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At least 127 records · Page 7

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NASA NTRS1975

A correlation test of the intrinsic interpretation of QSO absorption redshifts

It is noted that the general intrinsic interpretation of QSO absorption redshifts predicts a high probability of clustering for the expulsion velocities of clouds ejected from a QSO core and that a correlation function has been defined which depends on the probability of clustering of three or more expulsion velocities. A test of this correlation is formulated which utilizes data on pairs of QSOs with similar emission redshifts and at least three well established absorption redshifts with corresponding expulsion velocities greater than 0.02c. It is shown that the correlation should be positive (maximum value +100%) if the absorption systems are intrinsic and correlated, zero if there is no physical connection among these systems, and negative if a strong anticorrelation exists or QSOs tend to eject one or two clouds at well separated characteristic velocities. Data on five QSOs are analyzed, and large positive values are obtained for the correlations.

Opher, R.↗

NASA NTRS1984

The ultraviolet astronomy mission: Columbus

An ultraviolet astronomy mission (Columbus) is described. It exploits the spectral region between 900 and 1200A, which is extremely rich in containing the Lyman lines of hydrogen and deuterium and the Lyman band of their molecules, together with the resonance lines of many important ions. High resolving power and high sensitivity provide a unique capability for studying the brightest members of neighboring galaxies, the HeI and HeII absorption systems in quasars out to a red shift of 2, and the halos of intervening galaxies. Complementary focal plane instruments are planned in order to allow observations to longer (2000A) and shorter (100A) wavelengths. This wide coverage embraces the resonance lines of all the cosmically abundant elements and a wide range of temperature zones up to 100 million K.

Wilson, R.↗

NASA NTRS1988

Models of the quasar population. II - The effects of dust obscuration

A model for dust obscuration is proposed to account for the apparent lack of high-redshift quasars which are assumed to be obscured by dust in intervening galaxies. The distribution function for the optical depth as a function of redshift is constructed and used to determine the expected properties of quasars. The apparent magnitude distribution at three points is used to fit the model. Optical results are given for the comoving number density, redshift distribution, and mean color excess. If the theory is correct, evidence for dust may be seen in quasar spectra and a correlation between quasar continuum slopes and strength of metal line absorption systems should be observed.

Heisler, Julia↗

NASA NTRS1990

Strong associated C 4 absorption in low redshift quasars

IUE spectra of quasars were used to determine the frequency of occurrence of strong associated C 4 absorption systems at low red shifts. Four systems are found with rest frame equivalent width (REW) greater than 5 angstroms in the spectra of 38 quasars. This rate of occurrence of 0.12 is not significantly different from the rate of 0.064 determined for high red shift quasars. The detected strong associated systems are all in low red shift quasars which have been imaged from the ground. One of the quasars is unusual, having two nuclei, a close companion and distorted isotopes. Two of the others also have close companion galaxies at projected distances of under 100 kpc. The conclusion was made that a much larger sample is needed.

Tytler, David↗

NASA NTRS1990

What produces the ionizing background at large redshift?

Results are presented from mathematical and modeling studies to make estimates of the intensity of the ionizing background more definite and quantitative. It is shown that the observed quasars fail to emit sufficient ionizing radiation to satisfy the Gunn-Peterson test. Two possible sources for the additional source of UV radiation are examined: main-sequence, high mass stars in primordial galaxies and quasars that have not been observed because of dust obscuration. Both of these models satisfy the observational constraints with reasonable parameters. It is suggested that observations of He lines from Ly-alpha clouds or an analysis of the line ratios in metal line absorption systems may be used to determine which sources produce the ionizing background.

Miralda-Escude, Jordi↗

NASA NTRS1990

Development of non-intrusive instrumentation for NASA-Ames Ballistic Range and Shock Tunnel

The use of nonintrusive instrumentation in high-enthalpy facilities offers the opportunity to collect data that has previously been difficult or impossible to obtain. Three such instruments, now under development, are described: an OH absorption system which can measure OH species concentration and temperature, a holographic interferometer for flow visualization and density measurement at low density, and a telemetry system which allows the measurement of surface quantities on model in free-flight at hypervelocity. The challenges and opportunities encountered when employing these advanced diagnostics to high-enthalpy facilities are discussed.

