arXiv subjects

Charles R. Keeton

Publications and source records attributed to Charles R. Keeton.

87 records · Page 5Linked to original sources

arXiv2003

The Quintuple Quasar: Mass Models and Interpretation

The strange morphology of the six-component gravitational lens PMN J0134-0931 has resisted explanation. We present the first successful quantitative models for the system, based on the idea that there are two lens galaxies and two components of the background source. One source is quintuply imaged and corresponds to the five brightest observed radio components. The other source is triply imaged and corresponds to the sixth component, along with two others too faint to have been detected. The models reproduce the observed image positions and fluxes, and make falsifiable predictions about other properties of the system. Some of these predictions have been confirmed by high-resolution radio and optical observations, as described in the companion paper by Winn et al. (2003). Although we cannot determine the lens model uniquely with current data, we predict that the lens galaxies are spiral galaxies with roughly equal velocity dispersions sigma~120km/s and a projected separation of only 0.4" (2/h kpc at z_l=0.76). This system is the first known lens with five images of a single quasar, and the second with more than four images.

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arXiv2002

Analytic Cross Sections for Substructure Lensing

The magnifications of the images in a strong gravitational lens system are sensitive to small mass clumps in the lens potential; this effect has been used to infer the amount of substructure in galaxy dark matter halos. I study the theory of substructure lensing to identify important general features, and to compute analytic cross sections that will facilitate further theoretical studies. I show that the problem of a clump anywhere along the line of sight to a lens can be mapped onto an equivalent problem of a clump in a simple convergence and shear field; clumps at arbitrary redshifts are therefore not hard to handle in calculations. For clumps modeled as singular isothermal spheres (SIS), I derive simple analytic estimates of the cross section for magnification perturbations of a given strength. The results yield two interesting conceptual points. First, lensed images with positive parity are always made brighter by SIS clumps; images with negative parity can be brightened but are much more likely to be dimmed. Second, the clumps need not lie within the lens galaxy; they can be moved in redshift by several tenths and still have a significant lensing effect. Isolated small halos are expected to be common in hierarchical structure formation models, but it is not yet known whether they are abundant enough compared with clumps inside lens galaxies to affect the interpretation of substructure lensing.

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arXiv2002

The Redshift of a Lensing Galaxy in PMN J0134-0931

The Sloan Digital Sky Survey (SDSS) automatically targeted as a quasar candidate the recently discovered, gravitationally lensed, extremely reddened z=2.2 quasar PMN J0134-0931. The SDSS spectrum exhibits Ca II absorption at z=0.76451, which we identify as the redshift of a lensing galaxy. Hubble Space Telescope imaging shows that components CDE of the system are significantly redder than components A or B and detects faint galaxy emission between D and A+B. The redshift of the dust responsible for the reddening remains unconstrained with current data. However, we outline a model wherein lensing and differential reddening by a z=0.76451 galaxy pair can entirely explain this system. Detecting mm-wave molecular line absorption from the lensing galaxy or galaxies may be possible in PMN J0134-0931, just as in the lenses PKS1830-211 and B0218+357. Well-constructed optical quasar surveys like the SDSS can contribute to the detection and study of reddened quasars. (Expanded)

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arXiv2002

Rethinking Lensing and Lambda

Strong gravitational lensing has traditionally been one of the few phenomena said to oppose a large cosmological constant; many analyses of lens statistics have given upper limits on $Ω_Λ$ that are marginally inconsistent with the concordance cosmology. Those conclusions were based on models where the predicted number counts of galaxies at moderate redshifts (z~0.5-1) increased significantly with $Ω_Λ$. I argue that the models should now be calibrated by counts of distant galaxies. When this is done lens statistics lose most of their sensitivity to the cosmological constant.

