Galactic High-velocity Clouds Research Articles

A population of high-velocity absorption-line systems residing in the Local Group

Aims. We investigated the ionisation conditions and distances of Galactic high-velocity clouds (HVCs) in the Galactic halo and beyond in the direction of the Local Group (LG) barycentre and anti-barycentre, by studying spectral data of 29 extragalactic background sources obtained with the Cosmic Origins Spectropgraph (COS) installed on the Hubble Space Telescope (HST). Methods. We model column-densities of low, intermediate, and high ions such as Si II, C II, Si III, Si VI, and C IV, and use these data to construct a set of Cloudy ionisation models. Results. In total, we found 69 high-velocity absorption components along the 29 lines of sight. The components in the direction of the LG barycentre span the entire range of studied velocities, 100 ≲ |vLSR| ≲ 400 km s−1, while those in the anti-barycentre sample have velocities up to about 300 km s−1. For 49 components, we infer the gas densities. In the direction of the LG barycentre, the gas densities exhibit a wide range from log nH = −3.96 to −2.55, while in the anti-barycentre direction the densities are systematically higher, log nH > −3.25. The barycentre absorbers can be split into two groups based on their density: a high-density group with log nH > −3.54, which can be affected by the Milky Way radiation field, and a low-density group (log nH ≤ −3.54). The latter has very low thermal pressures of P/k < 7.3 K cm−3. Conclusions. Our study shows that part of the absorbers in the LG barycentre direction trace gas at very low gas densities and thermal pressures. These properties indicate that the absorbers are located beyond the virial radius of the Milky Way. Our study also confirms results from earlier, single-sightline studies, suggesting the presence of a metal-enriched intragroup medium filling the LG near its barycentre.

A new all-sky map of Galactic high-velocity clouds from the 21-cm HI4PI survey

High-velocity clouds (HVCs) are neutral or ionised gas clouds in the vicinity of the Milky Way that are characterised by high radial velocities inconsistent with participation in the regular rotation of the Galactic disc. Previous attempts to create a homogeneous all-sky HI map of HVCs have been hampered by a combination of poor angular resolution, limited surface brightness sensitivity and suboptimal sampling. Here, a new and improved HI map of Galactic HVCs based on the all-sky HI4PI survey is presented. The new map is fully sampled and provides significantly better angular resolution (16.2 versus 36 arcmin) and column density sensitivity (2.3 versus 3.7 * 10^18 cm^-2 at the native resolution) than the previously available LAB survey. The new HVC map resolves many of the major HVC complexes in the sky into an intricate network of narrow HI filaments and clumps that were not previously resolved by the LAB survey. The resulting sky coverage fraction of high-velocity HI emission above a column density level of 2 * 10^18 cm^-2 is approximately 15 per cent, which reduces to about 13 per cent when the Magellanic Clouds and other non-HVC emission are removed. The differential sky coverage fraction as a function of column density obeys a truncated power law with an exponent of -0.93 and a turnover point at about 5 * 10^19 cm^-2. HI column density and velocity maps of the HVC sky are made publicly available as FITS images for scientific use by the community.

An HST/COS legacy survey of high-velocity ultraviolet absorption in the Milky Way’s circumgalactic medium and the Local Group

To characterize the absorption properties of this circumgalactic medium (CGM) and its relation to the LG we present the so-far largest survey of metal absorption in Galactic high-velocity clouds (HVCs) using archival ultraviolet (UV) spectra of extragalactic background sources. The UV data are obtained with the Cosmic Origins Spectrograph (COS) onboard the Hubble Space Telescope (HST) and are supplemented by 21 cm radio observations of neutral hydrogen. Along 270 sightlines we measure metal absorption in the lines of SiII, SiIII, CII, and CIV and associated HI 21 cm emission in HVCs in the velocity range |v_LSR|=100-500 km s^-1. With this unprecedented large HVC sample we were able to improve the statistics on HVC covering fractions, ionization conditions, small-scale structure, CGM mass, and inflow rate. For the first time, we determine robustly the angular two point correlation function of the high-velocity absorbers, systematically analyze antipodal sightlines on the celestial sphere, and compare the absorption characteristics with that of Damped Lyman alpha absorbers (DLAs) and constrained cosmological simulations of the LG. Our study demonstrates that the Milky Way CGM contains sufficient gaseous material to maintain the Galactic star-formation rate at its current level. We show that the CGM is composed of discrete gaseous structures that exhibit a large-scale kinematics together with small-scale variations in physical conditions. The Magellanic Stream clearly dominates both the cross section and mass flow of high-velocity gas in the Milky Way's CGM. The possible presence of high-velocity LG gas underlines the important role of the local cosmological environment in the large-scale gas-circulation processes in and around the Milky Way (abridged).

