arXiv subjects

B. P. Wakker

Publications and source records attributed to B. P. Wakker.

At least 37 records · Page 2Linked to original sources

arXiv2005

A FUSE survey of high-latitude Galactic molecular hydrogen

Measurements of molecular hydrogen (H_2) column densities are presented for the first six rotational levels (J=0 to 5) for 73 extragalactic targets observed with FUSE. All of these have a final signal-to-noise ratio larger than \snlimit, and are located at galactic latitude |b|>20 deg. The individual observations were calibrated with the FUSE calibration pipeline CalFUSE version 2.1 or higher, and then carefully aligned in velocity. The final velocity shifts for all the FUSE segments are listed. H_2 column densities or limits are determined for the 6 lowest rotational (J) levels for each HI component in the line of sight, using a curve-of-growth approach at low column densities ~16.5), and Voigt-profile fitting at higher column densities. Detections include 73 measurements of low-velocity H_2 in the Galactic Disk and lower Halo. Eight sightlines yield non-detections for Galactic H_2. The measured column densities range from log N(H_2)=14 to log N(H_2)=20. Strong correlations are found between log N(H_2) and T_01, the excitation temperature of the H_2, as well as between log N(H_2) and the level population ratios (log (N(J')/N(J))). The average fraction of nuclei in molecular hydrogen (f(H_2)) in each sightline is calculated; however, because there are many HI clouds in each sightline, the physics of the transition from HI to H_2 can not be studied. Detections also include H2 in 16 intermediate-velocity clouds in the Galactic Halo (out of 35 IVCs). Molecular hydrogen is seen in one high-velocity cloud (the Leading Arm of the Magellanic Stream), although 19 high-velocity clouds are intersected; this strongly suggests that dust is rare or absent in these objects. Finally, there are five detections of H_2 in external galaxies.

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arXiv2004

FUSE Observations of Interstellar and Intergalactic Absorption Toward the X-ray Bright BL Lac Object Mrk 421

High-quality Far-Ultraviolet Spectroscopic Explorer (FUSE) observations at 20 km /s resolution of interstellar and intergalactic absorption from 910 to 1187 A are presented for the X-ray bright BL Lac object Mrk 421. In this study we consider the O VI absorption between -140 to 165 km /s and its relationship to the lower ionization absorption and to the strong absorption produced by O VII and O VIII at X-ray wavelengths. The O VI absorption extending from -140 to 60 km /s is associated with strong low ionization gas absorption and originates in the Galactic thick disk / halo. This O VI appears to be produced by a combination of processes, including conductive interfaces between warm and hot gas and possibly cooling Galactic Fountain gas and hot halo gas bubbles. The O VI absorption extending from 60 to 165 km /s has unusual ionization properties in that there is very little associated low ionization absorption, with the exception of C III. This absorption is not observed toward two foreground halo stars, implying that it occurs in gas more distant than 3.5 kpc from the Galactic disk. Over the 60 to 165 km/s velocity range, O VI and C III absorption have the same kinematic behavior. N(O VI)/N(C III) = 10+/-3 over the 60 to 120 km/s velocity range. Given the association of O VI with C III, it is unlikely that the high velocity O VI co-exists with the hotter gas responsible for the O VII and O VIII absorption. The O VI positive velocity absorption wing might be tracing cooler gas entrained in a hot Galactic Fountain outflow. The O VII and O VIII absorption observed by Chandra and XMM-Newton may trace the hot gas in a highly extended (~100 kpc) Galactic corona or hot gas in the Local Group.

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arXiv2004

C II Radiative Cooling of the Diffuse Gas in the Milky Way

The heating and cooling of the interstellar medium allow the gas in the ISM to coexist at very different temperatures in thermal pressure equilibrium. The heating cannot be directly determined, but the cooling can be inferred from observations of C II*, which is an important coolant in different environments. The amount of cooling can be measured through either the intensity of the 157.7 \micron [C II] emission line or the C II* absorption lines at 1037.018 Å and 1335.708 Å, observable with FUSE and HST/STIS, respectively. We present the results of a survey of these far-UV absorption lines in 43 objects situated at |b|>30. We derive the cooling rates and analyze the ionization structure, the depletion, and metallicity content from the column densities of C II*, S II, P II, Fe II, and H I 21-cm emission for the low-, intermediate-, and high-velocity clouds (LVCs, IVCs, and HVCs) along the different sightlines. 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 erg\,s^{-1} per H atom is -25.70^{+0.19}_{-0.36} dex. The corresponding total Galactic C II luminosity in the 157.7 \micron emission line is L~2.6x10^7 L_\sun. Combining N(C II*) with the intensity of H$α$ emission, we derive that ~50% of the C II* 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 sightlines. For the IVC IV Arch at z~1 kpc we find that on average the cooling is a factor 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^{+0.21}_{-0.53} dex. [Abridged]

