K. R. Sembach

Overview of the Far Ultraviolet Spectroscopic Explorer Mission

The Far Ultraviolet Spectroscopic Explorer satellite observes light in the far-ultraviolet spectral region, 905-1187 Angstrom, with a high spectral resolution. The instrument consists of four co-aligned prime-focus telescopes and Rowland spectrographs with microchannel plate detectors. Two of the telescope channels use Al :LiF coatings for optimum reflectivity between approximately 1000 and 1187 Angstrom, and the other two channels use SiC coatings for optimized throughput between 905 and 1105 Angstrom. The gratings are holographically ruled to correct largely for astigmatism and to minimize scattered light. The microchannel plate detectors have KBr photocathodes and use photon counting to achieve good quantum efficiency with low background signal. The sensitivity is sufficient to examine reddened lines of sight within the Milky Way and also sufficient to use as active galactic nuclei and QSOs for absorption-line studies of both Milky Way and extragalactic gas clouds. This spectral region contains a number of key scientific diagnostics, including O VI, H I, D I, and the strong electronic transitions of H-2 and HD.

A Far Ultraviolet Spectroscopic Explorer Survey of Interstellar Molecular Hydrogen in the Small and Large Magellanic Clouds

We describe a moderate-resolution Far Ultraviolet Spectroscopic Explorer (FUSE) survey of H2 along 70 sight lines to the Small and Large Magellanic Clouds, using hot stars as background sources. FUSE spectra of 67% of observed Magellanic Cloud sources (52% of LMC and 92% of SMC) exhibit absorption lines from the H2 Lyman and Werner bands between 912 and 1120 A. Our survey is sensitive to N(H2) ≥ 1014 cm-2; the highest column densities are log N(H2) = 19.9 in the LMC and 20.6 in the SMC. We find reduced H2 abundances in the Magellanic Clouds relative to the Milky Way, with average molecular fractions = 0.010 for the SMC and = 0.012 for the LMC, compared with = 0.095 for the Galactic disk over a similar range of reddening. The dominant uncertainty in this measurement results from the systematic differences between 21 cm radio emission and Lyα in pencil beam sight lines as measures of N(H I). These results imply that the diffuse H2 masses of the LMC and SMC are 8 × 106 and 2 × 106 M☉, respectively, 2% and 0.5% of the H I masses derived from 21 cm emission measurements. The LMC and SMC abundance patterns can be reproduced in ensembles of model clouds with a reduced H2 formation rate coefficient, R ~ 3 × 10-18 cm3 s-1, and incident radiation fields ranging from 10-100 times the Galactic mean value. We find that these high-radiation, low formation rate models can also explain the enhanced N(4)/N(2) and N(5)/N(3) rotational excitation ratios in the Clouds. We use H2 column densities in low rotational states (J = 0 and 1) to derive kinetic and/or rotational temperatures of diffuse interstellar gas, and we find that the distribution of rotational temperatures is similar to Galactic gas, with T01 = 82 ± 21 K for clouds with N(H2) ≥ 1016.5 cm-2. There is only a weak correlation between detected H2 and far-infrared fluxes as determined by IRAS, perhaps as a result of differences in the survey techniques. We find that the surface density of H2 probed by our pencil beam sight lines is far lower than that predicted from the surface brightness of dust in IRAS maps. We discuss the implications of this work for theories of star formation in low-metallicity environments.