The Milky Way halo as a QSO absorption-line system

We use archival UV absorption-line data from HST/STIS to statistically analyse the absorption characteristics of the high-velocity clouds (HVCs) in the Galactic halo towards more than 40 extragalactic background sources. We determine absorption covering fractions of low- and intermediate ions (OI, CII, SiIII, MgII, FeII, SiIII, CIV, and SiIV) in the range fc = 0.20 - 0.70. For detailed analysis we concentrate on SiII absorption components in HVCs, for which we investigate the distribution of column densities, b-values, and radial velocities. Combining information for SiII and MgII, and using a geometrical HVC model we investigate the contribution of HVCs to the absorption cross section of strong MgII absorbers in the local Universe. We estimate that the Galactic HVCs would contribute on average ~52 % to the total strong MgII cross section of the Milky Way, if our Galaxy were to be observed from an exterior vantage point. We further estimate that the mean projected covering fraction of strong MgII absorption in the Milky Way halo and disc from an exterior vantage point is fc(sMgII) = 0.31 for a halo radius of R = 61 kpc. These numbers, together with the observed number density of strong MgII absorbers at low redshift, indicate that the contribution of infalling gas clouds (i.e., HVC analogues) in the halos of Milky Way-type galaxies to the cross section of strong MgII absorbers is <34 %. These findings are in line with the idea that outflowing gas (e.g., produced by galactic winds) in the halos of more actively star-forming galaxies dominate the absorption-cross section of strong MgII absorbers in the local Universe.

HVC’s Favouring Star Forming Processes in galaxy discs: a basic calculation and some recent HI observations

Indirect evidence from detailed chemical evolution studies shows that infall of low metallicity gas appears to have proceeded at a rather constant rate during the lifetime of the Galactic disk. We explore two possible implications: (a) the effect of an infalling high velocity cloud (HVC) on interstellar cloud stability leading to enhanced star formation, and (b) the presence of these HVC’s around neighbouring galaxies, which would generalize this aspect of the evolution of galaxies in groups. (a) Using a simple analytical model we show how the overpressure produced in the galactic plane by HVC infall reduces the effective Jeans mass of the existing clouds, thus tending to enhance the SFR. (b) Using HI data from the THINGS survey we detect HI “fluff” around 4 galaxies of a total sample of 32 observed. Projecting some of the largest Galactic HVC’s to the distances of the THINGS galaxies we produce a useful estimate of the fraction of the HVC masses that we can detect at these distances, finding a value of order 10%, and go on to make tentative estimates about infall rates for nearby galaxies.

