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The timing and duration of the reionization epoch is crucial to the emergence and evolution of structure in the universe. The relative roles that star-forming galaxies, active galactic nuclei and quasars play in contributing to the metagalactic ionizing background across cosmic time remains uncertain. Deep quasar counts provide insights into their role, but the potentially crucial contribution from star-formation is highly uncertain due to our poor understanding of the processes that allow ionizing radiation to escape into the intergalactic medium (IGM). The fraction of ionizing photons that escape from star-forming galaxies is a fundamental free parameter used in models to "fine-tune" the timing and duration of the reionization epoch that occurred somewhere between 13.4 and 12.7 Gyrs ago (redshifts between 12 > z > 6). However, direct observation of Lyman continuum (LyC) photons emitted below the rest frame \ion{H}{1} ionization edge at 912 \AA\ is increasingly improbable at redshifts z > 3, due to the steady increase of intervening Lyman limit systems towards high z. Thus UV and U-band optical bandpasses provide the only hope for direct, up close and in depth, observations of the types of environment that favor LyC escape. By quantifying the evolution over the past 11 billion years (z < 3) of the relationships between LyC escape and local and global parameters ..., we can provide definitive information on the LyC escape fraction that is so crucial to answering the question of, how did the universe come to be ionized? Here we provide estimates of the ionizing continuum flux emitted by "characteristic" (L_{uv}^*) star-forming galaxies as a function of look back time and escape fraction, finding that at z = 1 (7.6 Gyrs ago) L_{uv}^* galaxies with an escape fraction of 1% have a flux of 10^{-19} ergs cm^{-2} s^{-1} \AA^{-1}., Comment: Science white paper submitted to NASA Solicitation NNH12ZDA008L: Science Objectives and Requirements for the Next NASA UV/Visible Astrophysics Mission Concepts. 6 pages 4 figures - (abstract abridged)

The primordial abundances of light elements produced in the standard theory of Big Bang nucleosynthesis (BBN) depend only on the cosmic ratio of baryons to photons, a quantity inferred from observations of the microwave background. The predicted primordial 7Li abundance is four times that measured in the atmospheres of Galactic halo stars. This discrepancy could be caused by modification of surface lithium abundances during the stars' lifetimes or by physics beyond the Standard Model that affects early nucleosynthesis. The lithium abundance of low-metallicity gas provides an alternative constraint on the primordial abundance and cosmic evolution of lithium that is not susceptible to the in situ modifications that may affect stellar atmospheres. Here we report observations of interstellar 7Li in the low-metallicity gas of the Small Magellanic Cloud, a nearby galaxy with a quarter the Sun's metallicity. The present-day 7Li abundance of the Small Magellanic Cloud is nearly equal to the BBN predictions, severely constraining the amount of possible subsequent enrichment of the gas by stellar and cosmic-ray nucleosynthesis. Our measurements can be reconciled with standard BBN with an extremely fine-tuned depletion of stellar Li with metallicity. They are also consistent with non-standard BBN., Comment: Published in Nature. Includes main text and Supplementary Information. Replaced with final title and abstract

Using high resolution, high signal-to-noise ultraviolet spectra of the z = 0.9754 quasar PG1148+549 obtained with the Cosmic Origins Spectrograph (COS) on the Hubble Space Telescope, we study the physical conditions and abundances of NeVIII+OVI absorption line systems at z(abs) =0.68381, 0.70152, 0.72478. In addition to NeVIII and OVI, absorption lines from multiple ionization stages of oxygen (OII, OIII, OIV) are detected and are well-aligned with the more highly ionized species. We show that these absorbers are multiphase systems including hot gas (T ~ 10^{5.7} K) that produces NeVIII and OVI, and the gas metallicity of the cool phase ranges from Z = 0.3 Z_{solar} to supersolar. The cool (~10^{4} K) phases have densities n_{H} ~ 10^{-4} cm^{-3} and small sizes (< 4kpc); these cool clouds are likely to expand and dissipate, and the NeVIII may be within a transition layer between the cool gas and a surrounding, much hotter medium. The NeVIII redshift density, dN/dz = 7^{+7}_{-3}, requires a large number of these clouds for every L > 0.1L* galaxy and a large effective absorption cross section (>~ 100 kpc), and indeed, we find a star forming ~L* galaxy at the redshift of the z(abs)=0.72478 system, at an impact parameter of 217 kpc. Multiphase absorbers like these NeVIII systems are likely to be an important reservoir of baryons and metals in the circumgalactic media of galaxies., Comment: Final published version (Astrophysical Journal)

Without a source of new gas, our Galaxy would exhaust its supply of gas through the formation of stars. Ionized gas clouds observed at high velocity may be a reservoir of such gas, but their distances are key for placing them in the Galactic halo and unraveling their role. We have used the Hubble Space Telescope to blindly search for ionized high-velocity clouds (iHVCs) in the foreground of Galactic stars. We show that iHVCs with 90 < |v_LSR| < 170 km/s are within one Galactic radius of the sun and have enough mass to maintain star formation, while iHVCs with |v_LSR|>170 km/s are at larger distances. These may be the next wave of infalling material., Comment: This paper is part of a set of three papers on circumgalactic gas observed with COS and STIS on HST, to be published in Science, together with related papers by Tripp et al. and Tumlinson et al., in the November 18, 2011 edition. This version has not undergone final copyediting. Please see Science online for the final printed version

Outflowing winds of multiphase plasma have been proposed to regulate the buildup of galaxies, but key aspects of these outflows have not been probed with observations. Using ultraviolet absorption spectroscopy, we show that "warm-hot" plasma at 10^{5.5} K contains 10-150 times more mass than the cold gas in a poststarburst galaxy wind. This wind extends to distances >68 kiloparsecs, and at least some portion of it will escape. Moreover, the kinematical correlation of the cold and warm-hot phases indicates that the warm-hot plasma is related to the interaction of the cold matter with a hotter (unseen) phase at >>10^{6} K. Such multiphase winds can remove substantial masses and alter the evolution of poststarburst galaxies., Comment: This paper is part of a set of three papers on circumgalactic gas observed with the Cosmic Origins Spectrograph on HST, to be published in Science, together with related papers by Tumlinson et al. and Lehner & Howk, in the November 18, 2011 edition. This version has not undergone final copyediting. Please see Science online for the final printed version

We present UV and optical observations from the Cosmic Origins Spectrograph on the Hubble Space Telescope and Keck of a z= 0.27395 Lyman limit system (LLS) seen in absorption against the QSO PG1630+377. We detect H I absorption with log N(HI)=17.06\pm0.05 as well as Mg II, C III, Si III, and O VI in this system. The column densities are readily explained if this is a multi-phase system, with the intermediate and low ions arising in a very low metallicity ([Mg/ H] =-1.71 \pm 0.06) photoionized gas. We identify via Keck spectroscopy and Large Binocular Telescope imaging a 0.3 L_* star-forming galaxy projected 37 kpc from the QSO at nearly identical redshift (z=0.27406, \Delta v = -26 \kms) with near solar metallicity ([O/ H]=-0.20 \pm 0.15). The presence of very low metallicity gas in the proximity of a near-solar metallicity, sub-L_* galaxy strongly suggests that the LLS probes gas infalling onto the galaxy. A search of the literature reveals that such low metallicity LLSs are not uncommon. We found that 50% (4/8) of the well-studied z < 1 LLSs have metallicities similar to the present system and show sub-L_* galaxies with rho < 100 kpc in those fields where redshifts have been surveyed. We argue that the properties of these primitive LLSs and their host galaxies are consistent with those of cold mode accretion streams seen in galaxy simulations., Comment: Accepted for publication in the Astrophysical Journal

