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All sourcesNASA NTRSDOE OSTI
NASA NTRS1993

Can quasars photoionize the intergalactic medium at high redshift?

The reionization of the intergalactic medium (IGM) by quasar sources at high redshift are discussed. The integrated UV background from observed QSO's, taking into account the hydrogen opacity associated with intervening Ly-alpha clouds and Lyman limit systems are computed. It is noted that the published data appear to indicate a significant underdensity of absorption systems in the Ly-alpha forest with column densities N(sub HI) greater than 10(exp 15) cm(sup -2). This deficit results in a reduction of the opacity of the universe by a factor of 1.5-3 at z = 3-5 relative to previous estimates. The QSO contribution to the metagalactic flux at the Lyman edge may be as large as J(sub 912)(z) is approximately 6((1 + z)/4.5)(sup 0.5) x 10(exp -22) erg cm(sup -2) s(sup -1) Hz(sup -1) sr(sup -1) for q(sup o) = O, and slightly lower for q(sub o) = 1/2. For a density of the diffuse component of the IGM of omega(sub D)(h(sub 50)(sup 2)) less than 0.025, QSO's could photoionize a smooth IGM sufficiently to satisfy the constraints imposed by the Gunn-Peterson effect. The epoch of reionization could be as recent as z is approximately greater than 5. As a result, neutral patches of IGM would be detectable in the spectra of high redshift quasars. The patches would appear as absorption line systems with typical column densities of 10(exp 19) - 10(exp 20) cm(sup -2), and velocity widths of 100 - 1000 km s(sup -1).

Meiksin, Avery↗
NASA NTRS1980

A thermodynamic analysis of a solar-powered jet refrigeration system

The article describes and analyzes a method of using solar energy to drive a jet refrigeration system. A new technique is presented in the form of a performance nomogram combining the energy and momentum equations to determine the performance characteristics. A numerical example, using water as the working fluid, is given to illustrate the nomogram procedure. The resulting coefficient of performance was found comparable with other refrigeration systems such as the solar-absorption system or the solar-Rankine turbocompressor system.

Lansing, F. L.↗
NASA NTRS1993

Clear evidence for the presence of O VI absorption in QSO metal systems

We have detected O VI 1031.93 A, 1037.62 A doublet absorption in a composite QSO spectrum formed from a large number of intervening C IV absorption systems. The detections constitute the first firm evidence for the presence of O VI in intervening QSO metal absorption systems. The equivalent width of the detected O VI absorption implies an O VI column density N(O VI) not less than 2.8 x 10 exp 14/sq cm. This value, together with the nondetection of the N V 1238.82 A, 1242.80 A doublet absorption, suggests that N(O VI)/N(N V) not less than 4.4. For collisionally ionized gas with a solar O to N abundance ratio in thermal equilibrium the above ratio requires a temperature T not less than 2.5 x 10 exp 5 K. It is found that C IV systems which show low-ionization species and those which do not both have associated O VI absorption, suggesting that O VI is probably present in all C IV systems. We also find that C IV systems which show low-ionization species on average have stronger high-ionization absorption lines than those which do not. A simple interpretation was given to explain this trend.

Lu, Limin↗
NASA NTRS1981

Galactic absorption line coronae

Models are developed to determine if gaseous coronas around galaxies give rise to the absorption systems in quasar spectra. A two-dimensional hydrodynamic code with radiative cooling is used to study the dynamics and thermodynamics of a corona, and the position and rate at which clouds form. Coronal gas is found to rise from an extended gaseous disk in two models with base coronal temperatures of 10 to the 6th K and 0.50 x 10 to the 6th K. Column densities of 10 to the 19th sq cm out to a radius of 7 kcp are also achieved with a total gas mass of 2.0 x 10 to the 9th and 1.7 x 10 to the 9th solar masses, respectively. The velocity structure of the model absorption line systems may be reproduced, and clouds in pressure equilibrium with the hot coronas have densities consistent with basic observational and theoretical restrictions.