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arXiv2002

Lensing and the Centers of Distant Early-Type Galaxies

Gravitational lensing provides a unique probe of the inner 10-1000 pc of distant galaxies (z=0.2-1). Lens theory predicts that every strong lens system should have a faint image near the center of the lens galaxy, which should be visible in radio lenses but have not been observed. We study these ``core'' images using models derived from the stellar distributions in nearby early-type galaxies. We find that realistic galaxies predict a remarkably wide range of core images, with lensing magnifications spanning some six orders of magnitude. More concentrated galaxies produce fainter core images, although not with any simple, quantitative, model independent relation. Some real galaxies have diffuse cores and predict bright core images (magnification mu>~0.1), but more common are galaxies that predict faint core images (mu<~0.001). Thus, stellar mass distributions alone are probably concentrated enough to explain the lack of observed core images, and may require observational sensitivity to improve by an order of magnitude before detections of core images become common. Two-image lenses will tend to have brighter core images than four-image lenses, so they will be the better targets for finding core images and exploiting these tools for studying the central mass distributions of distant galaxies.

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arXiv2002

A Catalog of Mass Models for Gravitational Lensing

Many different families of mass models are used in modern applications of strong gravitational lensing. I review a wide range of popular models, with two points of emphasis: (1) a discussion of strategies for building models suited to a particular lensing problem; and (2) a summary of technical results for a canonical set of models. All of the models reviewed here are included in publicly-available lensing software called gravlens.

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arXiv2002

Three-Dimensional Mapping of the Dark Matter

We study the prospects for three-dimensional mapping of the dark matter to high redshift through the shearing of faint galaxies images at multiple distances by gravitational lensing. Such maps could provide invaluable information on the nature of the dark energy and dark matter. While in principle well-posed, mapping by direct inversion introduces exceedingly large, but usefully correlated noise into the reconstruction. By carefully propagating the noise covariance, we show that lensing contains substantial information, both direct and statistical, on the large-scale radial evolution of the density field. This information can be efficiently distilled into low-order signal-to-noise eigenmodes which may be used to compress the data by over an order of magnitude. Such compression will be useful for the statistical analysis of future large data sets. The reconstructed map also contains useful information on the localization of individual massive dark matter halos, and hence the dark energy from halo number counts, but its extraction depends strongly on prior assumptions. We outline a procedure for maximum entropy and point-source regularization of the maps that can identify alternate reconstructions.

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arXiv2001

Lens Galaxies vs. CDM

By directly probing mass distributions, gravitational lensing offers several new tests of the CDM paradigm. Lens statistics place upper limits on the dark matter content of elliptical galaxies. Galaxies built from CDM mass distributions are too concentrated to satisfy these limits, so lensing extends the ``concentration problem'' in CDM to elliptical galaxies. The central densities of the model galaxies are too low on ~10 pc scales to agree with the lack of central images in observed lenses. The flux ratios of four-image lenses imply a substantial population of dark matter clumps with a typical mass ~10^6 Msun. Thus, lensing implies the need for a mechanism that reduces dark matter densities on kiloparsec scales without erasing structure on smaller scales.

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arXiv2001

Discovering Mass Clumps in Distant Galaxies with Lensing: The Case of B1422+231

Substructure in distant gravitational lens galaxies can be detected because it alters the brightnesses and shapes of the lensed images. The optical and radio flux ratios in the four-image lens B1422+231 imply that there is a \sim 10^4-10^7 Msun/h mass clump in the lens galaxy in front of image A; this is the first constraint on the mass of a particular clump lying in a distant galaxy and detected by its mass. The data also indicate that a small clump, perhaps a star, is passing in front of image B and making the optical flux ratios variable. Both of these hypotheses can be tested with new observations. B1422+231 demonstrates how data at different wavelengths can be used in individual lenses to probe individual mass clumps in distant galaxies.

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arXiv2001

Source Ellipticity and the Statistics of Lensed Arcs

The statistics of gravitationally lensed arcs, which can be used for a variety of cosmological tests, are sensitive to the intrinsic shapes of the source galaxies. I present an analytic formalism that makes it simple to include elliptical sources in analytic calculations of lens statistics. For cuspy lens models, sources with an axis ratio of 2:1 enhance the total number of arcs longer than 10:1 by a factor of order two, while modestly decreasing the number ratio of radial arcs to tangential arcs. Source ellipticity is therefore an important systematic effect in detailed quantitative studies, but it should not hinder cosmological applications such as attempts to constrain cluster dark matter profiles with arc statistics.