We present a survey for optically thick Lyman limit absorbers at z<2.6 using archival Hubble Space Telescope observations with the Faint Object Spectrograph and Space Telescope Imaging Spectrograph. We identify 206 Lyman limit systems (LLSs) increasing the number of catalogued LLSs at z<2.6 by a factor of ~10. We compile a statistical sample of 50 tau_LLS > 2 LLSs drawn from 249 QSO sight lines that avoid known targeting biases. The incidence of such LLSs per unit redshift, l(z)=dn/dz, at these redshifts is well described by a single power law, l(z) = C1 (1+z)^gamma, with gamma=1.33 +/- 0.61 at z<2.6, or with gamma=1.83 +/- 0.21 over the redshift range 0.2 < z < 4.9. The incidence of LLSs per absorption distance, l(X), decreases by a factor of ~1.5 over the ~0.6 Gyr from z=4.9 to 3.5; l(X) evolves much more slowly at low redshifts, decreasing by a similar factor over the ~8 Gyr from z=2.6 to 0.25. We show that the column density distribution function, f(N(HI)), at low redshift is not well fitted by a single power law index (f(N(HI)) = C2 N(HI)^(-beta)) over the column density range 13 < log N(HI) < 22 or log N(HI) >17.2. While low and high redshift f(N(HI)) distributions are consistent for log N(HI)>19.0, there is some evidence that f(N(HI)) evolves with z for log N(HI) < 17.7, possibly due to the evolution of the UV background and galactic feedback. Assuming LLSs are associated with individual galaxies, we show that the physical cross section of the optically thick envelopes of galaxies decreased by a factor of ~9 from z~5 to 2 and has remained relatively constant since that time. We argue that a significant fraction of the observed population of LLSs arises in the circumgalactic gas of sub-L* galaxies., Comment: Accepted by ApJ

We analyze images of BIMA 12CO (J = 1 --> 0), VLA HI, and Spitzer 3.6 and 24 \mum emission toward the edge-on galaxy NGC 891 and derive the radial and vertical distributions of gas and the radial distributions of stellar mass and recent star formation. We describe our method of deriving radial profiles for edge-on galaxies, assuming circular motion, and verify basic relationships between star formation rate and gas and stellar content, and between the molecular-to-atomic ratio and hydrostatic midplane pressure, that have been found in other galaxy samples. The Schmidt law index we find for the total gas (H2 + H I) is 0.85\pm0.55, but the Schmidt law provides a poor description of the SFR in comparison to a model that includes the influence of the stellar disk. Using our measurements of the thickness of the gas disk and the assumption of hydrostatic equilibrium, we estimate volume densities and pressures as a function of radius and height in order to test the importance of pressure in controlling the {\rho}H2/{\rho}HI ratio. The gas pressure in two dimensions P(r, z) using constant velocity dispersion does not seem to correlate with the {\rho}H2/{\rho}HI ratio, but the pressure using varying velocity dispersion appears to correlate with the ratio. We test the importance of gravitational instability in determining the sites of massive star formation, and find that the Q parameter using a radially varying gas velocity dispersion is consistent with self-regulation (Q - 1) over a large part of the disk., Comment: 19 pages, 19 figures, accepted for publication in AJ

There is now ample evidence that the interstellar thick disks of spiral galaxies are dusty. Although the majority of extraplanar gas in the first few kiloparsecs above the plane of a spiral galaxy is matter that has been expelled from the thin disk, the feedback-driven expulsion does not destroy dust grains altogether (and there is not yet any good measure suggesting it changes the dust-to-gas mass ratio). Direct optical imaging of a majority of edge-on spiral galaxies shows large numbers of dusty clouds populating the thick disk to heights z~2 kpc. These observations are likely revealing a cold, dense phase of the thick disk interstellar medium. New observations in the mid-infrared show emission from traditional grains and polycyclic aromatic hydrocarbons (PAHs) in the thick disks of spiral galaxies. PAHs are found to have large scale heights and to arise both in the dense dusty clouds traced through direct optical imaging and in the diffuse ionized gas. In this contribution, we briefly summarize these probes of dust in the thick disks of spiral galaxies. We also argue that not only can dust can be used to trace extraplanar material that has come from within the thick disk, but that its absence can be a marker for newly accreted matter from the circumgalactic or intergalactic medium. Thus, observations of dust can perhaps provide a quantitative measure of the importance of "outflow versus infall" in spiral galaxies., Comment: 8 pages; Invited review for the proceedings of "The Role of Disk-Halo Interaction in Galaxy Evolution: Outflow vs. Infall?" (Ed. M. de Avillez), in Espinho, Portugal, 18-22 August 2008 ; high resolution version at http://www.nd.edu/~jhowk/Papers/papers.html#conference

We report on an attempt to accurately wavelength calibrate four nights of data taken with the Keck HIRES spectrograph on QSO PHL957, for the purpose of determining whether the fine structure constant was different in the past. Using new software and techniques, we measured the redshifts of various Ni II, Fe II, Si II, etc. lines in a damped Ly-alpha system at z=2.309. Roughly half the data was taken through the Keck iodine cell which contains thousands of well calibrated iodine lines. Using these iodine exposures to calibrate the normal Th-Ar Keck data pipeline output we found absolute wavelength offsets of 500 m/s to 1000 m/s with drifts of more than 500 m/s over a single night, and drifts of nearly 2000 m/s over several nights. These offsets correspond to an absolute redshift of uncertainty of about Delta z=10^{-5} (Delta lambda= 0.02 Ang), with daily drifts of around Delta z=5x10^{-6} (Delta lambda =0.01 Ang), and multiday drifts of nearly Delta z=2x10^{-5} (0.04 Ang). The causes of the wavelength offsets are not known, but since claimed shifts in the fine structure constant would result in velocity shifts of less than 100 m/s, this level of systematic uncertainty makes may make it difficult to use Keck HIRES data to constrain the change in the fine structure constant. Using our calibrated data, we applied both our own fitting software and standard fitting software to measure (Delta alpha)/alpha, but discovered that we could obtain results ranging from significant detection of either sign, to strong null limits, depending upon which sets of lines and which fitting method was used. We thus speculate that the discrepant results on (Delta alpha)/alpha reported in the literature may be due to random fluctuations coming from under-estimated systematic errors in wavelength calibration and fitting procedure., Comment: 19 pages, 10 figures, to appear in ApJ, Dec 2009 Many corrections and changes made, some new figures

We present observations of the O VI system at z_abs = 0.495096 toward PKS 0405-123 (z_em = 0.5726) obtained with FUSE and STIS. In addition to strong O VI, with log N(O VI) = 14.47+/-0.02, and moderate H I this absorber shows C III, N IV, O IV, and O V, with upper limits for another seven ions. The large number of available ions allows us to test ionization models usually adopted with far less contraints. We find the ionic column densities cannot be matched by collisional ionization models, in or out of equilibrium. Photoionization models can match the observed column densities, including O VI. If one assumes photoionization by a UV background dominated by QSOs, the metallicity of the gas is [O/H] = -0.15, while a model for the UV background with contributions from ionizing photons escaping from galaxies gives [O/H] = -0.62. Both give [N/O] ~ -0.6 and [C/H] ~ -0.2 to -0.1. The choice of ionizing spectrum is poorly constrained. Multiphase models with a contribution from both photoionized gas (at T~10^4 K) and collisionally ionized gas (at T~(1-3)x10^5 K) can also match the observations giving very similar metallicities. The O VI in this system is not necessarily a reliable tracer of WHIM matter. We do not detect Ne VIII or Mg X absorption. The limit on Ne VIII/O VI < 0.21 (3 sigma), the lowest yet observed. Thus this absorber shows no firm evidence of the "warm-hot intergalactic medium" at T~(0.5-3)x10^6 K thought to contain a significant fraction of the baryons at low redshift. We present limits on the total column of warm-hot gas in this absorber as a function of temperature. This system would be unlikely to provide detectable X-ray absorption in the ions O VII or O VIII even if it resided in front of the brighter X-ray sources in the sky., Comment: Accepted for publication in MNRAS. 21 pages. Abstract abridged slightly