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arXiv2003

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

Using Far Ultraviolet Spectroscopic Explorer and Hubble Space Telescope observations of the QSO PG1259+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. A new Westerbork Synthesis Radio Telescope (WSRT) interferometer map of the H I 21 cm emission toward PG1259+593 indicates that the sight line passes through a compact concentration of neutral gas in Complex C. We find D/H = (2.2+/-0.7)x10^-5, O/H = (8.0+/-2.5)x10^-5, and D/O = 0.28+/-0.12. The metallicity of Complex C gas toward PG1259+593 is approximately 1/6 solar, as inferred from the oxygen abundance [O/H] = -0.79 (+0.12, -0.16). While we cannot rule out a value of D/H similar to that found for the local ISM, we can confidently exclude values as low as those determined recently for extended sight lines in the Galactic disk. 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. [Abbreviated abstract]

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arXiv2003

HI and OVI high-velocity clouds and the Local Group

I examine some of the evidence relevant to the idea that high-velocity clouds (HVCs) are gas clouds distributed throughout the Local Group, as proposed by Blitz et al. (1999) and Braun & Burton (1999). This model makes several predictions: a) the clouds have low metallicities; b) there should be no detectable Halpha emission; c) analogues near other galaxies should exist; and d) many faint HVCs in the region around M31 can be found. Low metallicities are indeed found in several HVCs, although they are also expected in several other models. Halpha emission detected in most HVCs and, when examined more closely, distant (D>200 kpc) HVCs should be almost fully ionized, implying that most HVCs with HI must lie near the Milky Way. No clear extragalactic analogues have been found, even though the current data appear sensitive enough. The final prediction (d) has not yet been tested. On balance there appears to be no strong evidence for neutral gas clouds distributed throughout the Local Group, but there may be many such clouds within 100 or so kpc from the Milky Way (and M31). On the other hand, some (but not all) of the high-velocity OVI recently discovered may originate in hot gas distributed throughout the Local Group.

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arXiv2003

Complex C: A Low-Metallicity High-Velocity Cloud Plunging into the Milky Way

(Abridged) We present a new high-resolution (7 km/s FWHM) echelle spectrum of 3C 351 obtained with STIS. 3C 351 lies behind the low-latitude edge of high-velocity cloud Complex C, and the new spectrum provides accurate measurements of O I, Si II, Al II, Fe II, and Si III absorption lines at the velocity of the HVC. We use collisional and photoionization models to derive ionization corrections; in both models we find that the overall metallicity Z = 0.1 - 0.3 Z_{solar} in Complex C, but nitrogen must be underabundant. The iron abundance indicates that Complex C contains very little dust. The absorbing gas probably is not gravitationally confined. The gas could be pressure-confined by an external medium, but alternatively we may be viewing the leading edge of the HVC, which is ablating and dissipating as it plunges into the Milky Way. O VI column densities observed with FUSE toward nine QSOs/AGNs behind Complex C support this conclusion: N(O VI) is highest near 3C 351, and the O VI/H I ratio increases substantially with decreasing latitude, suggesting that the lower-latitude portion of the cloud is interacting more vigorously with the Galaxy. The other sight lines through Complex C show some dispersion in metallicity, but with the current uncertainties, the measurements are consistent with a constant metallicity throughout the HVC. However, all of the Complex C sight lines require significant nitrogen underabundances. Finally, we compare the 3C 351 sight line to the sight line to the nearby QSO H1821+643 to search for evidence of outflowing Galactic fountain gas that could be mixing with Complex C. We find that the intermediate-velocity gas detected toward 3C 351 and H1821+643 has a higher metallicity and may well be a fountain/chimney outflow from the Perseus spiral arm.