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arXiv2001

CDM and Strong Lensing: Concord or Conflict?

(Abridged) Using the number and sizes of observed gravitational lenses, I derive upper limits on the dark matter content of elliptical galaxies. Galaxies built from Cold Dark Matter (CDM) mass distributions are too concentrated to comfortably satisfy these limits; SCDM is ruled out, and LCDM is only marginally consistent with the data. Thus, lensing adds to the evidence that CDM mass distributions are too concentrated on kiloparsec scales to agree with real galaxies, and extends the argument to elliptical galaxies. By contrast, the lack of central images in radio lenses implies that the central densities of CDM galaxies are too low on ~10 parsec scales, even if supermassive black holes are included. Self-interacting dark matter, or any other modification to regular cold dark matter, must simultaneously reduce the densities on kiloparsec scales and increase the densities on parsec scales in order to satisfy the unique constraints from lensing.

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arXiv2001

Computational Methods for Gravitational Lensing

Modern applications of strong gravitational lensing require the ability to use precise and varied observational data to constrain complex lens models. I discuss two sets of computational methods for lensing calculations. The first is a new algorithm for solving the lens equation for general mass distributions. This algorithm makes it possible to apply arbitrarily complicated models to observed lenses. The second is an evaluation of techniques for using observational data including positions, fluxes, and time delays of point-like images, as well as maps of extended images, to constrain models of strong lenses. The techniques presented here are implemented in a flexible and user-friendly software package called gravlens, which is made available to the community.

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arXiv2001

Lensing Constraints on the Cores of Massive Dark Matter Halos

The statistics of wide-separation (6'' < theta < 15'') gravitational lenses constrain the amount of mass in the cores of dark matter halos on group and cluster mass scales. For a family of halo models with a central cusp $ρ\propto r^{-alpha}$ (1.0 <= alpha <= 1.9), the lack of wide-separation lenses in the large Cosmic Lens All-Sky Survey yields an upper limit on the fraction of the halo mass that is contained within ~4% of the virial radius, f_{core} < 0.023 (95% confidence level, LCDM). This limit offers an important test of the cold dark matter paradigm. While the halo profiles derived from numerical simulations appear to be consistent with this upper limit, larger surveys currently underway such as the 2dF and SDSS should detect wide-separation lenses and thus provide a measurement of the core mass fraction in massive dark matter halos.

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arXiv1999

A reassessment of the data and models of the gravitational lens Q0957+561

We examine models of the mass distribution for the first known case of gravitational lensing. Several new sets of constraints are used, based on recent observations. We remodel the VLBI observations of the radio jets in the two images of Q0957+561, showing that the previously derived positions and uncertainties were incorrect. We use as additional constraints the candidate lensed pairs of galaxies identified recently with the Hubble Space Telescope. We explore a wider range of lens models than before, and find that the Hubble constant is not tightly constrained once elliptical lens models are considered. We also discuss the systematic uncertainties caused by the cluster containing the lens galaxy. We conclude that additional observations of the candidate lensed galaxies as well as direct measurements of the cluster mass profile are needed before an accurate value of the Hubble constant can be derived from this lens.

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arXiv1998

Q1208+1011: Search for the lensing galaxy

We present a high-resolution spectrum of the high redshift, lensed quasar Q1208+1101, obtained with the echellette spectrograph on the Multiple Mirror Telescope. We examine the new and published spectra and provide an updated list of high-confidence metal-line absorption systems at z=1.1349, 2.8626, 2.9118, 2.9136, 2.9149. Combining this with a simple model of the gravitational lens system allows us to constrain the possible lens redshifts. The high-redshift (z > 2.5) and low-redshift (z < 0.4) candidates can be ruled out with high confidence. The current spectra effectively probe about 40% of the redshift range in which the lens is expected. In that range, there is only one known metal-line absorption system, an MgII absorber at z=1.1349. We consider the possibility that this system is the lensing galaxy and discuss the implied parameters of the galaxy.

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