HST/COS OBSERVATIONS OF GALACTIC HIGH-VELOCITY CLOUDS: FOUR ACTIVE GALACTIC NUCLEUS SIGHT LINES THROUGH COMPLEX C

We report ultraviolet spectra of Galactic high-velocity clouds (HVCs) in Complex C, taken by the Cosmic Origins Spectrograph (COS) on the Hubble Space Telescope (HST), together with new 21 cm spectra from the Green Bank Telescope. The wide spectral coverage and higher signal-to-noise ratio, compared to previous HST spectra, provide better velocity definition of the HVC absorption, additional ionization species (including high ions), and improved abundances in this halo gas. Complex C has a metallicity of 10%–30% solar and a wide range of ions, suggesting dynamical and thermal interactions with hot gas in the Galactic halo. Spectra in the COS medium-resolution G130M (1133–1468 Å) and G160M (1383–1796 Å) gratings detect ultraviolet absorption lines from eight elements in low-ionization states (O i, N i, C ii, S ii, Si ii, Al ii, Fe ii, P ii) and three elements in intermediate- and high-ionization states (Si iii, Si iv, C iv, N v). Our four active galactic nucleus sight lines toward Mrk 817, Mrk 290, Mrk 876, and PG 1259+593 have high-velocity H i and O vi column densities, 19.39–20.05 and 13.58–14.10, with substantial amounts of kinematically associated photoionized gas. The high-ion abundance ratios are consistent with cooling interfaces between photoionized and collisionally ionized gas: N(C iv)/N(O vi) ≈ 0.3–0.5, N(Si iv)/N(O vi) ≈ 0.05–0.11, N(N v)/N(O vi) ≈ 0.07–0.13, and N(Si iv)/N(Si iii) ≈0.2.

HIGH-VELOCITY CLOUDS IN THE NEARBY SPIRAL GALAXY M 83

We present deep H i 21 cm and optical observations of the face-on spiral galaxy M 83 obtained as part of a project to search for high-velocity clouds (HVCs) in nearby galaxies. Anomalous-velocity neutral gas is detected toward M 83, with 5.6 × 107 M☉ of H i contained in a disk rotating 40–50 km s−1 more slowly in projection than the bulk of the gas. We interpret this as a vertically extended thick disk of neutral material, containing 5.5% of the total H i within the central 8 kpc. Using an automated source detection algorithm to search for small-scale H i emission features, we find eight distinct, anomalous-velocity H i clouds with masses ranging from 7 × 105 to 1.5 × 107M☉ and velocities differing by up to 200 km s−1 compared to the H i disk. Large on-disk structures are coincident with the optical spiral arms, while unresolved off-disk clouds contain no diffuse optical emission down to a limit of 27 r' mag per square arcsec. The diversity of the thick H i disk and larger clouds suggests the influence of multiple formation mechanisms, with a galactic fountain responsible for the slowly rotating disk and on-disk discrete clouds, and tidal effects responsible for off-disk cloud production. The mass and kinetic energy of the H i clouds are consistent with the mass exchange rate predicted by the galactic fountain model. If the HVC population in M 83 is similar to that in our own Galaxy, then the Galactic HVCs must be distributed within a radius of less than 25 kpc.

Distances to Galactic High-Velocity Clouds: Complex C

We report the first determination of a distance bracket for the high-velocity cloud (HVC) complex C. Combined with previous measurements showing that this cloud has a metallicity of 0.15 times solar, these results provide ample evidence that complex C traces the continuing accretion of intergalactic gas falling onto the Milky Way. Accounting for both neutral and ionized hydrogen as well as He, the distance bracket implies a mass of (3-14) × 106 M☉, and the complex represents a mass inflow of 0.1-0.25 M☉ yr-1. We base our distance bracket on the detection of Ca II absorption in the spectrum of the blue horizontal branch (BHB) star SDSS J120404.78+623345.6, in combination with a significant nondetection toward the BHB star BS 16034-0114. These results set a strong distance bracket of 3.7-11.2 kpc on the distance to complex C. A more weakly supported lower limit of 6.7 kpc may be derived from the spectrum of the BHB star BS 16079-0017.

Where Are the "Missing" Galactic Baryons?

