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NASA NTRS1988

Quasar microlensing and dark matter

The amplification of quasar brightness due to gravitational lensing by foreground objects is discussed. It is shown that a recently published sample of X-ray-selected quasars behind foreground galaxies shows a statistically significant brightening compared to a control sample. Correlations with galaxy redshift and impact parameter predicted by microlensing are also demonstrated. A technique is described to measure the mean density of the lenses from a small number of identified cases of microlensing. It is shown that, in this sample, amplification bias is important in determining the mean intensity enhancement and must be included in the density estimate. Assuming that at least two of the four intrinsically brightest quasars behind galaxies are indeed microlensed, the present data yield a formal lower limit on the mean density parameter of lenses Omega(l) greater than 0.25 at 95 percent confidence. These data also imply that a considerable quantity of dark matter exists in macroscopic objects outside the visible parts of galaxies but is still highly correlated with them.

Rix, Hans-Walter↗
NASA NTRS1988

An analytic treatment of gravitational microlensing for sources of finite size at large optical depths

Statistical methods are developed for gravitational lensing in order to obtain analytic expressions for the average surface brightness that include the effects of microlensing by stellar (or other compact) masses within the lensing galaxy. The primary advance here is in utilizing a Markoff technique to obtain expressions that are valid for sources of finite size when the surface density of mass in the lensing galaxy is large. The finite size of the source is probably the key consideration for the occurrence of microlensing by individual stars. For the intensity from a particular location, the parameter which governs the importance of microlensing is determined. Statistical methods are also formulated to assess the time variation of the surface brightness due to the random motion of the masses that cause the microlensing.

Deguchi, Shuji↗
NASA NTRS1990

An astrophysics data program investigation of cluster evolution

A preliminary status report is given on studies using the Einstein x ray observations of distant clusters of galaxies that are also candidates for gravitational lenses. The studies will determine the location and surface brightness distribution of the x ray emission from clusters associated with selected gravitational lenses. The x ray emission comes from hot gas that traces out the total gravitational potential in the cluster, so its distribution is approximately the same as the mass distribution causing gravitational lensing. Core radii and x ray virial masses can be computed for several of the brighter Einstein sources, and preliminary results are presented on A2218. Preliminary status is also reported on a study of the optical data from 0024+16. A provisional value of 1800 to 2200 km/s for the equivalent velocity dispersion is obtained. The ultimate objective is to extract the mass of the gravitational lens, and perhaps more detailed information on the distribution of matter as warranted. A survey of the Einstein archive shows that the clusters A520, A1704, 3C295, A2397, A1722, SC5029-247, A3186 and A370 have enough x ray counts observed to warrant more detailed optical observations of arcs for comparison. Mass estimates for these clusters can therefore be obtained from three independent sources: the length scale (core radius) that characterizes the density dropoff of the x ray emitting hot gas away from its center, the velocity dispersion of the galaxies moving in the cluster potential, and gravitational bending of light by the total cluster mass. This study will allow the comparison of these three techniques and ultimately improve the knowledge of cluster masses.

Kellogg, Edwin M.↗
NASA NTRS1991

An unusually strong Einstein ring in the radio source PKS1830 - 211

High-resolution radio images of PKS1830 - 211 are obtained to study the possibility that the double structure is a gravitationally lensed object. The VLBI observations, taken from interferometric radiotelescope networks, reveal an elliptical ring that connects two bright spots of similar composition. Because the lens and the lensed object are closely aligned, and because of the structure of the two spots, the source is concluded to be a radio Einstein ring. The source is found to be close to the galactic plane, and the lens and the lensed object are extragalactic. The source is also found to be unusually bright, suggesting that it is aligned with a bright background source or amplified by some mechanism related to a source that is not so bright.

Jauncey, D. L.↗
NASA NTRS1992

A gravitational lens candidate discovered with the Hubble Space Telescope

Evidence is reported for gravitational lensing of the high-redshift (z = 3.8) quasar 1208 + 101, observed as part of the Snapshot survey with the HST Planetary Camera. An HST V image taken on gyroscopes resolves the quasar into three point-source components, with the two fainter images having separations of 0.1 and 0.5 arcsec from the central bright component. A radio observation of the quasar with the VLA at 2 cm shows that, like most quasars of this redhsift, 1208 + 101 is radio quiet. Based on positional information alone, the probability that the observed optical components are chance superpositions of Galactic stars is small, but not negligible. Analysis of a combined ground-based spectrum of all three components, using the relative brightnesses of the HST image, supports the lensing hypothesis. If all the components are lensed images of the quasar, the observed configuration cannot be reproduced by simple lens models.