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arXiv2002

A FUSE Survey of Molecular Hydrogen in Intermediate-Velocity Clouds in the Milky Way Halo

Far Ultraviolet Spectroscopic Explorer (FUSE) data is used to investigate the molecular hydrogen (H_2) content of intermediate-velocity clouds (IVCs) in the lower halo of the Milky Way. We analyze interstellar absorption towards 56 (mostly extragalactic) background sources to study H_2 absorption in the Lyman- and Werner bands in 61 IVC components at H I column densities >10^19 cm^-2. For data with good S/N (~9 per resolution element and higher), H_2 in IVC gas is convincingly detected in 14 cases at column densities varying between ~10^14 and ~10^17 cm^-2. We find an additional 17 possible H_2 detections in IVCs in FUSE spectra with lower S/N. The molecular hydrogen fractions, f, vary between 10^-6 and 10^-3, implying a dense, mostly neutral gas phase that is probably related to the Cold Neutral Medium (CNM) in these clouds. If the H_2 stays in formation-dissociation equlibrium, the CNM in these clouds can be characterized by compact (D~0.1 pc) filaments with volume densities on the order of n_H~30 cm^-3. The relatively high detection rate of H_2 in IVC gas implies that the CNM in these clouds is ubiquitous. More dense regions with much higher molecular fractions may exist, but it would be difficult to detect them in absorption because of their small size.

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arXiv2002

Probing Baryons in Galactic Halos and Gas Near Galaxies

We describe an extensive FUSE survey of highly ionized oxygen in the vicinity of the Milky Way that serves as an example of the type of study that would be desirable for other galactic systems. Understanding the origin of hot gas in the vicinity of galaxies and its relationship to the intergalactic medium presents a major observational challenge. Ultraviolet absorption-line spectroscopy is currently the most direct means for comprehensive investigations of the gas in galactic environments, but even with present (and near-term) facilities the number of background objects available to probe nearby galaxy halos and low-redshift cosmological structures is limited. Studying these structures over a range of impact parameters and angular separations would provide fundamental information about the baryonic content of the hot gas, its physical conditions, and its origins. A large space telescope optimized for high resolution spectroscopy in the 900-3200 Angstrom wavelength region at a sensitivity sufficient to observe faint AGNs/QSOs at angular separations of <1 degree would be ideal for such studies.

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arXiv2002

Distribution and Kinematics of O VI in the Galactic Halo

FUSE spectra of 100 extragalactic objects are analyzed to obtain measures of O VI absorption along paths through the Milky Way thick disk/halo. Strong O VI absorption over the approximate velocity range from -100 to 100 km/s reveals a widespread but highly irregular distribution of thick disk O VI, implying the existence of substantial amounts of hot gas with T ~ 3x10^5 K in the Milky Way halo. Large irregularities in the distribution of the absorbing gas are found to be similar over angular scales extending from less than one to 180 degrees, indicating a considerable amount of small and large scale structure in the gas. The overall distribution of Galactic O VI is not well described by a symmetrical plane-parallel layer of patchy O VI absorption. The simplest departure from such a model that provides a reasonable fit to the observations is a plane-parallel patchy absorbing layer with a scale height of 2.3 kpc, and a 0.25 dex excess of O VI in the northern Galactic polar region. The O VI absorption has a Doppler parameter b = 30 to 99 km/s, with an average value of 60 km/s . Thermal broadening alone cannot explain the large observed profile widths. The average O VI absorption velocities toward high latitude objects range from -46 to 82 km/s, with a sample average of 0 km/s and a standard deviation of 21 km/s. O VI associated with the thick disk moves both toward and away from the plane with roughly equal frequency. 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. (abbreviated)

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arXiv2002

The FUSE survey of OVI absorption in and near the Galaxy

We present FUSE observations of OVI absorption in a sample of 100 extragalactic targets and 2 distant halo stars. We describe the details of the calibration, alignment in velocity, continuum fitting, and manner in which contaminants were removed (Galactic H2, absorption intrinsic to the background target and intergalactic Ly-beta lines). We searched for OVI absorption in the velocity range -1200 to 1200 km/s. With a few exceptions, we only find OVI between -400 and 400 km/s; the exceptions may be intergalactic OVI. We discuss the separation of the observed OVI absorption into components associated with the Galactic halo and components at high-velocity, which are probably located in the neighborhood of the Galaxy. 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 OVI absorption occurs along all sightlines - the few non-detections only occur in noisy spectra. We further show that high-velocity OVI is very common, having equivalent width >65 mAA in 50% of the sightlines and >30 mAA in 70% of the high-quality sightlines. The high-velocity OVI absorption has velocities relative to the LSR of +/-(100--330) km/s; there is no correlation between velocity and absorption strength. We present 50 km/s wide OVI channel maps. These show evidence for the imprint of Galactic rotation. They also highlight two known HI high-velocity clouds (complex~C and the Magellanic Stream). The channel maps further show that OVI at velocities <-200 km/s occurs along all sightlines in the region l=20-150, b<-30, while OVI at velocities >200 km/s occurs along all sightlines in the region l=180-300, b>20 (abbreviated).