Strawa, A. W.↗

NASA NTRS1990

The missing UV absorption lines of NGC 4151

Near simultaneous high dispersion long and short wavelength International Ultraviolet Explorer (IUE) observations of the Seyfert galaxy NGC 4151 are discussed. Previous observations revealed a narrow absorption system in Mg II not present in Ly alpha or C IV. The new observations confirm the presence of this system in Mg II and its absence in the other lines. Possible reasons for this are discussed. Future Hubble Space Telescope studies of NGC 4151 are discussed.

Leech, K. J.↗

NASA NTRS1990

Lyman-alpha emission from young galaxy disks

An imaging survey of Ly-alpha disk absorption systems for Ly-alpha emission from the absorbing objects, which are candidate young disk galaxies, has been conducted. Limits are set to the flux in Ly-alpha from these objects of approximately 10 exp -16 ergs/sq cm s (3 sigma), which corresponds to a limit on the star formation rate of 2-7 solar masses/yr, in the absence of dust. This suggests that although disk structures form early, rapid star formation does not occur immediately, or else occurs in episodic fashion.

Cohen, Ross D.↗

NASA NTRS1992

Infrared Pulse-laser Long-path Absorption Measurement of Carbon Dioxide Using a Raman-shifted Dye Laser

A pulsed laser source is effective in infrared laser long-path absorption measurements when the optical path length is very long or the reflection from a hard target is utilized, because higher signal-to-noise ratio is obtained in the detection of weak return signals. We have investigated the performance of a pulse-laser long-path absorption system using a hydrogen Raman shifter and a tunable dye laser pumped by a Nd:YAG laser, which generates second Stokes radiation in the 2-micron region.

Minato, Atsushi↗

NASA NTRS1992

Mg II absorption in the spectra of 103 QSOs - Implications for the evolution of gas in high-redshift galaxies

Spectra of 103 low- and intermediate-redshift QSOs with a resolution of 4-6 A were obtained in order to investigate the statistics of the M II 2796, 2803 absorption systems over the redshift range of 0.2-2.2. New emission-line redshifts, based mostly on Mg II 2800 and semiforbidden 1909, were determined for the QSOs. The mean number density of Mg II absorbers per unit redshift range decreased from 0.97 +/-0.10 for a rest equivalent width threshold of 0.3 A to 0.27 +/-0.05 for a threshold of 1.0 A. The results are in agreement with comparable previous work. The observed equivalent width distribution could be fitted with either exponential or power-law analytic distributions, with parameters in line with those found in earlier studies. Statistical tests show that the Mg II absorbers are uniformly distributed in both comoving coordinates and velocity relative to the QSO.

Steidel, Charles C.↗

NASA NTRS1993

Pruning the Lyman alpha forest of Q1331+170

A multitude of absorption lines seen shortward of QSO Ly-alpha emission, that cannot be traced to heavy element absorption systems, are assumed to be Ly-alpha lines arising in intervening clouds. Studies of these Ly-alpha clouds, typically done at 1 A or lower resolution, have shown N(H1) approximately equal to 10(exp 13) - 10(exp 17) sq cm and b approximately equal to 35 km/s. Sargent et al 1980, on the basis of a flat pair velocity correlation function (PVCF), argued that these clouds are intergalactic. But Crotts 1989 showed that the strong Ly-alpha lines are spatially clustered. High resolution studies of Webb 1987 and Rauch et al 1992 also report some evidence for weak clustering, but overall such high resolution studies have been rare. A study of the Ly-alpha forest of Q1331 + 170 over x(sub abs) = 1.60 - 2.19 based on 18 km/s resolution data at S/N approximately equal to 15, with metal-line deblending incorporated.

