大栗 真宗

We characterize the typical offset between the dark matter (DM) projected centre and the brightest cluster galaxy (BCG) in 10?000 Sloan Digital Sky Survey clusters. To place constraints on the centre of DM, we use an automated strong lensing (SL) analysis, mass-modelling technique which is based on the well-tested assumption that light traces mass. The cluster galaxies are modelled with a steep power law, and the DM component is obtained by smoothing the galaxy distribution fitting a low-order two-dimensional polynomial (via spline interpolation), while probing a whole range of polynomial degrees and galaxy power laws. We find that the offsets between the BCG and the peak of the smoothed light map representing the DM, ?, are distributed equally around zero with no preferred direction, and are well described by a log-normal distribution with < log(10)(Delta[h(-1) Mpc ])> = -1.895(-0.004)(+0.003) and sigma = 0.501 +/- 0.004 (95 per cent confidence levels) or < log(10)(Delta[arcsec])> = 0.564 +/- 0.005 and sigma = 0.475 +/- 0.007. Some of the offsets originate in prior misidentifications of the BCG or other bright cluster members by the cluster finding algorithm, whose level we make an additional effort to assess, finding that similar to 10 per cent of the clusters in the probed catalogue are likely to be misidentified, contributing to higher-end offsets in general agreement with previous studies. Our results constitute the first statistically significant high-resolution distributions of DM-to-BCG offsets obtained in an observational analysis, and importantly show that there exists such a typical non-zero offset, in the probed catalogue. The offsets show a weak positive correlation with redshift, so that higher separations are generally found for higher z clusters in agreement with the hierarchical growth of structure, which in turn could potentially help characterize the merger, relaxation and evolution history of clusters in future studies. In addition, the effective DM centre we adopt here, namely the peak of the smoothed light distribution representing the DM, can constitute a natural and alternative definition of cluster centres for optically selected cluster catalogues.

We present results from Chandra observations of the cluster lens SDSS J1029+2623 at z(l) = 0.58, which is a gravitationally lensed quasar with the largest known image separation. We clearly detect X-ray emission both from the lensing cluster and the three lensed quasar images. The cluster has an X-ray temperature of kT = 8.1(-1.2)(+2.0) keV and bolometric luminosity of L-X = 9.6 x 10(44) erg s(-1). Its surface brightness is centered near one of the brightest cluster galaxies, and it is elongated east-west. We identify a subpeak northwest of the main peak, which is suggestive of an ongoing merger. Even so, the X-ray mass inferred from the hydrostatic equilibrium assumption appears to be consistent with the lensing mass from the Einstein radius of the system. We find significant absorption in the soft X-ray spectrum of the faintest quasar image, which can be caused by an intervening material at either the lens or source redshift. The X-ray flux ratios between the quasar images (after correcting for absorption) are in reasonable agreement with those at optical and radio wavelengths, and all the flux ratios are inconsistent with those predicted by simple mass models. This implies that microlensing effect is not significant for this system and dark matter substructure is mainly responsible for the anomalous flux ratios.

We examine scatter and bias in weak lensing selected clusters, employing both an analytic model of dark matter haloes and numerical mock data of weak lensing cluster surveys. We pay special attention to effects of the diversity of dark matter distributions within clusters. We find that peak heights of the lensing convergence map correlate rather poorly with the virial mass of haloes. The correlation is tighter for the spherical overdensity mass with a higher mean interior density (e.g. M1000). We examine the dependence of the halo shape on the peak heights, and find that the root mean square scatter caused by the halo diversity scales linearly with the peak heights with the proportionality factor of 0.10.2. The noise originated from the halo shape is found to be comparable to the source galaxy shape noise and the cosmic shear noise. We find the significant halo orientation bias, i.e. weak lensing selected clusters on average have their major axes aligned with the line-of-sight direction, and that the orientation bias is stronger for higher signal-to-noise ratio (S/N) peaks. We compute the orientation bias using an analytic triaxial halo model and obtain results quite consistent with the ray-tracing results. We develop a prescription to analytically compute the number count of weak lensing peaks taking into account all the main sources of scatters in the peak heights. We find that the improved analytic predictions agree well with the simulation results for high-S/N peaks of ??5. We also compare the expected number count with our weak lensing analysis results for 4?deg2 of Subaru/Suprime-Cam observations and find a good agreement.