Bregman, J. N.↗
NASA NTRS1994

Far-ultraviolet absorption spectra of quasars: How to find missing hot gas and metals

We show that some high-redshift QSO absorption systems that reveal only the H I Lyman series lines at wavelengths visible from the ground maybe a new class of ultra-high-ionization metal line systems, with metal lines in the far-UV region which is now being explored with satellites. At high temperatures or in intense radiation fields metal systems will not show the usual C IV absorption, and O VI will become the most prominent metal absorber. At still higher ionization, O IV also becomes weak and the strongest metal lines are from Ne VIII, Mg X and Si XII, which have doublets in the rangs 500-800 A. Hence very high ionization metal systems will not show metal lines in existing spectra. Recent X-ray observations show that galaxy halos contain hot gas, so we predict that far-UV spectra of QSOs will also show this gas.

Verner, D. A.↗
NASA NTRS2000

Determining CDOM Absorption Spectra in Diverse Coastal Environments Using a Multiple Pathlength, Liquid Core Waveguide System. Measuring the Absorption of CDOM in the Field Using a Multiple Pathlength Liquid Waveguide System

We evaluated the accuracy, sensitivity and precision of a multiple pathlength, liquid core waveguide (MPLCW) system for measuring colored dissolved organic matter (CDOM) absorption in the UV-visible spectral range (370-700 nm). The MPLCW has four optical paths (2.0, 9.8, 49.3, and 204 cm) coupled to a single Teflon AF sample cell. Water samples were obtained from inland, coastal and ocean waters ranging in salinity from 0 to 36 PSU. Reference solutions for the MPLCW were made having a refractive index of the sample. CDOM absorption coefficients, a(sub CDOM), and the slope of the log-linearized absorption spectra, S, were compared with values obtained using a dual-beam spectrophotometer. Absorption of phenol red secondary standards measured by the MPLCW at 558 nm were highly correlated with spectrophotometer values (r > 0.99) and showed a linear response across all four pathlengths. Values of a(sub CDOM) measured using the MPLCW were virtually identical to spectrophotometer values over a wide range of concentrations. The dynamic range of a(sub CDOM) for MPLCW measurements was 0.002 - 231.5/m. At low CDOM concentrations (a(sub 370) < 0.1/m) spectrophotometric a(sub CDOM) were slightly greater than MPLCW values and showed larger fluctuations at longer wavelengths due to limitations in instrument precision. In contrast, MPLCW spectra followed an exponential to 600 nm for all samples. The maximum deviation in replicate MPLCW spectra was less than 0.001 absorbance units. The portability, sampling, and optical characteristics of a MPLCW system provide significant enhancements for routine CDOM absorption measurements in a broad range of natural waters.

Miller, Richard L.↗
NASA NTRS1978

Absorption in the spectra of quasi-stellar objects and BL Lacertae objects

Observations of absorption in the spectra of 64 QSOs and two BL Lac objects are reviewed. Criteria for selecting the absorption-line objects are discussed along with line identifications, properties of identified absorption systems, the observed absence of variability in the absorption lines, the redshift distribution, line locking, and absorption shortward of the Lyman limit. The possibility of noncosmological redshifts is noted, and two interpretations of the absorption-line data are considered: (1) the 'intrinsic' hypothesis that the absorptions arise in gas associated with the QSOs and (2) the 'intervening' hypothesis that they are produced by intervening galaxies, galactic halos, or intergalactic clouds intersected by the line of sight to a QSO. Direct observational tests for the location of the absorbing hydrogen in several individual objects are described. It is concluded that no conclusive observational evidence exists which is capable of establishing whether the absorption is intrinsic or intervening.

Perry, J. J.↗
NASA NTRS1995

The z = 0.8596 damped Ly-alpha absorbing galaxy toward PKS 0454+039

We present Hubble Space Telescope (HST) and ground-based data on the Z(sub abs) = 0.8596 metal-line absorption system along the line of sight to PKS 0454+0356. The system is a moderate-redshift damped Ly-alpha system, with N(H I) = (5.7 +/- 0.3) x 10(exp 20)/sq cm as measured from the Faint Object Spectrograph (FOS) spectrum. We also present ground-based images which we use to identify the galaxy which most probably gives rise to the damped system; the most likely candidate is relatively underluminous by QSO absorber standards M(sub B) approximately -19.0 for A(sub 0) = 0.5 and H(sub 0) = 50 km/s/Mpc) and lies approximately 8.5/h kpc in projection from the QSO sight line. Ground-based measurements of Zn II, Cr II, and Fe II absorption lines from this system allow us to infer abundances of (Zn/H) = -1.1, (Cr/H) = -1.2, and (Fe/H) = -1.2 indicating overall metallicity similar to damped systems at z is greater than 2, and that the depletion of Cr and Fe onto dust grains may be even less important than in many of the high-redshift systems of comparable metallicity. Limits previously placed on the 21 cm optical depth in the z = 0.8596 system, together with our new N(H I) measurement, suggest a very high spin temperature for the H I, T(sub s) is greater than 580 K.