Kulkarni, Varsha P.↗

NASA NTRS1993

Ly alpha and IR galaxy companions of high redshift damped Ly alpha QSO absorbers

We have used a Near-Infrared Camera and Multi-Object Spectrometer (NICMOS3) HgCdTe 256x256 array detector with the Infrared (IR) camera on the 2.3m telescope at Steward Observatory to image several Quasi-Stellar Object (QSO) fields. The limiting magnitude is K'(2.1 microns) = 21.0 - 21.5 mag per square arcsec for a 3 sigma detection in 3 hours of in-field chopping observations. Each QSO line-of-sight samples several known absorbers with Mg2(lambda)2796-2803 A and/or C4(lambda)1548-1551 A absorption doublets. The equivalent width distributions of the low and high ionization absorption lines of the absorber sample are identical to those of the parent population of all absorbers. This selection process, used already for a spectroscopic survey of Mg2 absorption lines in C4-selected absorption systems at high z, gives a methodical approach to observing, reduces the observer biases, and makes a more efficient use of telescope time. This selection guarantees that imaging of the sample of QSO fields will provide complete sampling of the whole population of high z QSO absorbers. Follow-up optical and IR spectroscopy of these objects is scheduled for redshift measurement and confirmation of the absorbing galaxies and the cluster members.

Caulet, Adeline↗

NASA NTRS1993

Magnetic field strengths in high-redshift galaxies - Can the galactic dynamo be tested?

The hypothesis that the population of strong H I absorption systems is responsible for a significant excess Faraday rotation measure in quasars is discussed, and it is concluded that the case is unproved, in contrast to a recent analysis by Wolf et al. (1991). The limitations and pitfalls inherent in attempts to derive firm magnetic field strengths from the existing integrated rotation measures of quasars are discussed, and it is shown that although it is premature to use integrated quasar rotation measures to either confirm or rule out particular mechanisms of magnetic field amplification in galaxy disks, the present observations may call for reexamination of current theories of large-scale magnetic field generation. The sources of rotation measure in the double quasar 0957+561 are also discussed.

Perry, Judith J.↗

NASA NTRS1993

Evolution of the gaseous content of the universe

Results of a new spectroscopic survey for damped Ly-alpha absorption systems conducted at ultraviolet wavelengths are combined with those of previous surveys conducted at optical wavelengths in order to examine the evolution of the gaseous content of the universe at z between 0 and 3.5. Implications for scenarios of galaxy formation and evolution are discussed.

Lanzetta, Kenneth M.↗

NASA NTRS1995

The Hubble Space Telescope quasar absorption line key project. v. redshift evolution of lyman limit absorption in the spectra of a large sample of quasars

Using a sample of 119 QSOs, containing objects we have selected having previously available high quality ground-based and IUE spectral observations, together with Hubble Space Telescope (HST) observations of 26 QSOs from Bahcall et al. (1993, 1995) and Impey et al. (1995) and new optical observations of 41 objects by Steidel & Sargent (1995), we study the redshift evolution of Lyman limit absorption systems (LLSs; tau greater than 1.0) over the reshift range 0.32 less than or equal to z(sub LLS) less than or equal to 4.11. The HST observations significantly improve the determination of the low redshift (0.4 less than or equal to z(sub LLS) less than or equal to 1.4) distribution. We find the effect which may have been responsible for the apparent strong evolution at a(sub LLS) greater than or equal to 2.5 found by Lanzetta (1991), which led him to consider a broken, not single power law as a better description of the redshift distribution of LLSs. After removing objects which may bias our sample, leaving a total of 169 QSOs, we find the distribution is well described by a single power law, and obtain for the number density as a function of redshift the form N(z) = N(sub 0)(1 + z)(exp gamma) with gamma = 1.50 =/- 0.39 and N(sub 0) = 0.25(sup -0.10)(sub +0.17), consistent with a constant comoving density of absorbers in a Firedmann universe with q(sub 0) = 0 but indicating evolution if q(sub 0) = 1/2.