Based on 19 high-resolution N-body/gasdynamical galaxy formation simulations in the ΛCDM cosmology, it is shown that for a galaxy like the Milky Way, in addition to the baryonic mass of the galaxy itself, about 70% extra baryonic mass should reside around the galaxy (inside of the virial radius), chiefly in the form of hot gas. Averaging over the entire field galaxy population, this "external" component amounts to 64%-85% of the baryonic mass of the population itself. These results are supported by the recent detection of very extended, soft X-ray emission from the halo of the quiescent, massive disk galaxy NGC 5746. Some of the hot gas may, by thermal instability, have condensed into mainly pressure-supported, warm clouds, similar to the Galactic high-velocity clouds (HVCs). Based on an ultra-high-resolution cosmological test simulation of a Milky Way-like galaxy (with a gas particle mass and gravity softening length of only 7.6 × 103 h-1 M☉ and 83 h-1 pc, respectively), it is argued that the hot gas phase dominates over the warm gas phase, in the halo. Finally, an origin of HVCs as "leftovers" from filamentary, "cold" accretion events, mainly occurring early in the history of galaxies, is proposed.

Absorption‐Line Study of Halo Gas in NGC 3067 toward the Background Quasar 3C 232

We present new H I 21 cm absorption data and ultraviolet spectroscopy from HST/STIS of the QSO/galaxy pair 3C 232/NGC 3067. The QSO sightline lies near the minor axis and 1.8 arcmin (11 kpc) above the plane of NGC 3067, a nearby luminous (cz = 1465 km/s, L = 0.5L*) starburst galaxy with a moderate star formation rate of 1.4 Solar masses per year. The UV spectra show that the Si IV and C IV doublets have the same three velocity components at cz = 1369, 1417, and 1530 km/s found in Ca II H & K, Na I D, Mg I, Mg II, and Fe II, implying that the low and high ionization gas are both found in three distinct absorbing clouds (only the strongest component at 1420 km/s is detected in H I 21 cm). The new Lyman alpha observation allows the first measurements of the spin and kinetic temperatures of halo gas: T_s = 435 +/- 140 K and T_k/T_s ~ 1. However, while a standard photoionization model can explain the low ions, the C IV and Si IV are explained more easily as collisionally-ionized boundary layers of the photoionized clouds. Due to their small inferred space velocity offsets (-260, -130, and +170 km/s) relative to the nucleus of NGC 3067 and the spatial coincidence of low and high ionization gas, we propose that these absorbers are analogous to Galactic high velocity clouds (HVCs). A comparison of the NGC 3067 clouds and Galactic HVCs finds similar H I column densities, kinematics, metallicities, spin temperatures, and inferred sizes. We find no compelling evidence that any halo gas along this sightline is escaping the gravitational potential of NGC 3067, despite its modest starburst.

The Hot Intergalactic Medium–Galaxy Connection: Two Strong OviAbsorbers in the Sight Line toward PG 1211+143

We present Hubble Space Telescope (HST) Space Telescope Imaging Spectrograph (STIS) and Far Ultraviolet Spectroscopic Explorer (FUSE) spectra of the QSO PG 1211+143 (zem = 0.081) and a galaxy survey of the surrounding field. This sight line shows two strong intergalactic absorption systems at cz 15,300 and 19,300 km s-1. This sight line addresses the nature and origin of the O VI absorbers and their connection to galaxies. We explore the relationship of these absorbers to the nearby galaxies and compare them to other O VI-bearing absorbers in diverse environments. At 15,300 km s-1, we find four distinct H I components and associated C II, C III, C IV, Si II, Si III, Si IV, N V, and O VI, lying near a spiral-dominated galaxy group with a bright member galaxy 137 h kpc from the sight line. The observed ions of C, Si, and N are likely to be photoionized, but the O VI is more consistent with collisional ionization. The ion ratios in this absorber resemble the highly ionized Galactic high-velocity clouds (HVCs); it may also trace the hot intragroup medium gas or the unbound wind of an undiscovered dwarf galaxy. At 19,300 km s-1, we find five H I components and associated C III, Si III, and collisionally ionized O VI lying 146 h kpc from an isolated galaxy. The properties of the O VI-bearing gas are consistent with an origin in strong shocks between low-metallicity gas (?2%-6% solar) and one or more of the warm photoionized components. It is likely that these absorbers are related to the nearby galaxies, perhaps by outflows or gas stripped from unseen satellite galaxies by interactions. However, we cannot reject completely the hypothesis that they reside in the same large-scale structure in which the galaxies are embedded but are otherwise not directly related.