We present Far Ultraviolet Spectroscopic Explorer (FUSE) and Space Telescope Imaging Spectrograph (STIS E140M) observations of the post-asymptotic giant branch star ZNG 1 in the globular cluster Messier 5 (l=3.9, b=+47.7; d=7.5 kpc, z=+5.3 kpc). High velocity absorption is seen in C IV, Si IV, O VI, and lower ionization species at LSR velocities of -140 and -110 km/s. We conclude that this gas is not circumstellar on the basis of photoionization models and path length arguments. Thus, the high velocity gas along the ZNG 1 sight line is the first evidence that highly-ionized HVCs can be found near the Galactic disk. We measure the metallicity of these HVCs to be [O/H]=+0.22\pm0.10, the highest of any known HVC. Given the clouds' metallicity and distance constraints, we conclude that these HVCs have a Galactic origin. This sight line probes gas toward the inner Galaxy, and we discuss the possibility that these HVCs may be related to a Galactic nuclear wind or Galactic fountain circulation in the inner regions of the Milky Way., Comment: 23 pages, 11 figures, 7 tables

We publish the Keck/HIRES and Keck/ESI spectra that we have obtained during the first 10 years of Keck observatory operations. Our full sample includes 42 HIRES spectra and 39 ESI spectra along 65 unique sightlines providing abundance measurements on ~85 damped Lya systems. The normalized data can be downloaded from the journal or from our supporting website: http://www.ucolick.org/~xavier/DLA/. The database includes all of the sightlines that have been included in our papers on the chemical abundances, kinematics, and metallicities of the damped Lya systems. This data has also been used to argue for variations in the fine-structure constant. We present new chemical abundance measurements for 10 damped Lya systems and a summary table of high-resolution metallicity measurements (including values from the literature) for 153 damped Lya systems at z>1.6. We caution, however, that this metallicity sample (and all previous ones) is biased to higher N(HI) values than a random sample., Comment: 55 pages, 11 figures. Accepted to ApJS. See http://www.ucolick.org/~xavier/DLA/ for the data

We explore the role of anisotropic thermal conduction on the evolution of supernova remnants through interstellar media with a range of densities via numerical simulations. We find that a remnant expanding in a dense environment can produce centre-bright hard x-ray emission within 20 kyr, and centre-bright soft x-ray emission within 60 kyr of the supernova event. In a more tenuous environment, the appearance of a centre-bright structure in hard x-rays is delayed until about 60 kyr. The soft x-ray emission from such a remnant may not become centre bright during its observable lifetime. This can explain the observations that show that mixed-morphology supernova remnants preferentially occur close to denser, molecular environments. Remnants expanding into denser environments tend to be smaller, making it easier for thermal conduction to make larger changes in the temperatures of their hot gas bubbles. We show that the lower temperatures make it very favorable to use high-stage ions as diagnostics of the hot gas bubbles in SNRs. In particular, the distribution of O VIII transitions from shell-bright at early epochs to centre-bright at later epochs in the evolution of an SNR expanding in a dense ISM when the physics of thermal conduction is included., Comment: 8 pages, 5 figures, submitted to Monthly Notices

As large optical quasar surveys for damped Lya become a reality and the study of star forming gas in the early Universe achieves statistical robustness, it is now vital to identify and quantify the sources of systematic error. Because the nature of optically-selected quasar surveys makes them vulnerable to dust obscuration, we have undertaken a radio-selected quasar survey for damped Lya systems to address this bias. We present the definition and results of this survey. We then combine our sample with the CORALS dataset to investigate the HI column density distribution function f(N) of damped Lya systems toward radio-selected quasars. We find that f(N) is well fit by a power-law f(N) = k_1 N^alpha_1, with log k_1 = 22.90 and alpha_1 = -2.18. This power-law is in excellent agreement with that of optically-selected samples at low N(HI), an important yet expected result given that obscuration should have negligible effect at these gas columns. However, because of the relatively small size of the radio-selected sample, 26 damped Lya systems in 119 quasars, f(N) is not well constrained at large N(HI) and the first moment of the HI distribution function, Omega_g, is, strictly speaking, a lower limit. The power-law is steep enough, however, that extrapolating it to higher column densities implies only a modest, logarithmic increase in Omega_g. The radio-selected value of Omega_g = 1.15 x 10^-3, agrees well with the results of optically-selected surveys. While our results indicate that dust obscuration is likely not a major issue for surveys of damped Lya systems, we estimate that a radio-selected sample of approximately 100 damped Lya systems will be required to obtain the precision necessary to absolutely confirm an absence of dust bias., Comment: 12 pages, 9 Figures. Accepted to ApJ April 11, 2006

We describe a new method for determining total gas-phase abundances for the Galactic ISM with minimal ionization uncertainties. For sight lines toward globular clusters containing both UV-bright stars and radio pulsars, one can measure column densities of HI and several metal ions using UV absorption measurements and of H II using radio dispersion measurements, thereby minimizing ionization uncertainties. We apply this method to the globular cluster Messier 3 sight line using FUSE and HST ultraviolet spectroscopy of the post-asymptotic giant branch star von Zeipel 1128 and radio observations by Ransom et al. of millisecond pulsars. Ionized hydrogen is 45+/-5% of the total along this sight line, the highest measured fraction along a high-latitude pulsar sight line. We derive total gas-phase abundances log N(S)/N(H) = -4.87+/-0.03 and log N(Fe)/N(H) = -5.27+/-0.05. Our derived sulfur abundance is in excellent agreement with recent solar system determinations of Asplund, Grevesse, & Sauval, but -0.14 dex below the solar system abundance typically adopted in studies of the ISM. The iron abundance is ~-0.7 dex below the solar system abundance, consistent with significant depletion. Abundance estimates derived by simply comparing S II and Fe II to H I are +0.17 and +0.11 dex higher, respectively, than our measurements. Ionization corrections to the gas-phase abundances measured in the standard way are, therefore, significant compared with the measurement uncertainties along this sight line. The systematic uncertainties associated with the uncertain contribution to the electron column density from ionized helium could raise these abundances by <+0.03 dex (+7%). [Abridged], Comment: To appear in the ApJ. 25 pages, including figures and text

Current measurements indicate that the deuterium abundance in diffuse interstellar gas varies spatially by a factor of ~4 among sightlines extending beyond the Local Bubble. One plausible explanation for the scatter is the variable depletion of D onto dust grains. To test this scenario, we have obtained high signal-to-noise, high resolution profiles of the refractory ion TiII along seven Galactic sightlines with D/H ranging from 0.65 to 2.1x10^-5. These measurements, acquired with the recently upgraded Keck/HIRES spectrometer, indicate a correlation between Ti/H and D/H at the >95% c.l. Therefore, our observations support the interpretation that D/H scatter is associated with differential depletion. We note, however, that Ti/H values taken from the literature do not uniformly show the correlation. Finally, we identify significant component-to-component variations in the depletion levels among individual sightlines and discuss complications arising from this behavior., Comment: 4 pages; Accepted to Astrophysical Journal Letters