Stengler-Larrea, Erik A.↗

NASA NTRS1996

Intergalactic Hydrogen Clouds at Low Redshift: Connections to Voids and Dwarf Galaxies

We provide new post-COSTAR data on one sightline (Mrk 421) and updated data from another (I Zw 1) from our Hubble Space Telescope (HST) survey of intergalactic Ly(alpha) clouds located along sightlines to four bright quasars passing through well-mapped galaxy voids (16000 km/s pathlength) and superclusters (18000 km/s). We report two more definite detections of low-redshift Ly(alpha) clouds in voids: one at 3047 km/s (heliocentric) toward Mrk 421 and a second just beyond the Local Supercluster at 2861 km/s toward I Zw 1, confirming our earlier discovery of Ly(alpha) absorption clouds in voids (Stocke et al., ApJ, 451, 24). We have now identified ten definite and one probable low-redshift neutral hydrogen absorption clouds toward four targets, a frequency of approximately one absorber every 3400 km/s above 10(exp 12.7/sq cm column density. Of these ten absorption systems, three lie within voids; the probable absorber also lies in a void. Thus, the tendency of Ly(alpha) absorbers to 'avoid the voids' is not as clear as we found previously. If the Ly(alpha) clouds are approximated as homogeneous spheres of 100 kpc radius, their masses are approximately 10(exp 9)solar mass (about 0.01 times that of bright L* galaxies) and they are 40 times more numerous, comparable to the density of dwarf galaxies and of low-mass halos in numerical CDM simulations. The Ly(alpha) clouds contribute a fraction Omega(sub cl)approximately equals 0.003/h(sub 75) to the closure density of the universe, comparable to that of luminous matter. These clouds probably require a substantial amount of nonbaryonic dark matter for gravitational binding. They may represent extended haloes of low-mass protogalaxies which have not experienced significant star formation or low-mass dwarf galaxies whose star formation ceased long ago, but blew out significant gaseous material.

Shull, J. Michael↗

NASA NTRS2002

Absorbing Outflows in AGN

The goal of this program was a comprehensive multiwavelength study of absorption phenomena in active galactic nuclei (AGN). These include a variety of associated absorption systems: X-ray warm absorbers, X-ray cold absorbers. UV absorbers with high ionization lines, MgII absorbers, red quasars and BALQSOs. The aim is to determine the physical conditions in the absorbing outflows, study their inter-relations and their role in AGN. We designed several observing programs to achieve this goal: X-ray spectroscopy, UV spectroscopy, FLAY spectroscopy and X-ray imaging. We were very successful towards achieving the goal over the five year period as shown through following observing programs and papers. Copies of a few papers are attached with this report.

Mathur, Smita↗

NASA NTRS1997

The X-Ray Warm Absorber in NGC 3516

The Seyfert 1 galaxy, NGC 3516 has been the subject of many absorption line studies at both ultra-violet and X-ray wavelengths. In the UV, strong, broad, variable associated metal line absorption with velocity width approximately 2000 km s(exp -1) is thought to originate in gas with N(sub H)greater than or approximately 10(exp 10)cm (exp -2) lying between 0.01 and 9 pc from the central active nucleus. The Ginga X-ray data are consistent with several possibilities: a warm absorber and a cold absorber combined either with partial covering or an unusually strong reflection spectrum. We present ROSAT observations of NGC 3516 which show a strong detection of a warm absorber dominated by a blend of 0 VII/O VIII edges at approximately 0.8 keV with N(sub H) approximately 7x10(exp 21)cm(exp -2), and U:8-12. We argue that NGC 3516 contains an outflowing 'XUV' absorber showing the presence of X-ray absorption edges which are consistent with the presence of broad absorption lines in the old IUE spectra and their disappearance in the new UV observations. Our dynamical model suggests that the O VII absorption edge will continue to weaken compared to the O VIII edge, an easily testable prediction with future missions like AXAF. Eventually the source would be transparent to the X-rays unless a new absorption system is produced.

Mathur, Smita↗