J. M. Shull

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.

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

We report the results of a FUSE study of high-velocity O VI absorption along complete sight lines through the Galactic halo in directions toward 100 extragalactic objects and two halo stars. The high-velocity O VI traces a variety of phenomena, including tidal interactions with the Magellanic Clouds, accretion of gas, outflowing material from the Galactic disk, warm/hot gas interactions in a highly extended Galactic corona, and intergalactic gas in the Local Group. We identify 84 high-velocity O VI features at ≥3 σ confidence at velocities of -500 106 K), low-density (n 10-4-10-5 cm-3) Galactic corona or Local Group medium. The existence of a hot, highly extended Galactic corona or Local Group medium and the prevalence of high-velocity O VI are consistent with predictions of current galaxy formation scenarios. Distinguishing between the various phenomena producing high-velocity O VI in and near the Galaxy will require continuing studies of the distances, kinematics, elemental abundances, and physical states of the different types of high-velocity O VI found in this study. Descriptions of galaxy evolution will need to account for the highly ionized gas, and future X-ray studies of hot gas in the Local Group will need to consider carefully the relationship of the X-ray absorption/emission to the complex high-velocity absorption observed in O VI.

THE GALAXY LUMINOSITY FUNCTION DURING THE REIONIZATION EPOCH

The new Wide Field Camera 3/IR observations on the Hubble Ultra-Deep Field started investigating the properties of galaxies during the reionization epoch. To interpret these observations, we present a novel approach inspired by the conditional luminosity function method. We calibrate our model to observations at z=6 and assume a non-evolving galaxy luminosity versus halo mass relation. We first compare model predictions against the luminosity function measured at z=5 and z=4. We then predict the luminosity function at z>=7 under the sole assumption of evolution in the underlying dark-matter halo mass function. Our model is consistent with the observed z>6.5 galaxy number counts in the HUDF survey and suggests a possible steepening of the faint-end slope of the luminosity function: alpha(z>8) 10^{-4}) might provide >75% of the total reionizing flux. Assuming escape fraction f_{esc}~0.2, clumping factor C~5, top heavy-IMF and low metallicity, galaxies below the detection limit produce complete reionization at z>8. For solar metallicity and normal stellar IMF, reionization finishes at z>6, but a smaller C/f_{esc} is required for an optical depth consistent with the WMAP measurement. Our model highlights that the star formation rate in sub-L_* galaxies has a quasi-linear relation to dark-matter halo mass, suggesting that radiative and mechanical feedback were less effective at z>6 than today.

Multiwavelength Observations of Short-Timescale Variability in NGC 4151. IV. Analysis of Multiwavelength Continuum Variability

For pt.III see ibid., vol.470, no.1, p.349-63 (1996). Combines data from the three preceding papers in order to analyze the multi wave-band variability and spectral energy distribution of the Seyfert 1 galaxy NGC 4151 during the 1993 December monitoring campaign. The source, which was near its peak historical brightness, showed strong, correlated variability at X-ray, ultraviolet, and optical wavelengths. The strongest variations were seen in medium-energy (~1.5 keV) X-rays, with a normalized variability amplitude (NVA) of 24%. Weaker (NVA=6%) variations (uncorrelated with those at lower energies) were seen at soft gamma-ray energies of ~100 keV. No significant variability was seen in softer (0.1-1 keV) X-ray bands. In the ultraviolet/optical regime, the NVA decreased from 9% to 1% as the wavelength increased from 1275 to 6900 Aring. These data do not probe extreme ultraviolet (1200 Aring to 0.1 keV) or hard X-ray (250 keV) variability. The phase differences between variations in different bands were consistent with zero lag, with upper limits of lsim0.15 day between 1275 Aring and the other ultraviolet bands, lsim0.3 day between 1275 Aring and 1.5 keV, and lsim1 day between 1275 and 5125 Aring. These tight limits represent more than an order of magnitude improvement over those determined in previous multi-wave-band AGN monitoring campaigns. The ultraviolet fluctuation power spectra showed no evidence for periodicity, but were instead well fitted with a very steep, red power law (ales-2.5)

Distribution and Kinematics of O VI in the Galactic Halo

Far-Ultraviolet Spectroscopic Explorer (FUSE) spectra of 100 extragalactic objects and two distant halo stars are analyzed to obtain measures of O VI λλ1031.93, 1037.62 absorption along paths through the Milky Way thick disk/halo. Strong O VI absorption over the velocity range from -100 to 100 km s-1 reveals a widespread but highly irregular distribution of O VI, implying the existence of substantial amounts of hot gas with T ~ 3 × 105 K in the Milky Way thick disk/halo. The integrated column density, log [N(O VI) cm-2], ranges from 13.85 to 14.78 with an average value of 14.38 and a standard deviation of 0.18. Large irregularities in the gas distribution are found to be similar over angular scales extending from 45°) range from -46 to 82 km s-1, with a high-latitude sample average of 0 km s-1 and a standard deviation of 21 km s-1. High positive velocity O VI absorbing wings extending from ~100 to ~250 km s-1 observed along 21 lines of sight may be tracing the flow of O VI into the halo. A combination of models involving the radiative cooling of hot fountain gas, the cooling of supernova bubbles in the halo, and the turbulent mixing of warm and hot halo gases is required to explain the presence of O VI and other highly ionized atoms found in the halo. The preferential venting of hot gas from local bubbles and superbubbles into the northern Galactic polar region may explain the enhancement of O VI in the north. If a fountain flow dominates, a mass flow rate of approximately 1.4 M⊙ yr-1 of cooling hot gas to each side of the Galactic plane with an average density of 10-3 cm-3 is required to explain the average value of log [N(O VI) sin |b|] observed in the southern Galactic hemisphere. Such a flow rate is comparable to that estimated for the Galactic intermediate-velocity clouds.