The interstellar thick disks of galaxies contain not only gas, but significant quantities of dust. Most of our knowledge of extraplanar dust in disk galaxies comes from direct broadband optical imaging of these systems, wherein the dust is identified due to the irregular extinction it produces against the thick disk and bulge stars. This observational technique is sensitive to only the most dense material, and we argue much of the material identified in this way traces a cold phase of the interstellar thick disks in galaxies. The presence of a cold, dense phase likely implies the interstellar pressures in the thick disks of spiral galaxies can be quite high. This dense phase of the interstellar medium may also fueling thick disk star formation, and H-alpha observations are now revealing H II regions around newly-formed OB stars associations in several galaxies. We argue that the large quantities of dust and the morphologies of the structures traced by the dust imply that much of the extraplanar material in disk galaxies must have been expelled from the underlying thin disk., Comment: 8 pages, 3 figures. To appear in the Proceedings of the Extra-planar Gas Conference, June 7-11, 2004, Dwingeloo (ASP Conf. Series, ed. R. Braun). High resolution PDF version can be found at http://kingpin.ucsd.edu/~howk/Papers/Proceedings/howk2005_dwingeloo.pdf

We present Keck/HIRES abundance measurements and metal-line kinematic profiles of the damped Lyman alpha systems (DLAs) towards the quasars Q0933+733 (z_abs=1.479) and Q0948+433 (z_abs=1.233). These two DLAs have among the five highest HI column densities at any redshift: N(HI)=4.2E21 cm^{-2}. The metal-line data, presented here for the first time, reveal that these DLAs are noteworthy for several other reasons as well. 1) The Q0933+733 DLA exhibits simple kinematic structure with unusually narrow velocity widths as measured from its unsaturated metal lines (delta v=16 km/s). At 2.6% solar, it has the second lowest metallicity at z<2. 2) The Q0948+433 DLA has among the strongest metal-line transitions of any known DLA. The saturated SiII1808 line implies a high metallicity ([Si/H]> -1) and a significant alpha-enhancement. The strong metal lines of this DLA have made possible the detection of TiII1910, CoII2012, and MgI2026. 3) We find that the relative gas-phase abundances of both DLAs follow the general trend seen at high redshift, e.g., enhanced Zn/Fe and Si/Fe, and sub-solar Mn/Fe, indicating that there is little evolution in the nucleosynthetic patterns of DLAs down to this epoch. 4) Their high HI column densities imply that these DLAs dominate the column density-weighted cosmic mean metallicity, , of the universe at z<1.5. Using the 15 DLAs with measured metallicities in the redshift interval 0.4 = -0.89^{+0.40}_{-0.33}, where the uncertainties are 95% confidence limits., Comment: 25 pages. Accepted for publication in the Astronomical Journal

We present results from an analysis of FUSE and HST/STIS spectroscopy of the z_em=0.57 quasar PKS0405-123. We focus on the intervening metal-line systems identified along the sightline and investigate their ionization mechanism, ionization state, and chemical abundances. We identify six OVI absorbers to a 3sigma equivalent width (EW) limit of 60mA. This implies an incidence dN/dz = 16^{+9}_{-6} consistent with previous OVI studies. In half of the OVI systems we report positive detections of CIII suggesting the gas is predominantly photoionized, has multiple ionization phases, or is in a non-equilibrium state. This contrasts with the general description of the warm-hot intergalactic medium (WHIM) as described by numerical simulations where the gas is predominantly in collisional ionization equilibrium. We have also searched the sightline for the NeVIII doublet (a better probe of WHIM gas with T>10^6K) over the redshift range 0.210^14 cm^-2 is [M/H] ~> -1.5 with large scatter. This significantly exceeds the median metallicity of CIV and OVI systems at z~3 and indicates enrichment of the intergalactic medium over the past 10Gyr. [abriged], Comment: 21 pages, 21 figures (mostly color). Accepted to ApJ, August 12, 2004

We discuss interstellar temperature determinations using the excitation equilibrium of the ^2P levels of Si II and C II. We show how observations of the ^2P_3/2 fine structure levels of Si II and C II (which have significantly different excitation energies, corresponding to ~413 and 92 K, respectively) can be used to limit gas kinetic temperatures. We apply this method to the z=4.224 damped Lyman-alpha system toward the quasar PSS1443+27. The lack of significant absorption out of the SiII ^2P_3/2 level and the presence of very strong C II ^2P_3/2 provides an upper limit to the temperature of the C II*-bearing gas in this system. Assuming a solar Si/C ratio, the observations imply a 2-sigma limit T<954 K for this absorber; a super-solar Si/C ratio gives stricter limits, T<524 K. The observations suggest the presence of a cold neutral medium; such cold gas may serve as the fuel for star formation in this young galaxy., Comment: Accepted to ApJ Letters; 13 pages, including figures and tables

We have used the Far Ultraviolet Spectroscopic Explorer to search for OVI 1031.926 Angstrom emission at four locations in the starburst superwind of M82. No OVI emission was detected at any of the four pointings, with upper limits less than or equal to the 0.3-2 keV X-ray flux. These observations limit the energy lost through radiative cooling of coronal phase (T~300,000 K) gas to roughly the same magnitude as that lost in the hot phase through X-ray emission, which has been shown to be small. The wind material retains most of its energy and should be able to escape from the gravitational potential of M82, enriching the intergalactic medium with energy and metals. The lack of coronal gas in the wind and observations of spatially correlated X-ray and H-alpha emission are consistent with a scenario in which the hot wind material over-runs cold clouds in the halo, or one where the H-alpha and X-ray emission arise at the interface between the hot wind and a cool shell of swept-up ISM, as long as the shock velocity is <150 km/s. The observed limits on the OVI/H-alpha and CIII/H-alpha flux ratios rule out shock heating as the source of the 10,000 K gas unless the shock velocity is <90 km/s., Comment: 5 pages, 3 figures, uses emulateapj.cls. ApJL in press (Oct 20, 2003)

We present Far Ultraviolet Spectroscopic Explorer and Space Telescope Imaging Spectrograph observations of high-ionization interstellar absorption toward HD 14434 [(l, b) = (135.1, -3.8); d ~ 2.3 kpc], an O5.5 V star in the Perseus OB1 Association. Intermediate-velocity interstellar Si IV and C IV absorption is present at V_LSR = -67 km/s, while low-ionization gas associated with the Perseus arm is detected at ~ -50 km/s. Neither N V nor O VI is detected at V_LSR = -67 km/s; although Al III and Fe III, tracers of warm ionized gas, are seen. The high-ion column densities in the -67 km/s component are log[N(C IV)] = 13.92 +/- 0.02 cm^-2, log[N(Si IV)] = 13.34 +/- 0.02 cm^-2, log[N(N V)] < 12.65 cm^-2, and log[N(O VI)] < 13.73 cm^-2 (3-sigma limits). The observed C IV/Si IV ratio of 3.8 +/- 0.3 in this intermediate-velocity cloud (IVC) is similar to the Galactic average (4.3 +/- 1.9). Our analysis of the Si IV andC IV line widths yields a temperature of T = 10,450 +/- 3,400 K for this component. At this low temperature, neither Si IV nor C IV can be produced via collisions. We investigate several photoionization models to explain the intermediate-velocity Si IV and C IV absorption toward HD 14434. Photoionization models employing cooling of a hot (T ~ 10^6 K) diffuse plasma as the source of ionizing radiation reproduces the observed properties of the IVC toward HD 14434 quite well. The hot plasma responsible for the ionizing radiation in these models may be attributed to hot gas contained in a supershell in or near the Perseus Arm or from a more generally distributed hot ionized medium., Comment: 29 pages, 6 figures, accepted for publication in August 1, 2003 edition of The Astrophysical Journal