Metal Abundances and Kinematics of Quasar Absorbers. II. Absorption Systems toward Q0347−3819 and APM BR J0307−4945

Detailed Monte Carlo inversion analysis of the spectral lines from three Lyman limit systems (LLSs) [N(H ) 1.0 × 1017 cm-2] and nine lower N(H ) systems [2 × 1014 cm-2 N(H ) 2 × 1016 cm-2] observed in the Very Large Telescope/UV-Visual Echelle Spectrograph spectra of Q0347-3819 (in the range 2.21 ≤ z ≤ 3.14) and of APM BR J0307-4945 (at z = 4.21 and 4.81) is presented. Combined with the results from a previous work, the analyzed LLSs show that they are a heterogeneous population originating in different environments. A functional dependence of the line-of-sight velocity dispersion σv on the absorber size L is confirmed: the majority of the analyzed systems follow the scaling relation σv ~ (NHL)0.3 (with NH being the total gas column density). This means that most absorbers may be related to virialized systems such as galaxies or their halos. Previously noted enhancement of the metal content in small-size systems is also confirmed: metallicities of Z ~ Z☉ are found in systems with L 0.4 kpc, whereas we observe much lower metal abundances in systems with larger linear sizes. For the first time in LLSs, a pronounced [α-element/iron peak] enrichment is revealed: the absorber at zabs = 2.21 shows [O/Fe] = 0.65 ± 0.11, [Si/Fe] = 0.51 ± 0.11, and [Mg/Fe] = 0.38 ± 0.11. Several absorption systems exhibit characteristics that are very similar to those observed in high-velocity clouds (HVCs) in the Milky Way and may be considered as high-redshift counterparts of Galactic HVCs.

The Population of Weak MgiiAbsorbers. II. The Properties of Single‐Cloud Systems

We present an investigation of MgII absorbers characterized as single-cloud weak systems at z~1. We measured column densities and Doppler parameters for MgII and FeII in 15 systems found in HIRES/Keck spectra at 6.6 km/s. Using these quantities and CIV, Lyman alpha and Lyman limit absorption observed with FOS/HST (resolution ~230 km/s) we applied photoionization models to each system to constrain metallicities, densities, ionization conditions, and sizes. We find that: (1) Single-cloud weak systems are optically thin in neutral hydrogen and may have their origins in a population of objects distinct from the optically thick strong MgII absorbers, which are associated with bright galaxies. (2) Weak systems account for somewhere between 25% to 100% of the z < 1 Lyman alpha forest clouds in the range 15.8<log N(HI)<16.8 cm^-2. (3) At least seven of 15 systems have two or more ionization phases of gas (multiphase medium). (4) We identify a subset of weak MgII absorber that we term ``iron-rich''. These clouds are not alpha-group enhanced and are constrained to have sizes of ~10 pc. At that size, to produce the observed redshift path density, they would need to outnumber L* galaxies by approximately six orders of magnitude. We discuss these results and the implications that the iron-rich systems require enrichment from Type Ia supernovae. Further, we address how star clusters or supernova remnants in dwarf galaxies might give rise to absorbers with the inferred properties. This would imply far larger numbers of such objects than are presently known, even locally. We compare the weak systems to the weak kinematic subsystems in strong MgII absorbers and to Galactic high velocity clouds. (abridged)

Molecular Hydrogen in High-Velocity Clouds

We present Far Ultraviolet Spectroscopic Explorer observations of interstellar molecular hydrogen (H2) in two Galactic high-velocity clouds (HVCs). Molecular hydrogen absorption is detected in the Magellanic Stream (abundance ~0.3 solar) toward the Seyfert galaxy Fairall 9 in the lowest three rotational states (J = 0-2) at vLSR = +190 km s-1, yielding a total H2 column density of log N(H2) = 16.40. In contrast, no H2 absorption is seen in the HVC Complex C (abundance ~0.1 solar) toward the quasar PG 1259+593 [log N(H2) ≤ 13.96 at vLSR = -130 km s-1], although both HVCs have similar H I column densities on the order of log N(H I) ≈ 20. Weak H2 absorption is detected in the Intermediate-Velocity Arch (IV Arch; abundance ~1.0 solar) toward PG 1259+593 [log N(H2) = 14.10 at vLSR = -55 km s-1 and log N(H I) = 19.5]. It thus appears that metal- and dust-poor halo clouds like Complex C are not able to form and maintain widely distributed H2, whereas metal- and dust-rich halo clouds like the IV Arch can maintain H2 even at low H I column densities.