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

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

We present Far Ultraviolet Spectroscopic Explorer (FUSE) observations of the O VI λλ1031.926, 1037.617 absorption lines associated with gas in and near the Milky Way, as detected in the spectra of a sample of 100 extragalactic targets and two distant halo stars. We combine data from several FUSE Science Team programs with guest observer data that were public before 2002 May 1. The sight lines cover most of the sky above Galactic latitude |b| > 25°—at lower latitude the ultraviolet extinction is usually too large for extragalactic observations. We describe the details of the calibration, alignment in velocity, continuum fitting, and manner in which several contaminants were removed—Galactic H2, absorption intrinsic to the background target and intergalactic Lyβ lines. This decontamination was done very carefully, and in several sight lines very subtle problems were found. We searched for O VI absorption in the velocity range -1200 to 1200 km s-1. With a few exceptions, we only find O VI in the velocity range -400 to 400 km s-1; the exceptions may be intergalactic O VI. In this paper we analyze the O VI associated with the Milky Way (and possibly with the Local Group). We discuss the separation of the observed O VI absorption into components associated with the Milky Way halo and components at high velocity, which are probably located in the neighborhood of the Milky Way. We describe the measurements of equivalent width and column density, and we analyze the different contributions to the errors. We conclude that low-velocity Galactic O VI absorption occurs along all sight lines—the few nondetections only occur in noisy spectra. We further show that high-velocity O VI is very common, having equivalent width >65 mA in 50% of the sight lines and equivalent width >30 mA in 70% of the high-quality sight lines. The central velocities of high-velocity O VI components range from |vLSR| = 100 to 330 km s-1; there is no correlation between velocity and absorption strength. We discuss the possibilities for studying O VI absorption associated with Local Group galaxies and conclude that O VI is probably detected in M31 and M33. We limit the extent of an O VI halo around M33 to be 200 km s-1 occurs along all sight lines in the region l = 180°-300°, b > 20°.

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.

Multiwavelength Monitoring of the BL Lacertae Object PKS 2155?304 in 1994 May. III. Probing the Inner Jet through Multiwavelength Correlations

In 1994 May, the BL Lac object PKS 2155-304 was observed continuously for ~10 days with the International Ultraviolet Explorer and the Extreme Ultraviolet Explorer and for 2 days with ASCA, as well as with ROSAT and with ground-based radio, infrared, and optical telescopes. The light curves show a well-defined X-ray flare followed by a broader, lower amplitude extreme-ultraviolet flare ~1 day later and a broad, low-amplitude UV flare ~2 days later. X-ray fluxes obtained at three well-separated times the preceding week indicate at least one previous flare of comparable amplitude or perhaps ongoing stochastic X-ray variations, and additional rapid variability was seen at the beginning of the IUE observation, when extremely sharp changes in UV flux occurred. The X-ray flux observed with ASCA flared by a factor of ~2 in about half a day and decayed roughly as fast. In contrast, the subsequent UV flare had an amplitude of only ~35% and lasted longer than 2 days. Assuming that the X-ray, EUV, and UV events are associated, the lags, the decrease of amplitude with wavelength, and the broadening of the temporal profile with wavelength are all qualitatively as expected for synchrotron emission from an inhomogeneous, relativistic jet. Because of the high quality of the data, we can rule out that the observed flares were caused by either a Fermi-type shock acceleration event or a pair cascade in a homogeneous synchrotron-emitting region. A homogeneous region is still possible if there was an instantaneous (t hours) injection of high-energy electrons that emit first at X-ray energies. Alternatively, the data are consistent with a compression wave or other disturbance crossing a region with stratified particle energy distributions. This kind of situation is expected to occur behind a shock front and/or in an inhomogeneous jet. The present light curves are in sharp contrast to the multiwavelength variability observed in 1991 November, when the amplitude was wavelength independent and the UV lagged the X-rays by less than ~3 hr. This means that the origin of rapid multiwavelength variability in this blazar is complex, involving at least two different modes.

Constraints on the Lyman continuum radiation from galaxies: First results with FUSE on Mrk 54

We present Far Ultraviolet Spectroscopic Explorer observations of the star-forming galaxy Mrk 54 at z =0 :0448. The Lyman continuum radiation is not detected above the H i absorption edge in our Galaxy. An upper limit is evaluated by comparison with the background measured in regions of the detector adjacent to the observed spectrum. A spectral window of 16 A, reasonably free of additional H i Lyman series line absorption, is used. No correction is needed for molecular hydrogen absorption in our Galaxy but a foreground extinction of 0.29 mag is accounted for. An upper limit of 6:15 10 16 erg cm 2 s 1 A 1 is obtained for the flux at900 A in the rest frame of Mrk 54. By comparison with the number of ionizing photons derived from the H flux, this limit translates into an upper limit of fesc < 0:062 for the fraction of Lyman continuum photons that escape the galaxy without being absorbed by interstellar material. This limit compares with the limits obtained in three other nearby galaxies and is compatible with the escape fractions predicted by models. The upper limits obtained in nearby galaxies contrasts with the detection of Lyman continuum flux in the composite spectrum of Lyman-break galaxies at z 3:4. The diculties and implications of a comparison are discussed.