Maoz, Dan↗
NASA NTRS1993

Statistics of highly magnified gravitational images in clusters of galaxies. I - Implications for the clusters. II - Implications for the sources

A study is presently conducted of the probability distribution of observing images of background galaxies that are gravitationally lensed in the form of elongated arcs by clusters of galaxies, as a function of arc width, curvature radius, and the elongation's orientation. An upper limit is obtained on the size of the cores which is smaller than cluster core radii obtained for the gas from the X-ray surface brightness. It is argued that lensing clusters tend to be elongated along the line of sight; projection effects will enhance surface density. In the second part of this work, it is shown that the number of arcs expected in lensing clusters is primarily dependent on the angular size of the sources, since lower magnification is required to generate an arc of fixed length for a larger source.

Miralda-Escude, Jordi↗
NASA NTRS1993

Statistics of arcs in clusters of galaxies

Samples of gravitational lens events in clusters show many large arcs compared to arclets, relative to what can be obtained by idealized singular lens models. We describe the probability of image magnification for point sources and for simple but more realistic gravitational lensing models that include a finite core size and an ellipticity. In addition, we explore the changes in the probability distribution of image magnifications, distortions, and angular extents for sources of different sizes as the parameters of the lenses are varied. A finite core in spherically symmetric lens models introduces a discontinuity in the probability distribution at which the relative number of highly magnified images is increased. In elliptical lenses, this discontinuity and its effect are replaced by a continuous increase in the probability of obtaining high-magnification images relative to singular spherically symmetric models. We also find that the finite size of the source causes a further increase in the expected number of images just below the maximum possible magnification.

Bergmann, Anton G.↗
NASA NTRS1994

Low Earth orbit durability evaluation of protected silicone for advanced refractive photovoltaic concentrator arrays

The need for efficient, cost effective sources of electrical power in space has led to the development of photovoltaic power systems which make use of novel refractive solar concentrators. These concentrators have been conceived in both point-focus and linear-focus designs. Current concentrator lenses are fabricated from flexible silicones with Fresnel facets along their inside surface. To insure the efficient operation of these power systems, the concentrator lenses must be durable and the silicone material must remain specularly transmitting over a reasonable lifetime in low Earth orbit (LEO) and other space environments. Because of the vulnerability of silicones to atomic oxygen and ultraviolet radiation in LEO these lenses have been coated with a multi-layer metal oxide protective coating. The objective of this research was to evaluate the LEO durability of the multilayer coated silicone for advanced refractive photovoltaic concentrator arrays with respect to optical properties and microstructure. Flat metal oxide coated silicone samples were exposed to ground-laboratory and in-space atomic oxyqen for durability evaluation.

Degroh, Kim K.↗
NASA NTRS1994

Galaxy mergers and gravitational lens statistics

We investigate the impact of hierarchical galaxy merging on the statistics of gravitational lensing of distant sources. Since no definite theoretical predictions for the merging history of luminous galaxies exist, we adopt a parameterized prescription, which allows us to adjust the expected number of pieces comprising a typical present galaxy at z approximately 0.65. The existence of global parameter relations for elliptical galaxies and constraints on the evolution of the phase space density in dissipationless mergers, allow us to limit the possible evolution of galaxy lens properties under merging. We draw two lessons from implementing this lens evolution into statistical lens calculations: (1) The total optical depth to multiple imaging (e.g., of quasars) is quite insensitive to merging. (2) Merging leads to a smaller mean separation of observed multiple images. Because merging does not reduce drastically the expected lensing frequency, it cannot make lambda-dominated cosmologies compatible with the existing lensing observations. A comparison with the data from the Hubble Space Telescope (HST) Snapshot Survey shows that models with little or no evolution of the lens population are statistically favored over strong merging scenarios. A specific merging scenario proposed to Toomre can be rejected (95% level) by such a comparison. Some versions of the scenario proposed by Broadhurst, Ellis, & Glazebrook are statistically acceptable.

Rix, Hans-Walter↗
NASA NTRS1994

Near-infrared images of MG 1131+0456 with the W. M. Keck telescope: Another dusty gravitational lens?

