B. P. Wakker

HIGHLY IONIZED HIGH-VELOCITY GAS IN THE VICINITY OF THE GALAXY

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 -500 106 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.

Distances and Metallicities of High- and Intermediate-Velocity Clouds

A table is presented that summarizes published absorption line measurements for the high- and intermediate-velocity clouds (HVCs and IVCs). New values are derived for N(H I) in the direction of observed probes, in order to arrive at reliable abundances and abundance limits (the H I data are described in Paper II). Distances to stellar probes are revisited and calculated consistently, in order to derive distance brackets or limits for many of the clouds, taking care to properly interpret nondetections. The main conclusions are the following. (1) Absolute abundances have been measured using lines of S II, N I, and O I, with the following resulting values: ~0.1 solar for one HVC (complex C), ~0.3 solar for the Magellanic Stream, ~0.5 solar for a southern IVC, and ~solar for two northern IVCs (the IV Arch and LLIV Arch). Finally, approximate values in the range 0.5-2 solar are found for three more IVCs. (2) Depletion patterns in IVCs are like those in warm disk or halo gas. (3) Most distance limits are based on strong UV lines of C II, Si II, and Mg II, a few on Ca II. Distance limits for major HVCs are greater than 5 kpc, while distance brackets for several IVCs are in the range 0.5-2 kpc. (4) Mass limits for major IVCs are 0.5-8 × 105 M☉, but for major HVCs they are more than 106 M☉. (5) The Ca II/H I ratio varies by up to a factor 2-5 within a single cloud, somewhat more between clouds. (6) The Na I/H I ratio varies by a factor of more than 10 within a cloud, and even more between clouds. Thus, Ca II can be useful for determining both lower and upper distance limits, but Na I only yields upper limits.

Distances to Galactic High-Velocity Clouds. I. Cohen Stream, Complex GCP, Cloud g1*

The high- and intermediate-velocity interstellar clouds (HVCs/IVCs) are tracers of energetic processes in and around the Milky Way. Clouds with near-solar metallicity about 1 kpc above the disk trace the circulation of material between disk and halo (the Galactic fountain). The Magellanic Stream consists of gas tidally extracted from the SMC, tracing the dark matter potential of the Milky Way. Several other HVCs have low metallicity and appear to trace the continuing accretion of infalling intergalactic gas. These assertions are supported by the metallicities (0.1 to 1 solar) measured for about 10 clouds in the past decade. Direct measurements of distances to HVCs have remained elusive, however. In this paper we present four new distance brackets, using VLT observations of interstellar Ca II H and K absorption toward distant Galactic halo stars. We derive distance brackets of 5.0 to 11.7 kpc for the Cohen Stream (likely to be an infalling low-metallicity cloud), 9.8 to 15.1 kpc for Complex GCP (also known as the Smith Cloud or HVC 40–15+100 and with still unknown origin), 1.0 to 2.7 kpc for an IVC that appears associated with the return flow of the fountain in the Perseus arm, and 1.8 to 3.8 kpc for cloud g1, which appears to be in the outflow phase of the fountain. Our measurements further demonstrate that the Milky Way is accreting substantial amounts of gaseous material, which influences the Galaxys current and future dynamical and chemical evolution.

Distribution and Kinematics of O VI in the Galactic Halo

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 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.

Physical Properties, Baryon Content, and Evolution of the Lyα Forest: New Insights from High-Resolution Observations at z ≲ 0.4*

We present a study of the Lyα forest at z 0.4, from which we conclude that at least 20% of the total baryons in the universe are located in the highly ionized gas traced by broad Lyα absorbers. The cool photoionized low-z intergalactic medium (IGM) probed by narrow Lyα absorbers contains about 30% of the baryons. We further find that the ratio of broad to narrow Lyα absorbers is higher at z 0.4 than at 1.5 z 3.6, implying that a larger fraction of the low-redshift universe is hotter and/or more kinematically disturbed. We base these conclusions on an analysis of seven QSOs observed with both FUSE and the HST STIS E140M ultraviolet echelle spectrograph. Our sample has 341 H I absorbers with a total unblocked redshift path of 2.064. The observed absorber population is complete for log N 13.2, with a column density distribution f(N) N. For narrow (b ≤ 40 km s-1) absorbers, β = 1.76 ± 0.06. The distribution of the Doppler parameter b at low redshift implies two populations: narrow (b ≤ 40 km s-1) and broad (b > 40 km s-1) Lyα absorbers (referred to as NLAs and BLAs, respectively). Both the NLAs and some BLAs probe the cool (T ~ 104 K) photoionized IGM. The BLAs also probe the highly ionized gas of the warm-hot IGM (T 105-106 K). The distribution of b has a more prominent high-velocity tail at z 0.4 than at 1.5 z 3.6, which results in median and mean b-values that are 15%-30% higher at low z than at high z. The ratio of the number density of BLAs to NLAs at z 0.4 is a factor of ~3 higher than at 1.5 z 3.6.