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arXiv2002

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

We report the results of an extensive FUSE study of high velocity OVI absorption along 102 complete sight lines through the Galactic halo. The high velocity OVI traces a variety of phenomena, including tidal interactions with the Magellanic Clouds, accretion of gas, outflow from the Galactic disk, warm/hot gas interactions in a highly extended Galactic corona, and intergalactic gas in the Local Group. We identify 85 high velocity OVI features at velocities of -500 < v(LSR) < +500 km/s along 59 of the 102 sight lines. Approximately 60% of the sky (and perhaps as much as 85%) is covered by high velocity H+ associated with the high velocity OVI. Some of the OVI is associated with known high velocity HI structures (e.g., the Magellanic Stream, Complexes A and C), while some OVI features have no counterpart in HI 21cm emission. The smaller dispersion in the OVI velocities in the GSR and LGSR reference frames compared to the LSR is necessary (but not conclusive) evidence that some of the clouds are extragalactic. Most of the OVI cannot be produced by photoionization, even if the gas is irradiated by extragalactic background radiation. Collisions in hot gas are the primary OVI ionization mechanism. We favor production of some of the OVI at the boundaries between warm clouds and a highly extended [R > 70 kpc], hot [T > 10^6 K], low-density [n < 10^-4 cm^-3] Galactic corona or Local Group medium. A hot Galactic corona or Local Group medium and the prevalence of high velocity OVI are consistent with predictions of galaxy formation scenarios. Distinguishing between the various phenomena producing high velocity OVI will require continuing studies of the distances, kinematics, elemental abundances, and physical states of the different types of high velocity OVI features found in this study. (abbreviated)

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arXiv2001

High-velocity clouds: a diverse phenomenon

In this contribution the current state of knowledge about the high-velocity clouds (HVCs) is summarized. Recent progress has shown that the HVCs are a diverse phenomenon. The intermediate-velocity clouds (IVCs) are likely to be part of a Galactic Fountain. The Magellanic Stream is a tidal remnant. HVC complex C (possibly complexes A and GCN) are low-metallicity clouds near the Galaxy; they could be remnants of the formation of the Galaxy or old tidal streams extracted from nearby dwarf galaxies. Having a substantial number of HI HVCs dispersed throughout the Local Group seems incompatible with the observed HI mass function of galaxies. Finally, FUSE finds high-velocity OVI, some of which is clearly associated with HI HVCs, but some which is not.

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arXiv2001

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 direction of the quasar PG 1259+593 (l=120,b=+58 deg). Absorption lines from CII, NI, NII, OI, AlII, SiII, PII, SII, ArI, FeII, and FeIII are used to study the atomic abundances in these two halo clouds at V_LSR=-130 km/s (Complex C) and V_LSR=-55 km/s (IV Arch). The OI/HI 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 (+0.125, -0.047) 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 (+1.21,-0.46) solar, which indicates a Galactic origin. We report the detection of an intermediate- velocity absorption component at +60 km/s that is not seen in HI 21cm 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.

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arXiv2001

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(HI) in the direction of observed probes, in order to arrive at reliable abundances and abundance limits (the HI 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 non-detections. The main conclusions are the following. 1) Absolute abundances have been measured using lines of SII, NI and OI, 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 CII, SiII and MgII, a few on CaII. Distance limits for major HVCs are >5 kpc, while distance brackets for several IVCs are in the range 0.5-2 kpc. 4) Mass limits for major IVCs are 0.5-8x10^5 M_sun, but for major HVCs they are >10^6 M_sun. 5) The CaII/HI ratio varies by up to a factor 2-5 within a single cloud, somewhat more between clouds. 6) The NaIHI ratio varies by a factor >10 within a cloud, and even more between clouds. Thus, CaII can be useful for determining both lower and upper distance limits, but NaI only yields upper limits.