On the Escape of Ionizing Radiation from Starbursts

Far-ultraviolet spectra obtained with FUSE show that the strong C II λ1036 interstellar absorption line is essentially black in five of the UV-brightest local starburst galaxies. Because the opacity of the neutral ISM below the Lyman edge will be significantly larger than in the C II line, these data provide strong constraints on the escape of ionizing radiation from these starbursts. Interpreted as a uniform, absorbing slab, the implied optical depth at the Lyman edge is huge (τ0 ≥ 102). Alternatively, the areal covering factor of opaque material is typically ≥94%. Thus, the fraction of ionizing stellar photons that escape the ISM of each galaxy is small: our conservative estimates typically yield fesc ≤ 6%. Inclusion of extinction due to dust will further decrease fesc. An analogous analysis of the rest-UV spectrum of the star-forming galaxy MS 1512-cB58 at z = 2.7 leads to similar constraints on fesc. These new results agree with the constraints provided by direct observations below the Lyman edge in a few other local starbursts. However, they differ from the recently reported properties of star-forming galaxies at z ≥ 3. We assess the idea that the strong galactic winds seen in many powerful starbursts clear channels through their neutral ISM. We show empirically that such outflows may be a necessary—but not sufficient—part of the process of creating a relatively porous ISM. We note that observations will soon document the cosmic evolution in the contribution of star-forming galaxies to the metagalactic ionizing background, with important implications for the evolution of the IGM.

FUSE Observations of Outflowing O VI in the Dwarf Starburst Galaxy NGC 1705

We report FUSE far-UV spectroscopy of the prototypical dwarf starburst galaxy NGC 1705. These data allow us for the —rst time to directly probe the coronal-phase (T a few times 105 K) gas that may dominate the radiative cooling of the supernova-heated interstellar medium (ISM) and thereby determine the dynamical evolution of the starburst-driven out—ows in dwarf galaxies. We detect a broad (D100 km s~1 FWHM) and blueshifted (*v 77 km s~1 )O VI j1032 absorption line arising in the previously known galactic out—ow. The mass and kinetic energy in the out—ow we detect is dominated by the warm (T D 104 K) photoionized gas which is also seen through its optical line emission. The kinematics of this warm gas are compatible with a simple model of the adiabatic expansion of a superbubble driven by the collective eUect of the kinetic energy supplied by supernovae in the starburst. However, the observed properties of the O VI absorption in NGC 1705 are not consistent with the simple superbubble model, in

Far Ultraviolet Spectroscopic Explorer and Space Telescope Imaging Spectrograph Observations of Intervening O VI Absorption Line Systems in the Spectrum of PG 0953+415* ** ***

We present Far Ultraviolet Spectroscopic Explorer (FUSE) and Space Telescope Imaging Spectrograph (STIS) observations of the intergalactic medium toward the bright QSO PG 0953+415 (zem = 0.239). The FUSE spectra extend from 905 to 1187 A and have a resolution of 25 km s-1, while the STIS spectra cover 1150-1730 A and have a resolution of 7 km s-1. Additional STIS observations at 30 km s-1 are obtained in selected wavelength ranges. An O VI system at z = 0.06807 is detected in H I Lyα, Lyβ, Lyγ, O VI λλ1031.93, 1037.62, N V λλ1238.82, 1242.80, C IV λλ1548.20, 1550.77, and C III λ977.02. The observed column densities can be modeled as a low-density intervening gas with a metallicity of 0.4 times solar in photoionization equilibrium with the ionizing extragalactic background radiation. The best fit is achieved with an ionization parameter, log U = -1.35, which implies nH ~ 10-5 cm-3 and a path length of ~80 kpc through the absorbing gas. H I Lyα absorption at z = 0.14232 spans a velocity range of 410 km s-1 with the strongest components near 0 and 80 km s-1 in the z = 0.14232 rest frame. In this system, O VI λλ1031.93, 1037.62 absorption is strong near 0 km s-1 and not detected at 80 km s-1. C III λ977.02 absorption is marginally detected at 80 km s-1 but is not detected at 0 km s-1. The observations place constraints on the properties of the z = 0.14232 system but do not discriminate between collisional ionization in hot gas versus photoionization in a very low density medium with an ionization parameter log U > -0.74. The z = 0.06807 and 0.14232 O VI systems occur at redshifts where there are peaks in the number density of intervening galaxies along the line of sight determined from WIYN redshift measurements of galaxies in the ~1° field centered on PG 0953+415. We combine our observations of PG 0953+415 with those for other QSOs to update the estimate of the low-redshift number density of intervening O VI systems. Over a total unobscured redshift path of Δz = 0.43, we detect six O VI systems with rest-frame equivalent widths of the O VI λ1031.93 line exceeding 50 mA, yielding dN/dz = 14 for z = 0.09. This implies a low-redshift value of the baryonic contribution to the closure density of the O VI systems of Ωb(O VI) > 0.002 h, assuming that the average metallicity in the O VI systems is 0.1 solar.