The Far Ultraviolet Spectroscopic Explorer Survey of O VI Absorption in and near the Galaxy

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 two distant halo stars. We combine data from several FUSE Science Team programs with guest observer data that were public before 2002 May 1. The sight lines 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 H2, absorption intrinsic to the background target and intergalactic Lyβ lines. This decontamination was done very carefully, and in several sight lines 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 discuss the separation of the observed O VI absorption into components associated with the Milky Way halo and components 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 sight lines—the few nondetections only occur in noisy spectra. We further show that high-velocity O VI is very common, having equivalent width >65 mA in 50% of the sight lines and equivalent width >30 mA in 70% of the high-quality sight lines. The central velocities of high-velocity O VI components range from |vLSR| = 100 to 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 M31 and M33. We limit the extent of an O VI halo around M33 to be 200 km s-1 occurs along all sight lines in the region l = 180°-300°, b > 20°.

Dependence of Gas-Phase Abundances in the Interstellar Medium on Column Density

Sight lines through high- and intermediate-velocity clouds allow measurements of ionic gas-phase abundances A at very low values of H I column density N(H I). Present observations cover over 4 orders of magnitude in N(H I). Remarkably, for several ions we find that the A versus N(H I) relation is the same at high and low column densities and that the abundances have a relatively low dispersion (factors of 2-3) at any particular N(H I). Halo gas tends to have slightly higher values of A than disk gas at the same N(H I), suggesting that part of the dispersion may be attributed to the environment. We note that the dispersion is largest for Na I; using Na I as a predictor of N(H I) can lead to large errors. Important implications of the low dispersions regarding the physical nature of the interstellar medium are (1) because of clumping, over sufficiently long path lengths N(H I) is a reasonable measure of the local density of most of the H atoms along the sight line; (2) the destruction of grains does not mainly take place in catastrophic events such as strong shocks but is a continuous function of the mean density; (3) the cycling of the ions becoming attached to grains and being detached must be rapid, and the two rates must be roughly equal under a wide variety of conditions; and (4) in gas that has a low average density the attachment should occur within denser concentrations.

C ii RADIATIVE COOLING OF THE DIFFUSE GAS IN THE MILKY WAY

The heating and cooling of the interstellar medium (ISM) allow the gas in the ISM to coexist at very different temperatures in thermal pressure equilibrium. The rate at which the gas cools or heats is therefore a fundamental ingredient for any theory of the ISM. The heating cannot be directly determined, but the cooling can be inferred from observations of ·C *, which is an important coolant in different environments. The amount of cooling can be measured through either the intensity of the 157.7 μm [C II] emission line or the C * absorption lines at 1037.018 and 1335.708 A, observable with the Far Ultraviolet Spectroscopic Explorer and the Space Telescope Imaging Spectrograph on board the Hubble Space Telescope, respectively. We present the results of a survey of these far-UV absorption lines in 43 objects situated at b 30°. Measured column densities of C *, S II, P II, and Fe II are combined with H I 21 cm emission measurements to derive the cooling rates (per H atom using H I and per nucleon using S II) and to analyze the ionization structure, depletion, and metallicity content of the low-, intermediate-, and high-velocity clouds (LVCs, IVCs, and HVCs) along the different sight lines. Based on the depletion and the ionization structure, the LVCs, IVCs, and HVCs consist mostly of warm neutral and ionized clouds. For the LVCs, the mean cooling rate in ergs s-1 per H atom is -25.70 dex (1 σ dispersion). With a smaller sample and a bias toward high H I column density, the cooling rate per nucleon is similar. The corresponding total Galactic C II luminosity in the 157.7 μm emission line is L ~ 2.6 × 107 L☉. Combining N(C *) with the intensity of Hα emission, we derive that ~50% of the C * radiative cooling comes from the warm ionized medium (WIM). The large dispersion in the cooling rates is certainly due to a combination of differences in the ionization fraction, in the dust-to-gas fraction, and physical conditions between sight lines. For the IVC Intermediate-Velocity (IV) Arch at z ~ 1 kpc we find that on average the cooling is a factor of 2 lower than in the LVCs that probe gas at lower z. For an HVC (complex C, at z > 6 kpc) we find the much lower rate of -26.99 dex, similar to the rates observed in a sample of damped Lyα absorber systems (DLAs). The fact that in the Milky Way a substantial fraction of the C II cooling comes from the WIM implies that this is probably also true in the DLAs. We also derive the electron density, assuming a typical temperature of the warm gas of 6000 K: for the LVCs, ne = 0.08 ± 0.04 cm-3, and for the IV Arch, ne = 0.03 ± 0.01 cm-3 (1 σ dispersion). Finally, we measured the column densities N(S II) and N(P II) in many sight lines and confirm that sulphur appears undepleted in the ISM. Phosphorus becomes progressively more deficient when log N(H ) > 19.7 dex, which can mean that either P becomes more depleted into dust as more neutral gas is present or P is always depleted by about -0.3 dex, but the higher value of P II at lower H I column density indicates the need for an ionization correction.