The interstellar abundances of D I, N I, and O I in the local ISM are studied us ing high-resolution spectra of four hot white dwarfs. The spectra of GD 246, WD 2331$-$475, HZ 21, and Lan 23 were obtained with the {\it Far Ultraviolet Spectroscopic Explorer} FUSE in the wavelength range 905-1187 A. The line of sight to GD 246 probes the Local Interstellar Cloud and at least one other H I cloud inside the Local Bubble, which contains most of the gas seen al ong this line of sight. The column densities of H I, C II*, S II, and Si II are measured using archival STIS echelle-mode observations. The H I column density is determined by fitting the strong damping wings of inte rstellar Lyman alpha using a model atmosphere to account for the stellar continuum. We combine the different abundance ratios computed here with previous published values to produce revised FUSE abundance ratios for DI/HI, OI/HI, NI/HI, DI/NI, DI/OI, and OI/NI., Comment: 12 figures, 48 pages

We present FUSE observations of the post-asymptotic giant branch star von Zeipel 1128 (l=42.5, b=+78.7; z=10.0 kpc), located in the globular cluster Messier 3. The FUSE observations cover the wavelength range 905-1187 Ang at ~20 km/s (FWHM) resolution. These data exhibit many photospheric and interstellar absorption lines, including absorption from ions associated with the warm neutral, warm ionized, and highly-ionized phases of the interstellar medium along this sight line. Ionized hydrogen represents >12%, most likely ~45%, of the total hydrogen column along this sight line, most of it associated with the warm ionized phase. The warm ionized and neutral media toward von Zeipel 1128 have very similar gas-phase abundances and kinematics: the neutral and ionized gases in this region of the thick disk are closely related. Strong O VI absorption is seen with the same central velocity as the warm ionized gas, though the O VI velocity dispersion is much higher (sigma=32 km/s). Virtually all of the O VI is found at velocities where lower-ionization gas is seen, suggesting the O VI and WNM/WIM probes are tracing different portions of the same structures (e.g., the O VI may reside in interfaces surrounding the WNM/WIM clouds). We see no evidence for interstellar absorption associated with the globular cluster Messier 3 itself nor with the circumstellar environment of von Zeipel 1128. Neither high velocity cloud absorption (with v(LSR)>125 km/s) nor high velocity-dispersion gas (with sigma~60 km/s) is seen toward von Zeipel 1128. [Abridged], Comment: Astrophysical Journal, in press; 39 pages, including 8 figures and 8 tables. Also available at http://kingpin.ucsd.edu/~howk/Papers/

We present Hubble Space Telescope Imaging Spectrograph (HST/STIS) and Far Ultraviolet Spectroscopic Explorer (FUSE) observations of high ion interstellar absorption along the sight line to HD 116852. At a distance of 4.8 kpc, HD 116852 is an O9 III star lying in the low Galactic halo, -1.3 kpc from the plane of the Galaxy in the direction l = 304.9, b = -16.1. The STIS E140H grating observations provide high-resolution (FWHM = 2.7 km/s) spectra of the resonance doublets of Si IV, C IV, and N V. These data are complemented by medium-resolution (FWHM = 20 km/s) FUSE spectra of O VI. We find evidence for three distinct types of highly ionized gas present in the data. First, two narrow absorption components are resolved in the Si IV and C IV profiles, at approximate LSR velocities of -36 and -10 km/s. These narrow components appear to be produced in gas associated with the Norma and Sagittarius spiral arms, at approximate z-distances of -1.0 and -0.5 kpc, respectively. Second, we detect an intermediate-width component in C IV and Si IV, at 17 km/s, which we propose could arise at the conductive interface at the boundary between a low column density neutral or weakly ionized cloud and the surrounding hot medium. Finally, a broad collisionally ionized component of gas responsible for producing the smooth N V and O VI profiles is observed; such absorption is also present to a lesser degree in the profiles of Si IV and C IV. The broad O VI absorption is observed at a velocity displaced from the broad C IV component by almost 20 km/s, an amount large enough to suggest that the two ions may not co-exist in the same physical location., Comment: 39 pages, 6 figures; accepted for publication in January 10, 2003 edition of The Astrophysical Journal

We report Far Ultraviolet Spectroscopic Explorer (FUSE) observations of interstellar O VI absorption in the halo of the Milky Way towards 12 early-type stars in the Large Magellanic Cloud (LMC) and 11 in the Small Magellanic Cloud (SMC). The mean column densities of O VI associated with the Galactic halo towards the LMC and SMC are log = 14.52(+0.10,-0.14) and 14.13(+0.14,-0.20), respectively, where the uncertainties represent the standard deviations of the individual measurements about the means. Significant variations in the O VI column densities are observed over all of the angular scales probed by our observations: 0.5-5.0 degrees towards the LMC and 0.05-3.3 degrees towards the SMC. The maximum factors by which the O VI varies between sight lines towards the LMC and SMC are ~2.8 and ~4.2, respectively. Though low-, intermediate-, and high-velocity clouds are present along most of the sight lines towards the LMC, the variations in O VI column densities are found in all of these O VI absorbing structures. The column density variations trace the structure of the hot ionized medium in the Galactic halo on scales of <<80 to 800 pc towards the LMC and <<6 to 400 pc towards the SMC (assuming the absorption arises within the first 5 kpc above the Galactic plane)., Comment: To appear in The Astrophysical Journal. 24 pages including 9 figures and 3 tables

We investigate the ionization state of the damped Lya system at z=2.62 toward GB1759+7539 through an analysis of ionic ratios sensitive to photoionization: ArI/SII, FeIII/FeII, NII/NI, AlIII/AlII. Approximately half of the metals arise in a mostly neutral velocity component with HI/H > 0.9, based on FeIII/FeII < 0.013. In contrast, the remaining half exhibits FeIII/FeII~0.3 indicative of a partially ionized medium with HI/H~0.5. These conclusions are supported by the observed NII/NI, AlIII/AlII, and ArI/SII ratios. We assess ionization corrections for the observed column densities through photoionization models derived from the CLOUDY software package. In the neutral gas, the ionization corrections are negligible except for ArI. However for the partially ionized gas, element abundance ratios differ from the ionic ratios by 0.1-0.3 dex for (SiII, SII, NiII, AlII)/FeII ratios and more for (NI, ArI)/FeII. Independent of the shape of the photoionizing spectrum and assumptions on the number of ionization phases, these ionization corrections have minimal impact (<0.1dex) on the total metallicity inferred for this damped Lya system. Measurements on the relative elemental abundances of the partially ionized gas, however, have a greater than ~0.15 dex uncertainty which hides the effects of nucleosynthesis and dust depletion. We caution the reader that this damped system is unusual for a number of reasons (e.g. a very low ArI/SII ratio) and we believe its ionization properties are special but not unique. Nevertheless, it clearly shows the value of examining photoionization diagnostics like FeIII/FeII in a larger sample of systems., Comment: 19 pages with 11 b&w figures. Visit the DLA database at http://kingpin.ucsd.edu/~hiresdla for more. Accepted to the Astrophysical Journal, Feb 5 2002

We present the results of a FUSE survey of O VI 1031.93 Angstrom and 1037.62 Angstrom absorption toward 18 OB stars in the Small Magellanic Cloud (SMC). We detect O VI along every sight line. The average value of the O VI column density in the SMC is log =14.53. This value is 1.7 times higher than the average value for the Milky Way halo (perpendicular to the Galactic plane) of log N_perp(O VI)=14.29 found by FUSE, even though the SMC has much lower metallicity than the Galaxy. The column density in the SMC is higher along sight lines that lie close to star-forming regions, in particular NGC 346 in the northern part of the SMC. The standard deviation of the column densities for sight lines outside of NGC 346 is +/- 27%, somewhat lower than the deviation seen in the Milky Way halo. The lowest O VI column densities, log N(O VI)~14.3, occur in the central region and in the southeastern ``Wing'' of the galaxy. Even these low column densities are as high as the Milky Way average, establishing the presence of a substantial, extended component of coronal gas in the SMC. The O VI absorption is always shifted to higher velocities than the main component of lower ionization gas. The O VI line widths are broader than expected for pure thermal broadening. We discuss several mechanisms that may be able to explain the observed properties of the hot gas, including supershells, a galactic fountain, and the infall of gas previously stripped from the SMC by tidal interactions with the Milky Way and the Large Magellanic Cloud. If a fountain produces the hot gas, the mass flux per unit surface area is ~0.02 M_sun/yr/kpc2., Comment: Accepted for publication in the ApJ. 23 pages, 5 figures, 5 tables