The Diversity of High‐ and Intermediate‐Velocity Clouds: Complex C versus IV Arch

We present Far Ultraviolet Spectroscopic Explorer (FUSE) and Space Telescope Imaging Spectrograph (STIS) observations of interstellar ultraviolet absorption lines in the Galactic high-velocity cloud Complex C and the Intermediate-Velocity Arch (IV Arch) in the direction of the quasar PG 1259+593 (l = 1206, b = +581). Absorption lines from C II, N I, N II, O I, Al II, Si II, P II, S II, Ar I, Fe II, and Fe III are used to study the atomic abundances in these two halo clouds at VLSR ~ -130 km s-1 (Complex C) and -55 km s-1 (IV Arch). The O I/H I ratio provides the best measure of the overall metallicity in the diffuse interstellar medium because ionization effects do not alter the ratio, and oxygen is at most only lightly depleted from the gas into dust grains. For Complex C, we find an oxygen abundance of 0.093 times solar, consistent with the idea that Complex C represents the infall of low-metallicity gas onto the Milky Way. In contrast, the oxygen abundance in the IV Arch is 0.98 times solar, which indicates a Galactic origin. We report the detection of an intermediate-velocity absorption component at +60 km s-1 that is not seen in H I 21 cm emission. The clouds along the PG 1259+593 sight line have a variety of properties, proving that multiple processes are responsible for the creation and circulation of intermediate and high-velocity gas in the Milky Way halo.

A targeted survey for H I clouds in galaxy groups

Five galaxy groups with properties similar to those of the Local Group have been surveyed for H I clouds with the Arecibo Telescope. In total 300 pointings have been observed on grids of approximately 2.5 x 1.5 Mpc(2) centred on the groups. The 4.5 sigma detection limit on the minimal detectable H I masses is approximately 7 x 10(6) M(circle dot) (H(0) = 65 km s(-1) Mpc(-1)). All detections could be attributed to optical galaxies; no significant detections of H I clouds have been made. This null result leads to the conclusion that the total H I mass of intragroup clouds must be less than 10 per cent of the total H I mass of galaxy groups and less than 0.05 per cent of the dynamical mass. The recent hypothesis that Galactic high-velocity clouds are Local Group satellite galaxies is highly inconsistent with these observations.

The Kinematics of Intermediate-Redshift M[CLC]g[/CLC] [CSC]ii[/CSC] Absorbers

We present 23 quasar absorption-line systems selected by the Mg II ??2796, 2803 doublet with Wr(2796) > 0.3 ? over the redshift range 0.4 ? z ? 1.2. The kinematics and profile morphologies are studied at 6 km s-1 resolution to a 5 ? equivalent width detection threshold of Wr(2796) = 0.015 ?. We are thus sensitive to very weak clouds isolated in velocity. The absorption profiles were segregated into kinematic subsystems, and the properties (velocities, velocity widths, column densities, etc.) were measured directly from the data and by Voigt profile modeling. Most absorbers are characterized by a dominant kinematic subsystem with Wr(2796) > 0.2 ? and velocity spreads ranging from 10 to 50 km s-1 in proportion to the system equivalent width. Additional kinematic subsystems have velocities separations as large as 400 km s-1 relative to the dominant subsystem. The equivalent widths and velocity spreads of these weaker subsystems are anticorrelated with their velocities, and their equivalent width distribution turns down from a power law below Wr(2796) 0.08 ?. These moderate- and high-velocity subsystems are inferred to have sub-Lyman limit H I, and therefore are probably not higher redshift analogs to Galactic high-velocity clouds (HVCs). Weak subsystems are asymmetrically distributed in velocity such that they are either all blueshifted or all redshifted with respect to the dominant subsystem. This implies, that although the kinematic morphologies vary greatly on a case-by-case basis, a given line of sight is apparently probing a well-defined spatial and kinematic structure. We investigate a simple kinematic model that relies on a rotating disk to explain the observed asymmetries. There are systematic differences, or trends, in both the subsystem-to-subsystem velocity clustering and in the overall kinematic morphologies with increasing equivalent width; we discuss how these may provide clues to the observed differential evolution in the equivalent width distribution of Mg II absorbers.

