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Results for “GRAVITATIONAL LENSES”

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NASA NTRS1992

Gravitational lensing by a massive black hole at the Galactic center

The manifestations of gravitational lensing by a massive black hole at the Galactic center, with particular attention given to lensing of stars in the stellar cluster that lie behind Sgr A*, and of Sgr A east, a nonthermal extended radio source which is known with certainty to lie behind the Galactic center. Lensing of the stellar cluster produces a deficit of stellar images within 10 mas of the center, and a surplus between 30 and 300 mas. The results suggest that the proper motion of the stars will produce brightness variations of stellar images on a time scale of a few years or less. Both images of such a source should be visible, and will rise and fall in luminosity together.

Wardle, Mark↗
NASA NTRS1981

On gravitational lenses and the cosmological evolution of quasars

A heuristic model for the effect of gravitational lenses on the apparent cosmological evolution of quasars is considered. The model satisfies the requirement of average flux conservation and has no net mean amplification. This requirement is shown to be numerically important in studying the effect. On the basis of the values of the evolution indicators calculated from the model, it is concluded that it is premature to assert that lensing plays an important role in affecting the apparent evolution. A qualitative, model independent observational test for the effect is suggested. The test estimates the distances where lensing is dominant. An application of this test to a complete sample of quasars indicates that lensing cannot completely account for the apparent evolution, except in an extreme situation.

Avni, Y.↗
NASA NTRS1987

Gravitational lenses and dark matter - Theory

Theoretical models are presented for guiding the application of gravitational lenses to probe the characteristics of dark matter in the universe. Analytical techniques are defined for quantifying the mass associated with lensing galaxies (in terms of the image separation), determining the quantity of dark mass of the lensing bodies, and estimating the mass density of the lenses. The possibility that heavy halos are made of low mass stars is considered, along with the swallowing of central images of black holes or cusps in galactic nuclei and the effects produced on a lensed quasar image by nonbaryonic halos. The observable effects of dense groups and clusters and the characteristics of dark matter strings are discussed, and various types of images which are possible due to lensing phenomena and position are described.

Gott, J. Richard, III↗
NASA NTRS1986

On the distance measure for gravitational lenses

A discussion is presented of some of the physical elements that are important for geometric optics in curved space-time, with a view toward interpreting data on gravitational lenses. This is important because each gravitational lens provides geometrical information about an arc that crosses a substantial portion of the universe. The propagation of photons along long arcs is highly sensitive to the mean density in the beam, averaged along the arc. Fluctuations about this mean are not important unless their scale is comparable to that of the universe. Tidal distortion is important if only a few lumps are involved. The variation in the geometrical optics is considerably greater if there is a nonzero cosmological constant.

Alcock, C.↗
NASA NTRS1986

The statistics of gravitational lenses. III - Astrophysical consequences of quasar lensing

The method of Schmidt and Green (1983) for calculating the luminosity function of quasars is combined with gravitational-lensing theory to compute expected properties of lensed systems. Multiple quasar images produced by galaxies are of order 0.001 of the observed quasars, with the numbers over the whole sky calculated to be (0.86, 120, 1600) to limiting B magnitudes of (16, 19, 22). The amount of 'false evolution' is small except for an interesting subset of apparently bright, large-redshift objects for which minilensing by starlike objects may be important. Some of the BL Lac objects may be in this category, with the galaxy identified as the parent object really a foreground object within which stars have lensed a background optically violent variable quasar.

Ostriker, J. P.↗
NASA NTRS1988

Arcs from gravitational lensing

The proposal made by Paczynski (1987) that the arcs of blue light found recently in two cluster cores are gravitationally lensed elongated images of background galaxies is investigated. It is shown that lenses that are circularly symmetric in projection produce pairs of arcs, in conflict with the observations. However, more realistic asymmetric lenses produce single arcs, which can become as elongated as the observed ones whenever the background galaxy is located on or close to a cusp caustic. Detailed computer simulations of lensing by clusters using a reasonable model of the mass distribution are presented. Elongated and curved lensed images longer than 10 arcsec occur in 12 percent of the simulated clusters. It is concluded that the lensing hypothesis must be taken seriously.