We present FUSE observations of interstellar O VI absorption towards 12 early-type stars in the Large Magellanic Cloud (LMC). O VI 1031.926 Ang absorption at LMC velocities is seen towards all 12 stars. The observed columns are in the range log N(O VI)=13.9 to 14.6, with a mean of 14.37. The observations probe several sight lines projected onto known superbubbles, but these show relatively little (if any) enhancement in O VI column density compared to sight lines towards relatively quiescent regions of the LMC. The observed LMC O VI absorption is broad, with Gaussian dispersions of 30 to 50 km/sec, implying temperatures T<(2-5)x10^6 K. The O VI absorption is typically displaced -30 km/sec from the corresponding low-ionization absorption associated with the bulk of the LMC gas. The properties of the LMC O VI absorption are very similar to those of the Milky Way halo. The average column density of O VI and the dispersion of the individual measurements about the mean are identical to those measured for the halo of the Milky Way, even though the metallicity of the LMC is a factor of ~2.5 lower than the Milky Way. Much of the LMC O VI may arise in a vertically-extended distribution similar to the Galactic halo. If the observed O VI absorption is tracing a radiatively cooling galactic fountain flow, the mass flow rate from one side of the LMC disk is of the order 1 Msun/yr, with a mass flux per unit area of the disk ~0.02 Msun/yr/kpc^2. (abridged), Comment: Accepted for publiction in the ApJ. 39 pages, including 9 figures and 6 tables. Version with full resolution figures available at http://fuse.pha.jhu.edu/~howk/Papers/

We present an atlas of 57 Large Magellanic Cloud (LMC) and 37 Small Magellanic Cloud (SMC) observations obtained with the Far Ultraviolet Spectroscopic Explorer (FUSE) satellite. The atlas highlights twelve interstellar absorption line transitions at a resolution of ~15 km/s. These transitions cover a broad range of temperatures, ionization states, and abundances. The species included are OVI, which probes hot (T~3x10^5 K) ionized gas; CIII and FeIII, which probe warm (T~10^4 K) ionized gas; SiII, PII, CII, FeII, and OI, warm neutral gas; and six different molecular hydrogen transitions, which trace cold (T<=500 K) gas. We include Schmidt Halpha CCD images of the region surrounding each sight line showing the morphology of warm ionized gas in the vicinity, along with continuum images near each FUSE aperture position. Finally, we present several initial scientific results derived from this dataset on the interstellar medium of the Magellanic Clouds and Galactic halo., Comment: 29 pages, 6 figures. Complete Atlas of 94 additional images (~800kB each) is available at http://fuse.pha.jhu.edu/~danforth/atlas Accepted to the ApJS March 2002

I summarize Far Ultraviolet Spectroscopic Explorer (FUSE) observations of interstellar O VI absorption towards 12 early-type stars in the Large Magellanic Cloud (LMC), the closest disk galaxy to the Milky Way. LMC O VI is seen towards all 12 stars with properties (average column densities, kinematics) very similar to those of the Milky Way halo, even though O/H in the LMC is lower by a factor of ~2.5. Sight lines projected onto known LMC superbubbles show little enhancement in O VI column density compared to sight lines towards quiescent regions of the LMC. The O VI absorption is displaced by \~-30 km/sec from the corresponding low-ionization absorption associated with the bulk of the LMC gas. The LMC O VI most likely arises in a vertically-extended distribution, and I discuss the measurements in the context of a halo composed of radiatively-cooling hot gas. In this case, the mass-flow rate from one side of the LMC disk is of the order 1 solar mass/yr., Comment: 4 pages, including figures. To appear in the Proceedings of the XVIIth IAP Colloquium: Gaseous Matter in Galactic and Intergalactic Space

We report a detection of far ultraviolet absorption from the supernova remnant SNR 0057 - 7226 in the Small Magellanic Cloud (SMC). The absorption is seen in the Far Ultraviolet Spectroscopic Explorer (FUSE) spectrum of the LBV/WR star HD 5980. Absorption from O VI 1032 and C III 977 is seen at a velocity of +300 km/s with respect to the Galactic absorption lines, +170 km/s with respect to the SMC absorption. The O VI 1038 line is contaminated by H_2 absorption, but is present. These lines are not seen in the FUSE spectrum of Sk80, only ~1' (~17 pc) away from HD 5980. No blue-shifted O VI 1032 absorption from the SNR is seen in the FUSE spectrum. The O VI 1032 line in the SNR is well described by a Gaussian with FWHM=75 km/s. We find log N(O VI)=14.33-14.43, which is roughly 50% of the rest of the O VI column in the SMC (excluding the SNR) and greater than the O VI column in the Milky Way halo along this sight line. The N(C IV)/N(O VI) ratio for the SNR absorption is in the range of 0.12-0.17, similar to the value seen in the Milky Way disk, and lower than the halo value, supporting models in which SNRs produce the highly ionized gas close to the plane of the Galaxy, while other mechanisms occur in the halo. The N(C IV)/N(O VI) ratio is also lower than the SMC ratio along this sight line, suggesting that other mechanisms contribute to the creation of the global hot ionized medium in the SMC. The O VI, C IV, and Si IV apparent column density profiles suggest the presence of a multi-phase shell followed by a region of higher temperature gas., Comment: 7 pages, 3 figures, 2 tables, uses emulateapj5.sty. Accepted for publication in ApJ Letters

We analyze extant data of Al++, Al+ and other low ions with the aim of studying the ionization properties of Damped Ly alpha systems (DLAs) from the analysis of the ratio R(Al++/Al+) = N(Al++)/N(Al+). We find good correlations logN(Al+)-logN(Si+) and logN(Al+)-logN(Fe+) that we use to indirectly estimate N(Al+) from N(Si+) and/or N(Fe+) measurements. In this way we determine the ratio R(Al++/Al+) for a sample of 20 DLAs. Contrary to common belief, the ratio can attain relatively high values (up to 0.6), suggesting that gas of intermediate ionization state plays an important role in DLAs. On the other hand, the lack of any trend between abundance ratios, such as Si/H and Si/Fe, and R(Al++/Al+) indicates that abundances are not severely influenced by ionization effects. We find a logR(Al++/Al+) - logN(H I) anticorrelation that we use, in conjunction with idealized photoionization equilibrium calculations, to constrain the ionization properties and to predict ionization corrections in DLAs. We consider two possible origins for the species of low and intermediate ionization state: (1) neutral regions devoid of Al++ and/or (2) partially ionized, (Al++)-bearing regions. The logR(Al++/Al+) - logN(H I) anticorrelation can be naturally explained in terms of a two-region model with a soft, stellar-type ionizing radiation field. We present abundance ionization corrections for 14 elements of astrophysical interest derived with different types of ionizing spectra. For most of these elements the corrections are generally below measurements errors, contrary to the predictions of recent models presented in the literature. We briefly discuss the potential effects of inaccuracies of the Al recombination rates used in the photoionization calculations., Comment: 20 pages including 16 figures, accepted for publication in the Astrophysical Journal, Vol. 557, No. 1, Aug 10, 2001