Images of the gravitational lens system MG 1131+0456 taken with the near-infrared camera on the W. M. Keck telescope in the J and K(sub s) bands show that the infrared counterparts of the compact radio structure are exceedingly red, with J - K greater than 4.2 mag. The J image reveals only the lensing galaxy, while the K(sub s) image shows both the lens and the infrared counterparts of the compact radio components. After subtracting the lensing galaxy from the K(sub s) image, the position and orientation of the compact components agree with their radio counterparts. The broad-band spectrum and observed brightness of the lens suggest a giant galaxy at a redshift of approximately 0.75, while the color of the quasar images suggests significant extinction by dust in the lens. There is a significant excess of faint objects within 20 sec of MG 1131+0456. Depending on their mass and redshifts, these objects could complicate the lensing potential considerably.

Larkin, J. E.↗
NASA NTRS1995

Surface temperature of a magnetized neutron star and interpretation of the ROSAT data. 1: Dipole fields

We model the temperature distribution at the surface of a magnetized neutron star and study the effects on the observed X-ray spectra and light curves. Generalrelativistic effects, i.e., redshift and lensing, are fully taken into account. Atmospheric effects on the emitted spectral flux are not included: we consider only blackbody emission at the local effective temperature. In this first paper we restrict ourselves to dipole fields. General features are studied and compared with the ROSAT data from the pulsars 0833 - 45 (Vela), 0656 + 14, 0630 + 178 (Geminga), and 1055 - 52, the four cases for which there is strong evidence that thermal radiation from the stellar surface is detected. The composite spectra we obtain are not very different from a blackbody spectrum at the star's effective temperature. We conclude that, as far as blackbody spectra are considered, temperature estimates using single-temperature models give results practically identical to our composite models. The change of the (composite blackbody) spectrum with the star's rotational phase is also not very large and may be unobservable inmost cases. Gravitational lensing strongly suppresses the light curve pulsations. If a dipole field is assumed, pulsed fractions comparable to the observed ones can be obtained only with stellar radii larger than those which are predicted by current models of neutron star struture, or with low stellar masses. Moreover, the shapes of the theoretical light curves with dipole fields do not correspond to the observations. The use of magnetic spectra may raise the pulsed fraction sufficiently but will certainly make the discrepancy with the light curve shapes worse: dipole fields are not sufficient to interpret the data. Many neutron star models with a meson condensate or hypersons predict very small radii, and hence very strong lensing, which will require highly nondipolar fields to be able to reproduce the observed pulsed fractions, if possible at all: this may be a new tool to constrain the size of neutron stars. The pulsed fractions obtained in all our models increase with photon energy: the strong decrease observed in Geminga at energies 0.3-0.5 keV is definitely a genuine effect of the magnetic field on the spectrum in contradistinction to the magnetic effects on the surface temperature considered her. Thus, a detailed analysis of thermal emission from the four pulsars we consider will require both complex surface field configurations and the inclusion of magnetic effects in the atmosphere (i.e., on the emitted spectrum).

Page, Dany↗
NASA NTRS1997

X-Ray Gas Temperatures in the Arc Clusters MS0440+204 and MS0302+1658

The cluster of galaxies MS0440+02, originally discovered through its X-ray emission, was part of an optical observational program to search for arcs and arclets in a complete sample of X-ray luminous, medium-distant clusters of galaxies. Mauna Kea CCD images of MS0440+02 showed a remarkable optical morphology. The core of the cluster contains 6 bright galaxies and numerous fainter ones embedded in a low surface brightness halo. Besides, MS0440+02 is the most spectacular example that we have found of an arc system in a compact condensed cluster, with arcs symmetrically distributed to draw almost perfect circles around the cluster center. Giant arcs are magnified images of distant galaxies, gravitationally distorted by massive foreground clusters. It is of great importance to compare the results of the lensing studies with those derived from X-ray observations, as the two are independent methods of studying the mass distribution. Thus MS0440+02 was the ideal target to obtain temperature measurement with ASCA and good spatial resolution X-ray observations with ROSAT. The X-ray data have been used in conjunction with Hubble Space Telescope observations to put more stringent constrains on the mass estimates. Most of the different wavelength datasets have been reduced and analyzed. Mass determinations have been separately obtained from galaxy virial motions and X-ray profile fitting using the cluster gas temperature as measured by the ASCA satellite. Assuming that the hot gas is in hydrostatic equilibrium and in a spherical potential, we find from the X-ray data a mass distribution profile that is well described by a Beta model. From the multiple images formed by gravitational lensing (HST data) using the modelling of the gravitational lensed arcs, we have derived Beta model. To reconcile the mass estimates we have explored the possibility of having a supercluster surrounding the MOS0440 cluster, that is a model with two isothermal spheres, one embedded inside the other. These results have been published or are in press.

