▪ Abstract  High-velocity clouds (HVCs) consist of neutral hydrogen (HI) at velocities incompatible with a simple model of differential galactic rotation; in practice one uses [Formula: see text] vLSR[Formula: see text] [Formula: see text]90 km/s to define HVCs. This review describes the main features of the sky and velocity distributions, as well as the available information on cloud properties, small-scale structure, velocity structure, and observations other than in 21-cm emission. We show that HVCs contain heavy elements and that the more prominent ones are more than 2 kpc from the Galactic plane. We evaluate the hypotheses proposed for their origin and reject those that account for only one or a few HVCs. At least three different hypotheses are needed: one for the Magellanic Stream and possibly related clouds, one for the Outer Arm Extension, and one (or more) for the other HVCs. We discuss the evidence for the accretion and the fountain model but cannot rule out either one.

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We present Far Ultraviolet Spectroscopic Explorer (FUSE) observations of the O VI λλ1031.926, 1037.617 absorption lines associated with gas in and near the Milky Way, as detected in the spectra of a sample of 100 extragalactic targets and 2 distant halo stars. We combine data from several FUSE Science Team programs with guest observer data that were public before 2002 May 1. The sightlines cover most of the sky above galactic latitude |b|>25 • -at lower latitude the ultraviolet extinction is usually too large for extragalactic observations. We describe the details of the calibration, alignment in velocity, continuum fitting, and manner in which several contaminants were removed -Galactic H 2 , absorption intrinsic to the background target and intergalactic Lyβ lines. This decontamination was done very carefully, and in several sightlines very subtle problems were found. We searched for O VI absorption in the velocity range −1200 to 1200 km s −1 . With a few exceptions, we only find O VI in the velocity range −400 to 400 km s −1 ; the exceptions may be intergalactic O VI. In this paper we analyze the O VI associated with the Milky Way (and possibly with the Local Group). We -2discuss the separation of the observed O VI absorption into components associated with the Milky Way halo and componet at high-velocity, which are probably located in the neighborhood of the Milky Way. We describe the measurements of equivalent width and column density, and we analyze the different contributions to the errors. We conclude that low-velocity Galactic O VI absorption occurs along all sightlinesthe few non-detections only occur in noisy spectra. We further show that high-velocity O VI is very common, having equivalent width >65 mÅ in 50% of the sightlines and equivalent width >30 mÅ in 70% of the high-quality sightlines. The high-velocity O VI absorption has velocities relative to the LSR of ±(100-330) km s −1 ; there is no correlation between velocity and absorption strength. We discuss the possibilities for studying O VI absorption associated with Local Group galaxies, and conclude that O VI is probably detected in M 31 and M 33. We limit the extent of an O VI halo around M 33 to be <100 kpc (at a 3σ detection limit of log N (O VI)∼14.0). Using the measured column densities, we present 50 km s −1 wide O VI channel maps. These show evidence for the imprint of Galactic rotation. They also highlight two known H I high-velocity clouds (complex C and the Magellanic Stream). The channel maps further show that O VI at velocities <−200 km s −1 occurs along all sightlines in the region l=20 • -150 • , b<−30 • , while O VI at velocities >200 km s −1 occurs along all sightlines in the region l=180 • -300 • , b>20 • .Subject headings: ISM: structure, Galaxy: halo, ultraviolet: ISM lists the alternative "Mrk", "PG", "Ton", "UGC" or Zwicky name for the object, if it exists. We established the following order of preference for the prefix indicating the source catalogue: "NGC", "3C", "Mrk", "PG", "Ton". This led us to prefer Mrk 116 over I Zw ...

Aims. Doubly ionized silicon (Si iii) is a powerful tracer of diffuse ionized gas inside and outside of galaxies. It can be observed in the local Universe in ultraviolet (UV) absorption against bright extragalactic background sources. We here present an extensive study of intervening Si iii-selected absorbers and study the properties of the warm circumgalactic medium (CGM) around low-redshift (z ≤ 0.1) galaxies. Methods. We analyzed the UV absorption spectra of 303 extragalactic background sources, as obtained with the Cosmic Origins Spectrograph (COS) on-board the Hubble Space Telescope (HST). We developed a geometrical model for the absorption-cross section of the CGM around the local galaxy population and compared the observed Si iii absorption statistics with predictions provided by the model. We also compared redshifts and positions of the absorbers with those of ∼64 000 galaxies using archival galaxy-survey data to investigate the relation between intervening Si iii absorbers and the CGM. Results. Along a total redshift path of ∆z ≈ 24, we identify 69 intervening Si iii systems that all show associated absorption from other low and high ions (e.g., H i, Si ii, Si iv, C ii, C iv). We derive a bias-corrected number density of dN/dz(Si iii) = 2.5 ± 0.4 for absorbers with column densities log N(Si iii) > 12.2, which is ∼3 times the number density of strong Mg ii systems at z = 0. This number density matches the expected cross section of a Si iii absorbing CGM around the local galaxy population with a mean covering fraction of f c = 0.69. For the majority (∼60 percent) of the absorbers, we identify possible host galaxies within 300 km s −1 of the absorbers and derive impact parameters ρ < 200 kpc, demonstrating that the spatial distributions of Si iii absorbers and galaxies are highly correlated.Conclusions. Our study indicates that the majority of Si iii-selected absorbers in our sample trace the CGM of nearby galaxies within their virial radii at a typical covering fraction of ∼70 percent. We estimate that diffuse gas in the CGM around galaxies, as traced by Si iii, contains substantially more (more than twice as much) baryonic mass than their neutral interstellar medium.

International audienceWe present high-resolution (R= 400 000) observations of interstellar Ca ii and Na i absorption lines towards seven stars in the direction of the southern opening of the recently identified Local Interstellar Chimney. These lines of sight probe the lower Galactic halo (0.3 ≲|z|≲ 2.5 kpc), without the complication of sampling dense foreground interstellar material. In addition to components with velocities expected from Galactic rotation, these stars also exhibit components with negative local standard of rest velocities, which are contrary to the sense of Galactic rotation for the sightlines observed. After a discussion of possible origins for these peculiar velocities, we conclude that at least some of them result from gas falling towards the Galactic plane from distances of |z|≳ 300 pc. The narrow linewidths are generally inconsistent with temperatures as high as the ∼6000 K generally assumed for the so-called Lockman layer. Rather, the picture that emerges is one of a scattered, generally infalling, population of high-|z| diffuse clouds, seemingly not very different from those encountered in the local interstellar medium. Overall, we argue that our results are most consistent with a ‘Galactic fountain’ model