We present Far Ultraviolet Spectroscopic Explorer observations of the atomic and molecular absorption in high velocity cloud HVC 287.5+22.5+240, which lies in front of the ultraviolet-bright nucleus of the Seyfert 1 galaxy NGC 3783. We detect H2, N I, N II, Si II, and Fe II absorption and set limits on the amount of absorption due to P III, Ar I, and Fe III. We extend the earlier metallicity and dust-depletion measurements made by Lu and collaborators by examining the relative gas-phase abundances of Si, P, S, and Fe. Corrections to the derived gas-phase abundances due to ionized gas in the HVC are less than 15%. The HVC has a metallicity of 0.2-0.4 solar, similar to that of the Small Magellanic Cloud. The relative abundance pattern for the elements studied resembles that of warm gas in the SMC, which supports the idea that this HVC is part of the tidally stripped Leading Arm of the Magellanic Stream. The abundance pattern implies that the HVC contains dust grains that have been processed significantly. It is likely that the grain mantles have been modified or stripped back to expose the grain cores. We have identified more than 30 lines of H2 arising in the HVC from rotational levels J = 0 to J = 3. Synthetic spectra and a curve-of-growth fit to these lines with b = 12 km/sec indicate that log N(H2) = 16.80+/-0.10 and f(H2) = 2N(H2)/[N(H I)+2N(H2)] = 0.0016. From an analysis of the H2 rotational populations, we find an absorption rate (at 1000 A) that is less than one-tenth the average value in the solar neighborhood. The presence of molecular gas in the HVC requires that either the H2 formed in situ or that molecules formed within the SMC survived tidal stripping. We favor the latter possibility because of the long formation time derived for molecules in this HVC., Comment: 28 pages (includes 6 figures). AASTeX preprint format. Accepted for publication in the February 2001 issue of the Astronomical Journal

We report empirical determinations of atomic oscillator strengths, or f-values, for 11 ground-state transitions of Fe II in the wavelength range 1050 to 1150 Ang. We use ultraviolet absorption line observations of interstellar material towards stars in the Galaxy and the Magellanic Clouds taken with Copernicus, the Goddard High Resolution Spectrograph on-board the Hubble Space Telescope, and the Far Ultraviolet Spectroscopic Explorer. We derive absolute oscillator strengths by a combination of the apparent optical depth, component fitting, and curve-of-growth fitting techniques. Our derived oscillator strengths are generally in excellent agreement with recent theoretical calculations by Raassen & Uylings using the orthogonal operator technique. However, we identify three of the eleven transitions studied here whose f-values seem to be incompatible with these calculations, by as much as a factor of two. We suggest revisions to these f-values based upon our analysis., Comment: To appear in the Astrophysical Journal. 31 pages, including 8 embedded tables and 10 embedded figures. Also available at http://fuse.pha.jhu.edu/~howk/Papers/

We use new Space Telescope Imaging Spectrograph (STIS) and archival Goddard High Resolution Spectrograph (GHRS) observations to study interstellar B II 1362 and O I 1355 absorption along seven sightlines. Our new column density measurements, combined with measurements of four sightlines from the literature, allow us to study the relative B/O abundances over a wide range of interstellar environments. We measure sightline-integrated relative gas-phase abundances in the range [B/O] = -1.00 to -0.17, and our data show the B/O abundances are anticorrelated with average sightline densities over the range log ~ -1.3 to +0.7. Detailed comparisons of the B II and O I line shapes show that the B/O ratio is significantly higher in warm interstellar clouds than in cool clouds. These results are consistent with the incorporation of boron into dust grains in the diffuse ISM. Since boron is likely incorporated into grains, we derive a lower limit to the present-day total (gas+dust) interstellar boron abundance of B/H > (2.5+/-0.9)x10^-10. The effects of dust depletion and ionization differences from element to element will make it very difficult to reliably determine 11B/10B along most interstellar sightlines., Comment: Accepted for publication in The Astrophysical Journal. 13 pages, including 2 tables and 4 figures. Also available at http://fuse.pha.jhu.edu/~howk/Papers/

We present a simple, effective approach for estimating the on-order backgrounds of spectra taken with the highest-resolution modes of the Space Telescope Imaging Spectrograph (STIS) on-board the Hubble Space Telescope. Our scheme for determining the on-order background spectrum uses polynomial fits to the inter-order scattered light visible in the two-dimensional STIS MAMA images. We present a suite of high-resolution STIS spectra to demonstrate that our background subtraction routine produces the correct overall zero point, as judged by the small residual flux levels in the centers of strongly-saturated interstellar absorption lines. Although there are multiple sources of background light in STIS echelle mode data, this simple approach works very well for wavelengths longward of Lyman-alpha. At shorter wavelengths, the smaller order separation and generally lower signal-to-noise ratios of the data can reduce the effectiveness of our background estimation procedure. Slight artifacts in the background-subtracted spectrum can be seen in some cases, particularly at wavelengths <1300 Ang. Most of these are caused by echelle scattering of strong spectral features into the inter-order light. We discuss the limitations of high-resolution STIS data in light of the uncertainties associated with our background subtraction procedure. We compare our background-subtracted STIS spectra with GHRS Ech-A observations of the DA white dwarf G191-B2B and GHRS first-order G160M observations of the early-type star HD 218915. We find no significant differences between the GHRS data and the STIS data reduced with our method in either case., Comment: To appear in The Astronomical Journal; 37 pages, including 2 embedded tables and 18 embedded figures. Version with high-resolution figures can be found at http://fuse.pha.jhu.edu/~howk/Papers/ . Software described in this paper is available to the general community; contact the authors for information

Observations of the B0.2V star tau Scorpii have revealed unusual stellar wind characteristics: red-shifted absorption in the far-ultraviolet O VI resonance doublet up to +250 km/s, and extremely hard X-ray emission implying gas at temperatures in excess of 10^7 K. We describe a phenomenological model to explain these properties. We assume the wind of tau Sco consists of two components: ambient gas in which denser clumps are embedded. The clumps are optically thick in the UV resonance lines primarily responsible for accelerating the ambient wind. The reduced acceleration causes the clumps to slow and even infall, all the while being confined by the ram pressure of the outflowing ambient wind. We calculate detailed trajectories of the clumps in the ambient stellar wind, accounting for a line radiation driving force and the momentum deposited by the ambient wind in the form of drag. We show these clumps will fall back towards the star with velocities of several hundred km/sec for a broad range of initial conditions. The infalling clumps produce X-ray emitting plasmas with temperatures in excess of (1-6)x10^7 K in bow shocks at their leading edge. The infalling material explains the peculiar red-shifted absorption wings seen in the O VI doublet. The required mass loss in clumps is 3% - 30% ofthe total mass loss rate. The model developed here can be generally applied to line-driven outflows with clumps or density irregularities. (Abstract Abridged), Comment: To appear in the ApJ (1 May 2000). 24 pages, including 6 embedded figures

We present new, deep optical images (BVI+H-alpha) of the interstellar medium (ISM) far above the plane of NGC 891. These sub-arcsecond images give a direct visual view of two physically distinct ``phases'' of the thick interstellar disk of this galaxy. A dense phase of the thick disk ISM is observed in our BVI images as highly-structured dust-bearing clouds viewed against the stellar light of the galaxy. These structures are traceable to heights |z|=2 kpc from the midplane. Very few highly-structured dust features are present at |z|>2 kpc. The more prominent dust structures have gas masses in excess of 10^5 solar masses, each having visual extinctions well in excess of unity. A warm ionized phase of the high-z ISM is observed through its well-studied H-alpha emission. Our images of the well-studied diffuse ionized medium, to date the highest-resolution observations of this phase of the ISM in NGC 891, show it is relatively smoothly distributed with some filamentary structure superposed on this smooth background. There is little correspondence between the H-alpha emitting material and the absorbing dust structures. These two phases of the multiphase high-z ISM are physically distinct. The H-alpha emission is being heavily extincted in many places by the dense dust-bearing medium. Our H-alpha observations show evidence for several discrete H II regions at large distances from the midplane (to |z|=2 kpc). The presence of these H II regions in the thick disk of NGC 891 suggests that on-going star formation may be present in some of the dense, high-z clouds visible in our images. (Abstract Abridged), Comment: Accepted for publication in The Astronomical Journal. 38 pages, including 6 embedded tables and 14 embedded figures. Interested readers are strongly encouraged to download a version with high-resolution figures from http://fuse.pha.jhu.edu/~howk/Papers/papers.html