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arXiv2001

HI spectra and column densities toward HVC and IVC probes

We show 21-cm line profiles in the direction of stars and extragalactic objects, lying projected on high- and intermediate-velocity clouds (HVCs and IVCs). About half of these are from new data obtained with the Effelsberg 100-m telescope, about a quarter are extracted from the Leiden-Dwingeloo Survey (LDS) and the remaining quarter were observed with other single-dish telescopes. HI column densities were determined for each HVC/IVC. Wakker (2001) (Paper I) uses these in combination with optical and ultraviolet high-resolution measurements to derive abundances. Here, an analysis is given of the difference and ratio of N(HI) as observed with a 9 arcmin versus a 35 arcmin beam. For HVCs and IVCs the ratio N(HI-9 arcmin)/N(HI-35 arcmin) lies in the range 0.2-2.5. For low-velocity gas this ratio ranges from 0.75 to 1.3 (the observed ratio is 0.85-1.4, but it appears that the correction for stray radiation is slightly off). The smaller range for the low-velocity gas may be caused by confusion in the line of sight, so that a low ratio in one component can be compensated by a high ratio in another -- for 11 low-velocity clouds fit by one component the distribution of ratios has a larger dispersion. Comparison with higher angular resolution data is possible for sixteen sightlines. Eight sightlines with HI data at 1 arcmin-2 arcmin resolution show a range of 0.75-1.25 for N(HI-2 arcmin)/N(HI-9 arcmin), while in eight other sightlines N(HI-Ly-alpha)/N(HI-9 arcmin) ranges from 0.74 to 0.98.

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arXiv2000

The FUSE Spectrum of PG0804+761: A Study of Atomic and MolecularGas in the Lower Galactic Halo and Beyond

We present an analysis of interstellar and intergalactic absorption lines in the FUSE spectrum of the low-redshift quasar PG0804+761 (z=0.100) at intermediate resolution (FWHM=25 km/s) in the direction l=138.3, b=31.0. With a good signal-to-noise ratio and the presence of several interesting Galactic and extragalactic absorption components along the sight line, this spectrum provides a good opportunity to demonstrate the ability of FUSE to do both interstellar and extragalactic science. Although the spectrum of PG0804+761 is dominated by strong absorption from local Galactic gas, we concentrate our study on absorption by molecular hydrogen and neutral neutral and ionized metals related to an intermediate-velocity cloud in the lower Galactic halo at -55 km/s, and on absorption from OVI extended to negative velocities. In the IVC, weak molecular hydrogen absorption is found in 5 lines for rotational levels 0 and 1, leading to a total H_2 column density of log N = 14.71(+-0.30). We derive an OI gas-phase abundance for the IVC of 1.03(+0.71-0.42) solar. Absorption by OVI is found at velocities as negative as -110 km/s, but no absorption from any species is found at velocities of -180 km/ where absorption from the nearby high-velocity Complex A would be expected. We suggest that the extended OVI absorption traces hot gas above the Perseus spiral arm. Finally, we find intergalactic absorption by an intervening HI Ly betax absorber at z=0.019 and absorption by HI, CIII and OVI in an associated system at z=0.102. No intervening OVI absorbers are seen in the spectrum of PG0804+761.

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arXiv2000

Dependence of Gas Phase Abundances in the ISM on Column Density

Sightlines through high- and intermediate-velocity clouds allow measurements of ionic gas phase abundances, A, at very low values of HI column density, N(HI). Present observations cover over 4 orders of magnitude in N(HI). Remarkably, for several ions we find that the A vs N(HI) relation is the same at high and low column density and that the abundances have a relatively low dispersion (factors of 2-3) at any particular N(HI). Halo gas tends to have slightly higher values of A than disk gas at the same N(HI), suggesting that part of the dispersion may be attributed to the environment. We note that the dispersion is largest for NaI; using NaI as a predictor of N(HI) can lead to large errors. Important implications of the low dispersions regarding the physical nature of the ISM are: (a) because of clumping, over sufficiently long pathlengths N(HI) is a reasonable measure of the_local_ density of_most_ of the H atoms along the sight line; (b) 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; (c) 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; (d) in gas that has a low average density the attachment should occur within denser concentrations.

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arXiv2000

FUSE Spectroscopy of High Velocity Cloud Complex C

We present Far Ultraviolet Spectroscopic Explorer (FUSE) observations of the sightline 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)_solar. This value of N(Fe II)/N(H I) implies that the metallicity of complex C along this sightline may be higher than that along the Mrk 290 sightline (0.1 solar) found by Wakker et al. (1999). 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 significan t 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 non-detections 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 H_2 absorption is seen in the high velocity cloud to a limit N(H_2)<2.0x10^14 cm^-2. Future FUSE observations of extragalactic objects behind Galactic high velocity clouds will allow us to better constrain models of HVC origins.

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