Steidel, Charles C.↗
NASA NTRS1962

Design comparison of lunar return configurations

Parabolic manned re-entry design comparisons are made between two classes of slender and blunt configurations capable of conventional horizontal and vertical earth landings. Aerodynamic modulation using flaps and control augmentation requirements are discussed in terms of their effect upon re-entry trajectories and landing footprints. Within the environment defined by the re-entry trajectories, the convective and radiative modes of heat transfer are compared, their differences discussed, and the problems associated with the configurations for planetary return missions indicated. Thermal protection systems are discussed and the factors influencing the selection of a given system for the re-entry environment are indicated. Comparison between reflective and absorptive systems are made and the merits of each for a given flight time indicated. A thermo-structural analysis is presented which shows the trade-oft between structural operating temperature and heat protection system thickness requirements. Total heat shield and structural weights are compared for the landing footprints presented and their variation with range shown. Finally, the areas of uncertainty in the aero-thermo structural design analysis are indicated for each class of configuration studied, and the weight differences between conventional horizontal landing and vertical descent configurations are given.

Parabolic flight↗
NASA NTRS1992

Low-redshift Lyman-alpha absorption lines and the dark matter halos of disk galaxies

Ultraviolet observations of the low-redshift quasar 3C 273 using the Hubble Space Telescope have revealed many more Lyman-alpha absorption lines than would be expected from extrapolation of the absorption systems seen toward QSOs at z about 2. It is shown here that these absorption lines can plausibly be produced by gas at large radii in the disks of spiral and irregular galaxies; the gas is confined by the dark matter halos and ionized and heated by the extragalactic radiation field. This scenario does not require the extragalactic ionizing radiation field to decline as rapidly with decreasing z as the QSO emissivity. Observations of Ly-alpha absorption through the halos of known galaxies at low redshift will constrain both the extragalactic background and the properties of galactic halos.

Maloney, Philip↗
NASA NTRS2011

Manned Mission Planning Considerations when Using a Non-Regenerable CO2 Removal System

As the commercial spacecraft industry increases in size, there will be a corresponding increase in the number of manned spacecraft built and operationally flown each year. Industry teams for these new spacecraft will have multiple design and operational choices to make for each of these spacecraft s subsystems. The carbon dioxide (CO2) removal subsystem of the environmental control and life support system is one that presents such challenges. This paper seeks to aid industry in making design and operations choices by providing a document containing lessons learned by the Space Shuttle Program s Operations team, with specific focus given to the non-regenerable CO2 removal system currently used by the Space Shuttle. Carbon dioxide, one of the key byproducts of respiration, can lead to injury and death if allowed to build up in a spacecraft s habitable environment. Therefore, any spacecraft s environmental control and life support system must contain a method for removing this hazard. These removal systems can either be non-regenerable or regenerable. While this paper defines the difference between these types and presents a generic comparison of their capabilities, the focus is specifically on the Space Shuttle s CO2 removal systems. This will include a short discussion of the Space Shuttle s regenerable amine solid absorption system that was part of the Extended Duration Orbiter (EDO) modification, however, emphasis is given to the non-regenerable Lithium Hydroxide canister system used as the prime removal method employed by the Space Shuttle, including a discussion on why this method was chosen over a regenerable system. A full exposition is given on all of the considerations required for mission planning when using a non-regenerable CO2 removal system. Key discussion items include: airflow lessons learned, recent physiological issues related to short term and long term exposure and how lower levels may be more harmful than previously thought, lithium hydroxide dust issues, mass vs. stowage comparisons, cost, complexity considerations, operational margin considerations, timing considerations with respect to the crew s schedule, and logistical management. The conclusion of the paper finalizes the discussion and contains a listing of considerations in regards to CO2 management using a non-regenerable removal system in the near future.