ITAL]Far Ultraviolet Spectroscopic Explorer[/ITAL] Spectroscopy of High-Velocity Cloud Complex C

We present Far Ultraviolet Spectroscopic Explorer (FUSE) observations of the sight line toward the Seyfert 1 galaxy Markarian 876, which passes through high-velocity cloud (HVC) complex C. This sight line demonstrates the ability of FUSE to measure ionic absorption lines in Galactic HVCs. High-velocity absorption is clearly seen in both members of the O VI doublet. This is the first detection of O VI in a neutral hydrogen HVC. One component of HVC complex C is resolved in multiple Fe II lines from which we derive N(Fe II)/N(H I) = 0.48 (Fe/H)☉. This value of N(Fe II)/N(H I) implies that the metallicity of complex C along this sight line may be higher than that along the Mrk 290 sight line (0.1 solar) found by Wakker et al. On the other hand, if the metallicity of complex C is also 0.1 solar along this line of sight, the observed value of N(Fe II)/N(H I) suggests there may be a significant amount of H+ along the line of sight. In any case, little, if any, iron can be depleted into dust grains if the intrinsic metallicity of complex C is subsolar. Absorption from complex C is also seen in C II, N I, and N II, and upper limits based on nondetections can be determined for Ar I, P II, and Fe III. Although molecular hydrogen in the Milky Way is obvious in the FUSE data, no H2 absorption is seen in the HVC to a limit N(H2) < 2.0 × 1014 cm-2. Future FUSE observations of extragalactic objects behind Galactic HVCs will allow us to better constrain models of HVC origins.

The Space Density of Primordial Gas Clouds near Galaxies and Groups and their Relation to Galactic High-Velocity Clouds

The Arecibo H i Strip Survey probed the halos of approximately 300 cataloged galaxies and the environments of approximately 14 groups with sensitivity to neutral hydrogen masses >/=107 M middle dot in circle. The survey detected no objects with properties resembling the high-velocity clouds (HVCs) associated with the Milky Way or Local Group. If the HVCs were typically MHi=107.5 M middle dot in circle objects distributed throughout groups and galaxy halos at distances of approximately 1 Mpc, the survey should have made approximately 70 HVC detections in groups and approximately 250 detections around galaxies. The null detection implies that HVCs are deployed at typical distances of </=200 kpc from the galaxies or group barycenters. If the clouds are in virial equilibrium, their average dark matter fraction must be 98% or higher.

INTERSTELLAR ABUNDANCES FROM ABSORPTION-LINE OBSERVATIONS WITH THE HUBBLE SPACE TELESCOPE

▪ Abstract The Goddard High-Resolution Spectrograph (GHRS) aboard the Hubble Space Telescope (HST) has yielded precision abundance results for a range of interstellar environments, including gas in the local medium, in the warm neutral medium, in cold diffuse clouds, and in distant halo clouds. Through GHRS studies, investigators have determined the abundances of elements such as C, N, O, Mg, Si, S, and Fe in individual interstellar clouds. These studies have provided new information about the composition of interstellar dust grains, the origin of the Galactic high-velocity cloud system, and the processes that transport gas between the disk and the halo. Precision measurements of the interstellar D to H ratio and of the abundances of r- and s-process elements have also provided fiducial reference values for cosmological and stellar evolutionary observations and theoretical models.