Recent optical and submillimeter observations have begun to probe the existence of dust grains in the halos of spiral galaxies. I review our own work in this area which employs high-resolution optical images of edge-on spiral galaxies to trace high-z dust in absorption against the background stellar light of the galaxies. We have found that a substantial fraction of such galaxies (>50%) show extensive webs of dust-bearing clouds to heights z>2 kpc. Extraplanar dust in galaxies is statistically correlated with extraplanar diffuse ionized gas, though there is no evidence for a direct, physical relationship between these two phases of the high-z interstellar medium. The dense high-z clouds individually have masses estimated to be >10^5} to 10^6 solar masses. The detailed properties of the observed dust structures suggest the clouds seen in our images may represent the dense phase of a multiphase ISM at high-z. Such dense clouds can have an important effect on the observed light distribution in spiral galaxies. I discuss the effects such high-z dust can have on quantitative measures of the vertical structure of stars and ionized gas in edge-on systems., Comment: To appear in "Toward a New Millennium in Galaxy Morphology," eds. D.L. Block, I. Puerari, A. Stockton, & D. Ferreira (Kluwer, Dordrecht). 8 pages; 3 embedded figures. Version with high-resolution figures available at http://fuse.pha.jhu.edu/~howk/Papers/papers.html

We present model calculations of ionization fractions for elements in the warm (log T = 4), low-density photoionized interstellar medium (WIM) of the Milky Way. We model the WIM as a combination of overlapping low-excitation H II regions having n(HII)/n(H) > 0.8. Our standard model incorporates an intrinsic elemental abundance pattern similar to that found for warm neutral clouds in the Galaxy and includes the effects of interstellar dust grains. The radiation field is characterized by an ionizing spectrum of a star with T ~ 35,000 K and an ionization parameter log(q) ~ -4.0. The emergent emission line strengths are in agreement with the observed ratios of [SII]/Halpha, [NII]/Halpha, [SII]/[NII], [OI]/Halpha, [OIII]/Halpha, and He I/Halpha in the Galactic WIM. Although the forbidden emission-line intensities depend strongly upon the input model parameters, the ionization fractions of the 20 elements studied in this work are robust over a wide range of physical conditions considered in the models. These ionization fractions have direct relevance to absorption-line determinations of the elemental abundances in the warm neutral and ionized gases in the Milky Way and other late-type galaxies. We demonstrate a method for estimating the WIM contributions to the observed column densities of singly and doubly ionized atoms used to derive abundances in the warm neutral gas. We apply this approach to study the gas-phase abundances of the warm interstellar clouds toward the halo star HD 93521., Comment: To appear in The Astrophysical Journal. 20 pages + 8 tables and 9 figures. Uses aaspp4 or aasms4 style files

Recent high-resolution observations of metal absorption lines in high-redshift damped Ly-alpha systems have shown that Al III, a tracer of moderately-ionized gas, very often has a velocity structure indistinguishable from that of low-ionization gas. Regions of ionized and neutral hydrogen in these systems are likely cospatial. The higher-ionization Si IV and C IV absorption shows a much weaker or non-existent correlation with the low ionization material, implying that the regions traced by Al III are photoionized by a soft (stellar) spectrum, by a hard (power law) spectrum with a very low ionization parameter, or a combination of both. We discuss the ionization of the damped Ly-alpha systems and use photoionization equilibrium models to make quantitative estimates of its effects on abundance studies in these systems. We show that ionization effects may be large enough to account for the observed dispersion in absolute metal abundances in damped Ly-alpha systems, causing systematically higher abundances in lower column density systems. The observed Si^+/Fe^+ and Zn^+/Cr^+ ratios may systematically overestimate the intrinsic Si/Fe and Zn/Cr ratios, respectively, if ionized gas is present in these systems, thereby mimicking the effects of alpha-element enrichment or dust depletion., Comment: To appear in The Astrophysical Journal Letters. 10 pages, including 3 embedded figures

We present ultraviolet interstellar absorption line measurements for the sightline towards the O9.5 V star mu Columbae obtained with the Goddard High Resolution Spectrograph (GHRS) on board the Hubble Space Telescope. These archival data represent the most complete GHRS interstellar absorption line measurements for any line of sight towards an early-type star. The 3.5 km/s resolution of the instrument allow us to accurately derive the gas-phase column densities of many important ionic species in the diffuse warm neutral medium using a combination of apparent column density and component fitting techniques, and we study in detail the contamination from ionized gas along this sightline. The low-velocity material shows gas-phase abundance patterns similar to the warm cloud (cloud A) towards the disk star zeta Oph, while the component at v = +20.1 km/s shows gas-phase abundances similar to those found in warm halo clouds. We find the velocity-integrated gas-phase abundances of Zn, P, and S relative to H along this sightline are indistinguishable from solar system abundances. We discuss the implications of our gas-phase abundance measurements for the composition of interstellar dust. The relative ionic column density ratios of the intermediate velocity components show the imprint both of elemental incorporation into grains and (photo)ionization. The components at v = -30 and -48 km/s along this sightline likely trace shocked gas with very low hydrogen column densities. Appendices include a new derivation of the GHRS instrumental line spread function, and a new very accurate determination of the total H I column along this sightline. (Abridged), Comment: Accepted for publication in the Astrophysical Journal. 80 pages including 19 embedded figures and 12 embedded tables. Version with higher resolution figures can be downloaded from http://fuse.pha.jhu.edu/~howk/Papers/papers.html

We present high resolution BV images of 12 edge-on spiral galaxies observed with the WIYN 3.5-m telescope. These images were obtained to search for extraplanar (|z| > 0.4 kpc) absorbing dust structures similar to those previously found in NGC 891 (Howk & Savage 1997). Our imaged galaxies include a sample of seven massive L_*-like spiral galaxies within D<25 Mpc that have inclinations i > 87 deg from the plane of the sky. We find that five of these seven systems show extraplanar dust, visible as highly-structured absorbing clouds against the background stellar light of the galaxies. The more prominent structures are estimated to have associated gas masses >10^5 M_sun; the implied potential energies are > 10^(52) ergs. All of the galaxies in our sample that show detectable halpha emission at large z also show extraplanar dust structures. None of those galaxies for which extraplanar halpha searches were negative show evidence for extensive high-z dust. The existence of extraplanar dust is a common property of massive spiral galaxies. We discuss several mechanisms for shaping the observed dust features, emphasizing the possibility that these dusty clouds represent the dense phase of a multiphase medium at high-z in spiral galaxies. The correlation between high-z dust and extraplanar Halpha emission may simply suggest that both trace the high-z interstellar medium in its various forms (or phases), the existence of which may ultimately be driven by vigorous star formation in the underlying disk. (Abstract abridged), Comment: 26 pages; 15 jpeg figures. To appear in The Astronomical Journal, May 1999. Gzipped tar files of high-resolution figures in postscript and jpeg formats are available at http://www.astro.wisc.edu/~howk/Papers/papers.html#survey