Arthur F. Davidsen

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.

The Rest-Frame Extreme-Ultraviolet Spectral Properties of Quasi-stellar Objects

We use a sample of 332 Hubble Space Telescope spectra of 184 quasi-stellar objects (QSOs) with z > 0.33 to study the typical ultraviolet spectral properties of QSOs, with emphasis on the ionizing continuum. Our sample is nearly twice as large as that from previous work by W. Zheng and colleagues and provides much better spectral coverage in the extreme-ultraviolet (EUV). The overall composite continuum can be described by a power law with index αEUV = -1.76 ± 0.12 (fν ∝ να) between 500 and 1200 A. The corresponding results for subsamples of radio-quiet and radio-loud QSOs are αEUV = -1.57 ± 0.17 and αEUV = -1.96 ± 0.12, respectively. We also derive αEUV for as many individual objects in our sample as possible, totaling 39 radio-quiet and 40 radio-loud QSOs. The typical individually measured values of αEUV are in good agreement with the composites. We find no evidence for evolution of αEUV with redshift for either radio-loud or radio-quiet QSOs. However, we do find marginal evidence for a trend toward harder EUV spectra with increasing luminosity for radio-loud objects. An extrapolation of our radio-quiet QSO spectrum is consistent with existing X-ray data, suggesting that the ionizing continuum may be represented by a single power law. The resulting spectrum is roughly in agreement with models of the intergalactic medium photoionized by the integrated radiation from QSOs.

A Composite HST Spectrum of Quasars

We construct a composite quasar spectrum from 284 HST FOS spectra of 101 quasars with redshifts z > 0.33. The spectrum covers the wavelengths between 350 and 3000 A in the rest frame, with a peak S/N level of ~130 per A at ~1200 A. Since ~90% of the sample quasars have redshift z 1.5 quasars, for which significant corrections for the accumulated Lyman-series line and continuum absorption have been applied. There is a significant steepening of the continuum slope around 1050 A. The continuum between 1050 and 2200 A can be modeled as a power law fν να with α = -0.99 ± 0.05. For the full sample the power-law index in the extreme ultraviolet (EUV) between 350 and 1050 A is α = -1.96 ± 0.15. For the radio-loud subsample (60 objects), the EUV spectral index is α -2.2, while for the radio-quiet subsample (41 objects) it is α -1.8. The continuum flux in the wavelengths near the Lyman limit shows a depression of ~10%. The break in the power-law index and the slight depression of the continuum near the Lyman limit are features expected in Comptonized accretion-disk spectra. Comptonization produces a power-law tail in the wavelength band shortward of ~1000 A and smears out the Lyman-limit edge of the intrinsic accretion-disk spectrum. In the EUV waveband, we detect several possible emission features, including one around 690 A that may be O III + N III produced by the Bowen fluorescence effect. Comparing our composite spectrum with one made at higher redshifts by Francis et al., we find that the equivalent widths of Lyα and high-ionization emission lines are larger in our sample, reflecting a known luminosity dependence. The equivalent widths of low-ionization lines do not exhibit such a dependence, suggesting that the quasar EUV continuum above ~50 eV is steeper at higher luminosity. Radio-quiet quasars appear to show a slightly harder continuum and lower ionization levels in their emission lines.

Evolution of structure in the intergalactic medium and the nature of the ly-alpha forest

We have performed a detailed statistical study of the evolution of structure in a photoionized intergalactic medium (IGM) using analytical simulations to extend the calculation into the mildly nonlinear density regime found to prevail at z = 3. Our work is based on a simple fundamental conjecture: that the probability distribution function of the density of baryonic diffuse matter in the universe is described by a lognormal (LN) random field. The LN distribution has several attractive features and follows plausibly from the assumption of initial linear Gaussian density and velocity fluctuations at arbitrarily early times. Starting with a suitably normalized power spectrum of primordial fluctuations in a universe dominated by cold dark matter (CDM), we compute the behavior of the baryonic matter, which moves slowly toward minima in the dark matter potential on scales larger than the Jeans length. We have computed two models that succeed in matching observations. One is a nonstandard CDM model with Ω = 1, h = 0.5, and Γ = 0.3, and the other is a low-density flat model with a cosmological constant (LCDM), with Ω = 0.4, ΩΛ = 0.6, and h = 0.65. In both models, the variance of the density distribution function grows with time, reaching unity at about z = 4, where the simulation yields spectra that closely resemble the Lyα forest absorption seen in the spectra of high-z quasars. The calculations also successfully predict the observed properties of the Lyα forest clouds and their evolution from z = 4 down to at least z = 2, assuming a constant intensity for the metagalactic UV background over this redshift range. However, in our model the forest is not due to discrete clouds, but rather to fluctuations in a continuous intergalactic medium. At z = 3, typical clouds with measured neutral hydrogen column densities NH I = 1015.3, 1013.5, and 1011.5 cm-2 correspond to fluctuations with mean total densities approximately 10, 1, and 0.1 times the universal mean baryon density. Perhaps surprisingly, fluctuations whose amplitudes are less than or equal to the mean density still appear as clouds because in our model more than 70% of the volume of the IGM at z = 3 is filled with gas at densities below the mean value. We find that the column density distribution of Lyα forest lines can be fitted to f(NH I