The Deuterium-to-Hydrogen Ratio in a Low-Metallicity Cloud Falling onto the Milky Way

Using Far Ultraviolet Spectroscopic Explorer (FUSE) and Hubble Space Telescope observations of the QSO PG 1259+593, we detect D I Lyman series absorption in high-velocity cloud Complex C, a low-metallicity gas cloud falling onto the Milky Way. This is the first detection of atomic deuterium in the local universe in a location other than the nearby regions of the Galactic disk. We construct a velocity model for the sight line based on the numerous O I absorption lines detected in the ultraviolet spectra. We identify eight absorption-line components, two of which are associated with the high-velocity gas in Complex C at ≈-128 and ≈-112 km s-1. A new Westerbork Synthesis Radio Telescope (WSRT) interferometer map of the H I 21 cm emission toward PG 1259+593 indicates that the sight line passes through a compact concentration of neutral gas in Complex C. We use the WSRT data together with single-dish data from the Effelsberg 100 m radio telescope to estimate the H I column density of the high-velocity gas and to constrain the velocity extents of the H I Lyman series absorption components observed by FUSE. We find N(H I) = (9.0 ± 1.0) × 1019 cm-2, N(D I) = (2.0 ± 0.6) × 1015 cm-2, and N(O I) = (7.2 ± 2.1) × 1015 cm-2 for the Complex C gas (68% confidence intervals). The corresponding light-element abundance ratios are D/H = (2.2 ± 0.7) × 10-5, O/H = (8.0 ± 2.5) × 10-5, and D/O = 0.28 ± 0.12. The metallicity of Complex C gas toward PG 1259+593 is approximately 1/6 solar, as inferred from the oxygen abundance [O/H] = -0.79 ±. While we cannot rule out a value of D/H similar to that found for the local ISM (i.e., D/H ~ 1.5 × 10-5), we can confidently exclude values as low as those determined recently for extended sight lines in the Galactic disk (D/H < 1 × 10-5). Combined with the sub-solar metallicity estimate and the low nitrogen abundance, this conclusion lends support to the hypothesis that Complex C is located outside the Milky Way, rather than inside in material recirculated between the Galactic disk and halo. The value of D/H for Complex C is consistent with the primordial abundance of deuterium inferred from recent Wilkinson Microwave Anisotropy Probe observations of the cosmic microwave background and simple chemical evolution models that predict the amount of deuterium astration as a function of metallicity.

O VI ABSORBERS TRACING HOT GAS ASSOCIATED WITH A PAIR OF GALAXIES AT z = 0.167*

High signal-to-noise observations of the QSO PKS 0405-123 (z em = 0.572) with the Cosmic Origins Spectrograph from 1134 to 1796 ? with a resolution of ~17?km?s?1 are used to study the multi-phase partial Lyman limit system (LLS) at z = 0.16716, which has previously been studied using relatively low signal-to-noise spectra from STIS and FUSE. The LLS and an associated H I-free broad O VI absorber likely originate in the circumgalactic gas associated with a pair of galaxies at z = 0.1688 and 0.1670 with impact parameters of 116 h ?1 70 and 99 h ?1 70. The broad and symmetric O VI absorption is detected in the z = 0.16716 rest frame with v = ?278?? 3?km?s?1, log N(O VI) = 13.90?? 0.03, and b = 52?? 2?km?s?1. This absorber is not detected in H I or other species with the possible exception of N V. The broad, symmetric O VI profile and the absence of corresponding H I absorption indicate that the circumgalactic gas in which the collisionally ionized O VI arises is hot (log T?~ 5.8-6.2). The absorber may represent a rare but important new class of low-z intergalactic medium absorbers. The LLS has strong asymmetrical O VI absorption with log N(O VI) = 14.72?? 0.02 spanning a velocity range from ?200 to +100?km?s?1. The high and low ions in the LLS have properties resembling those found for Galactic highly ionized high-velocity clouds where the O VI is likely produced in the conductive and turbulent interfaces between cool and hot gas.