On the Physical Origin of O VI Absorption-Line Systems

We present a unified analysis of the O VI absorption lines seen in the disk and halo of the Milky Way, high-velocity clouds, the Magellanic Clouds, starburst galaxies, and the intergalactic medium. We show that these disparate systems define a simple relationship between the O VI column density and absorption-line width that is independent of the oxygen abundance over the range O/H ~ 10% to twice solar. We show that this relation is exactly that predicted theoretically as a radiatively cooling flow of hot gas passes through the coronal temperature regime?independent of its density or metallicity (for O/H 0.1 solar). Since most of the intergalactic O VI clouds obey this relation, we infer that they cannot have metallicities less than a few percent solar. In order to be able to cool radiatively in less than a Hubble time, the intergalactic clouds must be smaller than ~1 Mpc in size. The implied global heating rate of the warm/hot intergalactic medium is consistent with available sources. We show that the cooling column densities for the O IV, O V, Ne V, and Ne VI ions are comparable to those seen in O VI. This is also true for the Li-like ions Ne VIII, Mg X, and Si XII (if the gas is cooling from T 106 K). All these ions have strong resonance lines in the extreme-ultraviolet spectral range and would be accessible to Far Ultraviolet Spectroscopic Explorer at z 0.2-0.8. We also show that the Li-like ions can be used to probe radiatively cooling gas at temperatures 1 order of magnitude higher than where their ionic fraction peaks. We calculate that the H-like (He-like) O, Ne, Mg, Si, and S ions have cooling columns of ~1017 (a few times 1016) cm-2. The properties of the O VII, O VIII, and Ne IX X-ray absorption lines toward PKS 2155-304 may be consistent with a scenario of radiatively cooling gas in the Galactic disk or halo.

Abundances of Deuterium, Nitrogen, and Oxygen in the Local Interstellar Medium: Overview of First Results from the FUSE Mission

Observations obtained with the Far Ultraviolet Spectroscopic Explorer (FUSE) have been used to determine the column densities of D i ,N i, and O i along seven sight lines that probe the local interstellar medium (LISM) at distances from 37 to 179 pc. Five of the sight lines are within the Local Bubble, and two penetrate the surrounding H i wall. Reliable values of N(H i) were determined for five of the sight lines from Hubble Space Telescope (HST) data, International Ultraviolet Explorer (IUE) data, and published Extreme Ultraviolet Explorer (EUVE) measurements. The weighted mean of D i/H i for these five sight lines is ð1:52 � 0:08 Þ� 10 � 5 (1 � uncertainty in the mean). It is likely that the D i/H i ratio in the Local Bubble has a single value. The D i/O i ratio for the five sight lines within the Local Bubble is ð3:76 � 0:20 Þ� 10 � 2 .I t is likely that O i column densities can serve as a proxy for H i in the Local Bubble. The weighted mean for O i/H i for the seven FUSE sight lines is ð3:03 � 0:21 Þ� 10 � 4 , comparable to the weighted mean ð3:43 � 0:15 Þ� 10 � 4 reported for 13 sight lines probing larger distances and higher column densities. The FUSE weighted mean of N i/H i for five sight lines is half that reported by Meyer and colleagues for seven sight lines with larger distances and higher column densities. This result combined with the variability of O i/N i (six sight lines) indicates that at the low column densities found in the LISM, nitrogen ionization balance is important. Thus, unlike O i ,N i cannot be used as a proxy for H i or as a metallicity indicator in the LISM. Subject headings: cosmology: observations — Galaxy: abundances — ISM: abundances — ISM: evolution — ultraviolet: ISM

Resolving the Structure of Ionized Helium in the Intergalactic Medium with the Far Ultraviolet Spectroscopic Explorer