DeSimpelaere, Edward↗
NASA NTRS1994

Conditions in the z = 0.692 absorber toward 3CR 286

We present Hubble Space Telescope/Faint Object Spectrograph (HST/FOS) ultraviolet and ground-based optical spectra of the z = 0.692 21 cm absorption system in the quasi-stellar object (QSO) 3CR 286. The strength of the damped Lyman-alpha absorption implies an H I column density, N(H I) approximately 2 x 10(exp 21)/sq cm. We derive a high spin temperature for the H I gas, T(sub s) greater than or approximately equal to 10(exp 3) K, as has been found for other high-redshift 21 cm absorbing systems; at least 80% of the H I is hotter than 1200 K. Curve-of-growth analysis yields Mg(+) and Fe(+) abundances which are approximately 1-2 dex below solar values; the Ca(+) abundance is even lower implying some depletion onto dust grains. The H2 fraction is not high. We speculate that the high inferred T(sub s) for the gas may reflect continuing active star formation at the 5-8 Gyr look-back time to the absorbing galaxy.

Cohen, Ross D.↗
NASA NTRS1987

The nature of the BL Lacertae object AO 0235 + 164

Emission features in the optical spectrum of the highly variable BL Lac object AO 0235 + 164 have been detected which are identified as Mg II, forbidden Ne V, and forbidden O II at a redshift of 0.9399, consistent with a cosmological interpretation of the two absorption systems at z = 0.524 and z = 0.851. The spectrum also shows emission features associated with the z = 0.524 system at a projected separation of 20 kpc from the apparent central concentration of the material producing that absorption. This implies a highly extended and very luminous line-emitting region for the material at z = 0.524. A comparison of the new data with previous spectroscopic observations of the BL Lac object when it was faint suggests that the Mg II emission may be variable on a time scale of approximately 2 yr.

Cohen, Ross D.↗
NASA NTRS1990

Ca II and Na I absorption in the QSO S4 0248 + 430 due to an intervening galaxy

Observations of the QSO S4 0248 + 430 and a nearby anonymous galaxy are presented. Two absorption components are found in both Ca II H and K and Na I D1 and D2 at z(a) = 0.0515, 0.0523. Column densities of log N(Ca II) = 13.29, 13.50, and log N(Na I) = 13.79, 14.18 are found for z(a) = 0.0515, 0.0523 absorption systems, respectively. The column density ratios imply considerable calcium depletion and disk-type absorbing gas. At least one and possibly both absorption components are produced by high-velocity gas. A broadband image of the field shows an asymmetrical armlike feature or possible tidal tail covering and extending past the position of the QSO. The presence of this extended feature and the apparent difference between the absorption velocities and galaxy rotation velocity suggest that the absorbing gas is not ordinary disk gas, but rather is a result of tidal disruption.

Womble, Donna S.↗
NASA NTRS1992

Quasar absorption lines with a nonzero cosmological constant

Quasar absorption lines in flat universes with nonzero cosmological constant Lambda are examined and compared with more conventional zero Lambda universes. Various evolution effects for intergalactic absorbers and the observed number density evolution of each absorption system are examined in order to discriminate between evolution effects and the cosmological models. An interesting interaction between Lambda effects and cosmic absorption phenomena is explored. Equations describing IGM absorption statistics are developed for nonzero Lambda cosmologies, both for unevolving absorber populations and some of the more popular physical models for the IGM and intergalactic clouds which include the effects of the absorber evolution.