Gioia, Isabella M.↗
NASA NTRS1998

X-ray Spectra of Distant Clusters

The masses of galaxy clusters are dominated by dark matter, and a robust determination of their temperatures and masses has the potential of indicating how much dark matter exists on large scales in the universe, and the cosmological parameter Omega. X-ray observations of galaxy clusters provide a direct measure of both the gas mass in the intra-cluster medium, and also the total gravitating mass of the cluster. We used new and archival ASCA and ROSAT observations to measure these quantities for a sample of intermediate redshift clusters which have also been subject to intensive dynamical studies, in order to compare the mass estimates from different methods. We used data from 12 of the CNOC cluster sample at 0.18 less than z less than 0.55 for this study. A direct comparison of dynamical mass estimates from Carlberg, Yee & Ellingson (1997) yielded surprisingly good results. The X-ray/dynamical mass ratios have a mean of 0.96+/- 0.10, indicating that for this sample, both methods are probably yielding very robust mass estimates. Comparison with mass estimates from gravitational lensing studies from the literature showed a small systematic with weak lensing estimates, and large discrepancies with strong lensing estimates. This latter is not surprising, given that these measurements are made close to the central core, where optical and Xray estimates are less certain, and where substructure and the effects of individual galaxies will be more pronounced. These results are presented in Lewis, Ellingson, Morris \& Carlberg, 1998, submitted to the Astrophysical Journal.

Ellingson, E.↗
NASA NTRS1999

The Sunyaev-Zel'dovich Effect in Abell 370

We present interferometric measurements of the Sunyaev-Zel'dovich (SZ) effect towards the galaxy cluster Abell 370. These measurements, which directly probe the pressure of the cluster's gas, show the gas is strongly aspherical, on agreement with the morphology revealed by x-ray and gravitational lensing observations. We calculate the cluster's gas mass fraction by comparing the gas mass derived from the SZ measurements to the lensing-derived gravitational mass near the critical lensing radius. We also calculate the gas mass fraction from the SZ data by deriving the total mass under the assumption that the gas is in hydrostatic equilibrium (HSE). We test the assumptions in the HSE method by comparing the total cluster mass implied by the two methods. The Hubble constant derived for this cluster, when the known systematic uncertainties are included, has a very wide range of values and therefore does not provide additional constraints on the validity of the assumptions. We examine carefully the possible systematic errors in the gas fraction measurement. The gas fraction is a lower limit to the cluster's baryon fraction and so we compare the gas mass fraction, calibrated by numerical simulations to approximately the virial radius, to measurements of the global mass fraction of baryonic matter, OMEGA(sub B)/OMEGA(sub matter). Our lower limit to the cluster baryon fraction is f(sub B) = (0.043 +/- 0.014)/h (sub 100). From this, we derive an upper limit to the universal matter density, OMEGA(sub matter) <= 0.72/h(sub 100), and a likely value of OMEGA(sub matter) <= (0.44(sup 0.15, sub -0.12)/h(sub 100).

Grego, Laura↗
NASA NTRS2000

The Sunyaev-Zeldovich Effect in Abell 370

We present interferometric measurements of the Sunyaev-Zeldovich (SZ) effect toward the galaxy cluster Abell 370. These measurements, which directly probe the pressure of the cluster's gas, show the gas distribution to be strongly aspherical, as do the X-ray and gravitational lensing observations. We calculate the cluster's gas mass fraction in two ways. We first compare the gas mass derived from the SZ measurements to the lensing-derived gravitational mass near the critical lensing radius. We also calculate the gas mass fraction from the SZ data by deprojecting the three-dimensional gas density distribution and deriving the total mass under the assumption that the gas is in hydrostatic equilibrium (HSE). We test the assumptions in the HSE method by comparing the total cluster mass implied by the two methods and find that they agree within the errors of the measurement. We discuss the possible system- atic errors in the gas mass fraction measurement and the constraints it places on the matter density parameter, Omega(sub M).