HST FOS spectroscopy of M87: Evidence for a disk of ionized gas around a massive black hole

_{{r H} {r I}}

Narrowband HST images of M87: Evidence for a disk of ionized gas around a massive black hole

-->)NH I

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

−{β}_{{r H} {r I}}

The Hopkins Ultraviolet Telescope - Performance and calibration during the Astro-1 mission

-->, with β = 1.46 in the range 12.5 0.32 A, also in good agreement with observations. We fit Voigt profiles to our simulated lines and find a distribution of b parameters that matches that obtained from similar fits to real spectra. The effective opacity in the Lyα forest is found from the model to be τeff = 0.26[(1 + z)/4]3.1, again in good agreement with observations. The exponents for the evolution of the number of lines and the effective opacity do not simply differ by 1.0, as in the standard cloud picture, owing to saturation effects for the stronger lines. This also explains why the effective opacity evolves more slowly than (1 + z)4.5, which is expected for the Gunn-Peterson effect in a uniform medium with J = constant (and Ω = 1). We compare the spectral filling factor for our CDM and LCDM models with observations of the Lyα forest in HS 1700+64 and find good agreement. Similar calculations for a standard CDM model and a cold plus hot dark matter model fail to match the observed spectral filling factor function. We have also compared a number of predictions of our analytical model with results of numerical hydrodynamical calculations (Miralda-Escude et al. 1996) and again find them to be in good agreement. The lognormal hypothesis, coupled with otherwise attractive CDM-dominated cosmological models, appears to provide a plausible and useful description of the distribution of photoionized intergalactic gas and provides a new estimate of the baryonic density of the universe.

The optical counterpart of GX 1+4: A symbiotic star

Using the Faint Object Spectrograph (FOS) on the Hubble Space Telescope (HST) to observe the central region of M87, we have obtained spectra covering approximately 4600-6800 A at a spectral dispersion approximately 4.4 A per resolution element through the .26 sec diameter entrance aperture. One spectrum was obtained centered on the nucleus of M87 and two centered 0.25 sec off the nucleus at position angles of 21 deg and 201 deg, thus sampling the anticipated major axis of the disklike structure (described in a companion Letter) expected to lie approximately perpendicular to the axis of the M87 jet. Pointing errors for these observations are estimated to be less than 0.02 sec. Radial velocities of the ionized gas in the two positions 0.25 sec on either side of the nucleus are measured to be approx. equals +/- 500 km/s relative to the M87 systemic velocity. These observations plus emission-line spectra obtained at two additional locations near the nucleus show the ionized gas to be in Keplerian rotation about a mass M = (2.4 +/- 0.7) x 10(exp 9) solar mass within the inner 0.25 sec of M87. Our results provide strong evidence for the presence of a supermassive nuclear black hole in M87.

Performance and Preliminary Calibration of the Hopkins Ultraviolet Telescope on the Astro-2 Mission

We present Hubble Space Telescope Wide Field/Planetary Camera-2 (HST WFPC2) narrowband H-alpha + (N II) images of M87 which show a small disk of ionized gas with apparent spiral structure surrounding the nucleus of M87. The jet projects approximately 19.5 deg from the minor axis of the disk, which suggests that the jet is approximately normal to the disk. In a companion Letter, Harms et al. measure the radial velocities at r = +/- 0.25 sec along a line perpendicular to the jet, showing that one side of the disk is approaching at 500 +/- 50 km/s and the other side of the disk is receding at 500 +/- 50 km/s. Absorption associated with the disk and the sense of rotation imply that the apparent spiral arms trail the rotation. The observed radial velocites corrected for a 42 deg inclination of the disk imply rotation at +/- 750 km/s. Analysis of velocity measurements at four positions near the nucleus gives a total mass of approximately 2.4 +/- 0.7 x 10(exp 9) solar mass within 18 pc of the nucleus, and a mass-to-light ratio (M/L)(sub I) = 170. We conclude that there is a disk of ionized gas feeding a massive black hole in the center of M87.