Turner, Edwin L.↗
NASA NTRS1995

NE VIII lambda 774 and time variable associated absorption in the QSO UM 675

We discuss measurements of Ne VIII lambda 774 absorption and the time variability of other lines in the z(sub a) approximately equal z(sub e) absorption system of the z(sub e) = 2.15 QSO UM 675 (0150-203). The C IV lambda 1549 and N V 1240 doublets at z(sub a) = 2.1340 (shifted approximately 1500 km/s from z(sub e) strengthened by a factor of approximately 3 between observations by Sargent, Boksenberg and Steidel (1981 November) and our earliest measurements (1990 November and December). We have no information on changes in other z(sub a) approximately equal z(sub e) absorption lines. Continued monitoring since 1990 November shows no clear changes in any of the absorptions between approximately 1100 and 1640 A rest. The short timescale of the variability (less than or approximately equal to 2.9 yr rest) strongly suggests that the clouds are dense, compact, close to the QSO, and photoionized by the QSO continuum. If the line variability is caused by changes in the ionization, the timescale requires densities greater than approximately 4000/cu cm. Photoionization calculations place the absorbing clouds within approximately 200 pc of the continuum source. The full range of line ionizations (from Ne VIII lambda 774 to C III lambda 977) in optically thin gas (no Lyman limit) implies that the absorbing regions span a factor of more than approximately 10 in distance or approximately 100 in density. Across these regions, the total hydrogen (H I + H II) column ranges from a few times 10(exp 18)/sq cm in the low-ionization gas to approximately 10(exp 20)/sq cm where the Ne VIII doublet forms. The metallicity is roughly solar or higher, with nitrogen possibly more enhanced by factors of a few. The clouds might contribute significant line emission if they nearly envelop the QSO. The presence of highly ionized Ne VIII lambda 774 absorption near the QSO supports recent studies that link z(sub a) approximately equal to z(sub e) systems with X-ray 'wamr absorbers. We show that the Ne VIII absorbing gas would itself produce measurable warm absorption -- characterized by bound-free O VII or O VIII edegs near 0.8 keV -- if the column densities were N(sub H) greater than or approximately equal to 10(exp 21)/sq cm (for solar abundances).

Hamann, Fred↗
NASA NTRS1995

Radiative transfer in a clumpy universe: The colors of high-redshift galaxies

We assess the effects of the stochastic attenuation produced by intervening QSO absorption systems on the broadband colors of galaxies at cosmological distances. We compute the H I opacity of a clumpy universe as a function of redshift, including scattering in resonant lines, such as Lyman alpha, Lyman beta, Lyman gamma, and higher order members, and Lyman-continuum absorption. Both the numerous, optically thin Lyman-alpha forest clouds and the rarer, optically thick Lyman limit systems are found to contribute to the obscuration of background sources. We study the mean properties of primeval galaxies at high redshift in four broad optical passbands, U(sub n), B, G, and R. Even if young galaxies radiated a significant amount of ionizing photons, the attenuation due to the accumulated photoelectric opacity along the path is so severe that sources beyond z approximately 3 will drop out of the U(sub n) image together. We also show that the observed B-R color of distant galaxies can be much redder than expected from a stellar population. At z approximately 3.5, the blanketing by discrete absorption lines in the Lyman series is so effective that background galaxies appear, on average, 1 mag fainter in B. By z approximately 4, the observed B magnitude increment due to intergalactic absorption exceeds 2 mag. By modeling the intrinsic UV spectral energy distribution of star-forming galaxies with a stellar population synthesis code, we show that the (B-R)(sub AB) approximately 0 criterion for identifying 'flat-spectrum,' metal-producing galaxies is biased against objects at z greater than 3. The continuum blanketing from the Lyman series produces a characteristic staircase profile in the transmitted power. We suggest that this cosmic Lyman decrement might be used as a tool to identify high-z galaxies.

Madau, Piero↗
NASA NTRS1997

Damped Ly alpha absorbers at high redshift: Large disks or galactic building blocks?

The nature of the physical structures giving rise to damped Lyman alpha absorption systems (DLAS) at high redshifts is investigated. The proposal that rapidly rotating large disks are the only viable explanation for the observed asymmetric profiles of low ionization absorption lines is examined. Using hydrodynamic simulations of galaxy formation, it is demonstated that irregular protogalactic clumps can reproduce the observed velocity width distribution and asymmetries of the absorption profiles equally well. The velocity broadening in the simulated clumps is due to a mixture of rotation, random motions, infall and merging. The observed velocity width correlates with the virial velocity for the dark matter halo of the forming protogalactic clump. The typical virial velocity of the halos required to lead to the DLAS population is approximately 100 km/s. It is concluded that the evidence that DLAS at high redshift are related to large, rapidly rotating disks, is not compelling.

Haehnelt, Martin G.↗