Grego, Laura↗
NASA NTRS2001

Horizontal vestibuloocular reflex evoked by high-acceleration rotations in the squirrel monkey. IV. Responses after spectacle-induced adaptation

The horizontal angular vestibuloocular reflex (VOR) evoked by sinusoidal rotations from 0.5 to 15 Hz and acceleration steps up to 3,000 degrees /s(2) to 150 degrees /s was studied in six squirrel monkeys following adaptation with x2.2 magnifying and x0.45 minimizing spectacles. For sinusoidal rotations with peak velocities of 20 degrees /s, there were significant changes in gain at all frequencies; however, the greatest gain changes occurred at the lower frequencies. The frequency- and velocity-dependent gain enhancement seen in normal monkeys was accentuated following adaptation to magnifying spectacles and diminished with adaptation to minimizing spectacles. A differential increase in gain for the steps of acceleration was noted after adaptation to the magnifying spectacles. The gain during the acceleration portion, G(A), of a step of acceleration (3,000 degrees /s(2) to 150 degrees /s) increased from preadaptation values of 1.05 +/- 0.08 to 1.96 +/- 0.16, while the gain during the velocity plateau, G(V), only increased from 0.93 +/- 0.04 to 1.36 +/- 0.08. Polynomial fits to the trajectory of the response during the acceleration step revealed a greater increase in the cubic than the linear term following adaptation with the magnifying lenses. Following adaptation to the minimizing lenses, the value of G(A) decreased to 0.61 +/- 0.08, and the value of G(V) decreased to 0.59 +/- 0.09 for the 3,000 degrees /s(2) steps of acceleration. Polynomial fits to the trajectory of the response during the acceleration step revealed that there was a significantly greater reduction in the cubic term than in the linear term following adaptation with the minimizing lenses. These findings indicate that there is greater modification of the nonlinear as compared with the linear component of the VOR with spectacle-induced adaptation. In addition, the latency to the onset of the adapted response varied with the dynamics of the stimulus. The findings were modeled with a bilateral model of the VOR containing linear and nonlinear pathways that describe the normal behavior and adaptive processes. Adaptation for the linear pathway is described by a transfer function that shows the dependence of adaptation on the frequency of the head movement. The adaptive process for the nonlinear pathway is a gain enhancement element that provides for the accentuated gain with rising head velocity and the increased cubic component of the responses to steps of acceleration. While this model is substantially different from earlier models of VOR adaptation, it accounts for the data in the present experiments and also predicts the findings observed in the earlier studies.

NASA Discipline Neuroscience↗
NASA NTRS2004

Tunable-Bandwidth Filter System

A tunable-bandwidth filter system (TBFS), now undergoing development, is intended to be part of a remote sensing multispectral imaging system that will operate in the visible and near infrared spectral region (wavelengths from 400 to 900 nm). Attributes of the TBFS include rapid tunability of the pass band over a wide wavelength range and high transmission efficiency. The TBFS is based on a unique integration of two pairs of broadband Raman reflection holographic filters with two rotating spherical lenses. In experiments, a prototype of the TBFS, was shown to be capable of spectral sampling of images in the visible range over a 200 nm spectral range with a spectral resolution of 30 nm. The figure depicts the optical layout of a prototype of the TBFS as part of a laboratory multispectral imaging system for the spectral sampling of color test images in two orthogonal polarizations. Each pair of broadband Raman reflection holographic filters is mounted at an equatorial plane between two halves of a spherical lens. The two filters in each pair are characterized by steep spectral slopes (equivalently, narrow spectral edges), no ripple or side lobes in their pass bands, and a few nanometers of non-overlapping wavelength range between their pass bands. Each spherical lens and thus the filter pair within it is rotated in order to rapidly tune its pass band. The rotations of are effected by electronically controlled, programmable, high-precision rotation stages. The rotations are coordinated by electronic circuits operating under overall supervision of a personal computer in order to obtain the desired variation of the overall pass bands with time. Embedding the filters inside the spherical lenses increases the range of the hologram incidence angles, making it possible to continuously tune the pass and stop bands of the filters over a wider wavelength range. In addition, each spherical lens also serves as part of the imaging optics: The telephoto lens focuses incoming light to a field stop that is also a focal point of each spherical lens. A correcting lens in front of the field stop compensates for the spherical aberration of the spherical lenses. The front surface of each spherical lens collimates the light coming from the field stop. After the collimated light passes through the filter in the spherical lens, the rear surface of the lens focuses the light onto a charge-coupled-device image detector.

Bailey, John W.↗