”