The neutral hydrogen (H i) and ionized helium (Heii) absorption in the spectra of quasars are unique probes of structure in the early universe. We present Far-Ultraviolet Spectroscopic Explorer observations of the line of sight to the quasar HE2347-4342 in the 1000 to 1187 angstrom band at a resolving power of 15,000. We resolve the He ii Lyman α (Lyα) absorption as a discrete forest of absorption lines in the redshift range 2.3 to 2.7. About 50 percent of these features have H icounterparts with column densities N Hi > 1012.3 per square centimeter that account for most of the observed opacity in He iiLyα. The He ii to H i column density ratio ranges from 1 to >1000, with an average of ∼80. Ratios of <100 are consistent with photoionization of the absorbing gas by a hard ionizing spectrum resulting from the integrated light of quasars, but ratios of >100 in many locations indicate additional contributions from starburst galaxies or heavily filtered quasar radiation. The presence of He ii Lyα absorbers with no H icounterparts indicates that structure is present even in low-density regions, consistent with theoretical predictions of structure formation through gravitational instability.

Far-Ultraviolet Spectroscopy of the Intergalactic and Interstellar Absorption toward 3C 273

We present Far Ultraviolet Spectroscopic Explorer observations of the molecular, neutral atomic, weakly ionized, and highly ionized components of the interstellar and intergalactic material toward the quasar 3C 273. We identify Lyb absorption in eight of the known intergalactic Lya absorbers along the sight line with rest-frame equivalent widths Re—ned estimates of the H I column den- W r (Lya) Z 50 mAe . sities and Doppler parameters (b) of the clouds are presented. We —nd a range of b B 16¨46 km s~1. We detect multiple H I lines (LybLyh) in the 1590 km s~1 Virgo absorber and estimate log N(H I) which is 10 times more H I than in all of the other absorbers along the sight line combined. 15.85 ^ 0.08, The Doppler width of this absorber, b B 16 km s~1, implies K. We detect O VI absorption T ( 15,000 at 1015 km s~1 at the 2¨3 p level that may be associated with hot, X-rayemitting gas in the Virgo Cluster. We detect weak C III and O VI absorption in the intergalactic medium at z 0.12007; this absorber is predominantly ionized and has N(H)/N(H I) 4000Z~1, where Z is the metallicity. Strong Galactic interstellar O VI is present between (100 and 100 km s~1, with an additional high-velocity wing con- taining about 13% of the total O VI between D100 and D240 km s~1. The Galactic O VI ,N V, and C IV lines have similar shapes, with roughly constant ratios across the (100 to 100 km s~1 velocity range. The high-velocity O VI wing is not detected in other species. Much of the interstellar high ion absorption probably occurs within a highly fragmented medium within the Loop IV remnant or in the outer cavity walls of the remnant. Multiple hot gas production mechanisms are required. The broad O VI absorption wing likely traces the expulsion of hot gas out of the Galactic disk into the halo. A —ux limit of 5.4 ) 10~16 ergs cm~2 s~1 on the amount of diUuse O VI emission present oU the 3C 273 sight

Deuterium Abundance toward WD 2211–495: Results from the FUSE Mission*

We present a deuterium abundance analysis of the line of sight toward the white dwarf WD 2211-495 observed with the Far Ultraviolet Spectroscopic Explorer (FUSE). Numerous interstellar lines are detected on the continuum of the stellar spectrum. A thorough analysis was performed through the simultaneous fit of interstellar absorption lines detected in the four FUSE channels of multiple observations with different slits. We excluded all saturated lines in order to reduce possible systematic errors on the column density measurements. We report the determination of the average interstellar D/O and D/N ratios along this line of sight at the 95% confidence level: D/O = (4.0 ? 1.2) ? 10-2 and D/N = (4.4 ? 1.3) ? 10-1. In conjunction with FUSE observations of other nearby sight lines, the results of this study will allow a deeper understanding of the present-day abundance of deuterium in the local interstellar medium and its evolution with time.