Grossman, Scott A.↗
NASA NTRS1987

Gravitational lensing of supernovae by dark matter candidates of mass M greater than about 0.001 solar masses

A review is presented concerning the gravitational lensing of supernovae by intervening condensed objects, including dark matter candidates such as dim stars and black holes. the expansion of the supernova beam within the lens produces characteristic time-dependent amplification and polarization which depend upon the mass of the lens. The effects of the shearing of the beam due to surrounding masses are considered, although the study of these effects is confined to isolated masses whose size is much less than that of the supernova (about 10 to the 15th cm). Equations for the effects of lensing and graphs comparing these effects in different classes of supernovae are compared. It is found that candidates for lensing would be those supernovae at least as bright as their parent galaxy, or above the range of luminosities expected for their spectral class.

Wagoner, Robert V.↗
NASA NTRS1987

Gravitational lenses and large scale structure

Four possible statistical tests of the large scale distribution of cosmic material are described. Each is based on gravitational lensing effects. The current observational status of these tests is also summarized.

Turner, Edwin L.↗
NASA NTRS1992

Gravitationally lensed arcs in two X-ray selected clusters of galaxies at z = 0.426 and z = 0.583

The discovery of two clusters of galaxies that contain new gravitationally lensed images (arcs) of high-redshift background galaxies is reported. Both clusters, MS1621.5+2640 and MS2053.7-0449, were selected from the Einstein Extended Medium Sensitivity Survey based on their high X-ray luminosity. V-band CCD images of the clusters taken in 0.5 arcsec seeing are presented, and it is shown that the arcs in both clusters are thin; they remain unresolved in width, but are fragmented along their length. The arc in MS1621.5+2640 has a small radius of curvature (about 7 arcsec), and spans about 70 deg centered near a 2.6 mJy radio galaxy that is not the brightest cluster galaxy. The arc in MS2053.7-0449 spans 40 deg at a radius of about 16 arcsec with a center slightly offset from the optically dominant cluster galaxy. The arc fragments into two clumps that appear to be stretched images of background galaxies.

Luppino, G. A.↗
NASA NTRS1991

Gravitational lensing by clusters of galaxies - Constraining the mass distribution

The possibility of placing constraints on the mass distribution of a cluster of galaxies by analyzing the cluster's gravitational lensing effect on the images of more distant galaxies is investigated theoretically in the limit of weak distortion. The steps in the proposed analysis are examined in detail, and it is concluded that detectable distortion can be produced by clusters with line-of-sight velocity dispersions of over 500 km/sec. Hence it should be possible to determine (1) the cluster center position (with accuracy equal to the mean separation of the background galaxies), (2) the cluster-potential quadrupole moment (to within about 20 percent of the total potential if velocity dispersion is 1000 km/sec), and (3) the power law for the outer-cluster density profile (if enough background galaxies in the surrounding region are observed).

Miralda-Escude, Jordi↗
NASA NTRS1985

Are some BL Lac objects artefacts of gravitational lensing?

It is proposed here that a significant fraction of BL Lac objects are optically violently variable quasars whose continuum emission has been greatly amplified, relative to the line emission, by pointlike gravitational lenses in intervening galaxies. Several anomalous physical and statistical properties of BL Lacs can be understood on the basis of this model, which is immediately testable on the basis of absorption line studies and by direct imaging.

Ostriker, J. P.↗
NASA NTRS1995

The Luminous IRAS Source FSC10214+4724 is Gravitationally Lensed

An IRAS source appears to be one of the most luminous objects known in the universe. This source has been suggested to be either a gas- rich protogalaxy or a dust embedded quasar. A strong suggestion that this source is gravitationally lensed is modeled, and implications are discussed.

IRAS Space Object Quasar Protogalaxy Astronomy↗
NASA NTRS1990

Gravitational lensing limits on the cosmological constant in a flat universe

Inflationary cosmological theories predict, and some more general aesthetic criteria suggest, that the large-scale spatial curvature of the universe k should be accurately zero (i.e., flat), a condition which is satisfied when the universe's present mean density and the value of the cosmological constant Lambda have certain pairs of values. Available data on the frequency of multiple image-lensing of high-redshift quasars by galaxies suggest that the cosmological constant cannot make a dominant contribution to producing a flat universe. In particular, if the mean density of the universe is as small as the baryon density inferred from standard cosmic nucleosynthesis calculations or as determined from typical dynamical studies of galaxies and galaxy clusters, then a value of Lambda large enough to produce a k = 0 universe would result in a substantially higher frequency of multiple-image lensing of quasars than has been observed so far. Shortcomings of the available lens data and uncertainties concerning galaxy properties allow some possibility of escaping this conclusion, but systematic searches for a gravitational lenses and continuing investigations of galaxy mass distributions should soon provide decisive information. It is also noted that nonzero-curvature cosmological models can account for the observed frequency of galaxy-quasar lens systems and for a variety of other constraints.

Turner, Edwin L.↗
NASA NTRS1992

Gravitational lensing effects in a time-variable cosmological 'constant' cosmology

A scalar field phi with a potential V(phi) varies as phi exp -alpha(alpha is greater than 0) has an energy density, behaving like that of a time-variable cosmological 'constant', that redshifts less rapidly than the energy densities of radiation and matter, and so might contribute significantly to the present energy density. We compute, in this spatially flat cosmology, the gravitational lensing optical depth, and the expected lens redshift distribution for fixed source redshift. We find, for the values of alpha of about 4 and baryonic density parameter Omega of about 0.2 consistent with the classical cosmological tests, that the optical depth is significantly smaller than that in a constant-Lambda model with the same Omega. We also find that the redshift of the maximum of the lens distribution falls between that in the constant-Lambda model and that in the Einstein-de Sitter model.

Ratra, Bharat↗
NASA NTRS1996

Hubble Space Telescope Observations of the Luminous IRAS Source FSC 10214+4724: A Gravitationally Lensed Infrared Quasar

With a redshift of 2.3, the IRAS source FSC 10214+4724 is apparently one of the most luminous objects known in the universe. We present an image of FSC 10214+4724 at 0.8 pm obtained with the Hubble Space Telescope (HST) WFPC2 Planetary Camera. The source appears as an unresolved (less then 0.06) arc 0.7 long, with significant substructure along its length. The center of curvature of the arc is located near an elliptical galaxy 1.18 to the north. An unresolved component 100 times fainter than the arc is clearly detected on the opposite side of this galaxy. The most straightforward interpretation is that FSC 10214+4724 is gravitationally lensed by the foreground elliptical galaxy, with the faint component a counter-image of the IRAS source. The brightness of the arc in the HST image is then magnified by approx. 100, and the intrinsic source diameter is approx. 0.0l (80 pc) at 0.25 microns rest wavelength. The bolometric luminosity is probably amplified by a smaller factor (approx. 30) as a result of the larger extent expected for the source in the far-infrared. A detailed lensing model is presented that reproduces the observed morphology and relative flux of the arc and counterimage and correctly predicts the position angle of the lensing galaxy. The model also predicts reasonable values for the velocity dispersion, mass, and mass-to-light ratio of the lensing galaxy for a wide range of galaxy redshifts. A redshift for the lensing galaxy of -0.9 is consistent with the measured surface brightness profile from the image, as well as with the galaxy's spectral energy distribution. The background lensed source has an intrinsic luminosity approx. 2 x 10(exp 13) L(solar mass) and remains a highly luminous quasar with an extremely large ratio of infrared to optical/ultraviolet luminosity.

Eisenhardt, Peter R.↗
NASA NTRS1994

Implications for gravitational lensing and the dark matter content in clusters of galaxies from spatially resolved x-ray spectra

A simple method for deriving well-behaved temperature solutions to the equation of hydrostatic equilibrium for intracluster media with X-ray imaging observations is presented and applied to a series of generalized models as well as to observations of the Perseus cluster and Abell 2256. In these applications the allowed range in the ratio of nonbaryons to baryons as a function of radius is derived, taking into account the uncertainties and crude spatial resolution of the X-ray spectra and considering a range of physically reasonable mass models with various scale heights. Particular attention is paid to the central regions of the cluster, and it is found that the dark matter can be sufficiently concentrated to be consistent with the high central mass surface densities for moderate-redshift clusters from their gravitational lensing properties.

Loewenstein, M.↗