We measure the small-scale (comoving separation ) two-point correlation function of quasars using a sample of 26 spectroscopically confirmed binary quasars at 0.6 < z < 2.2 from the Sloan Digital Sky Survey Quasar Lens Search (SQLS). Thanks to careful candidate selections and extensive follow-up observations of the SQLS, which are aimed at constructing a complete quasar lens sample, our sample of binary quasars is also expected to be nearly complete within a specified range of angular separations and redshifts. The measured small-scale correlation function rises steeply towards smaller scales, which is consistent with earlier studies based on incomplete or smaller binary quasar samples. We find that the quasar correlation function can be fitted by a power law reasonably well over 4 orders of magnitude, with the best-fitting slope of ?(r) ? r-1.92. We interpret the measured correlation function within the framework of the halo occupation distribution (HOD). We propose a simple model that assumes a constant fraction of quasars that appear as satellites in dark matter haloes, and find that measured small-scale clustering signals constrain the satellite fraction to fsat=0.054+0.017-0.016 for a singular isothermal sphere number density profile of satellites. We note that the HOD modelling appears to underpredict clustering signals at the smallest separations of rp similar to 10 h-1 kpc unless we assume very steep number density profiles (such as a NavarroFrenkWhite profile with the concentration parameter cvir?30), which may be suggestive of enhanced quasar activities by direct interactions.

We present results from strong lens modelling of 10 000 Sloan Digital Sky Survey (SDSS) clusters, to establish the universal distribution of Einstein radii. Detailed lensing analyses have shown that the inner mass distribution of clusters can be accurately modelled by assuming light traces mass, successfully uncovering large numbers of multiple images. Approximate critical curves and the effective Einstein radius of each cluster can therefore be readily calculated, from the distribution of member galaxies and scaled by their luminosities. We use a subsample of 10 well-studied clusters covered by both SDSS and Hubble Space Telescope (HST) to calibrate and test this method, and show that an accurate determination of the Einstein radius and mass can be achieved by this approach blindly, in an automated way, and without requiring multiple images as input. We present the results of the first 10 000 clusters analysed in the range 0.1 < z < 0.55 and compare them to theoretical expectations. We find that for this all-sky representative sample the Einstein radius distribution is lognormal in shape, with , , and with higher abundance of large ?e clusters than predicted by ? cold dark matter. We visually inspect each of the clusters with (zs= 2) and find that similar to 20 per cent are boosted by various projection effects detailed here, remaining with similar to 40 real giant-lens candidates, with a maximum of (zs= 2) for the most massive candidate, in agreement with semi-analytic calculations. The results of this work should be verified further when an extended calibration sample is available.

We present the final statistical sample of lensed quasars from the Sloan Digital Sky Survey (SDSS) Quasar Lens Search (SQLS). The well-defined statistical lens sample consists of 26 lensed quasars brighter than i = 19.1 and in the redshift range of 0.6 < z < 2.2 selected from 50,826 spectroscopically confirmed quasars in the SDSS Data Release 7 (DR7), where we restrict the image separation range to 1 '' < theta < 20 '' and the i-band magnitude differences in two images to be smaller than 1.25 mag. The SDSS DR7 quasar catalog also contains 36 additional lenses identified with various techniques. In addition to these lensed quasars, we have identified 81 pairs of quasars from follow-up spectroscopy, 26 of which are physically associated binary quasars. The statistical lens sample covers a wide range of image separations, redshifts, and magnitudes, and therefore is suitable for systematic studies of cosmological parameters and surveys of the structure and evolution of galaxies and quasars.

We present a statistical analysis of the final lens sample from the Sloan Digital Sky Survey Quasar Lens Search (SQLS). The number distribution of a complete subsample of 19 lensed quasars selected from 50,836 source quasars is compared with theoretical expectations, with particular attention given to the selection function. Assuming that the velocity function of galaxies does not evolve with redshift, the SQLS sample constrains the cosmological constant to Omega(Lambda) = 0.79(-0.07)(+0.06)(stat.)(-0.06)(+0.06)(syst.) for a flat universe. The dark energy equation of state is found to be consistent with w = -1 when the SQLS is combined with constraints from baryon acoustic oscillation (BAO) measurements or results from the Wilkinson Microwave Anisotropy Probe (WMAP). We also obtain simultaneous constraints on cosmological parameters and redshift evolution of the galaxy velocity function, finding no evidence for redshift evolution at z less than or similar to 1 in any combinations of constraints. For instance, number density evolution quantified as nu(n) d ln phi(*)/d ln(1 + z) and the velocity dispersion evolution nu(sigma) d ln sigma(*)/d ln(1 + z) are constrained to nu(n) = 1.06(-1.39)(+1.36)(stat.)(-0.64)(+0.33)(syst.) and nu(s) = -0.05(-0.16)(+0.19)(stat.)(-0.03)(+0.03)(syst.), respectively, when the SQLS result is combined with BAO and WMAP for flat models with a cosmological constant. We find that a significant amount of dark energy is preferred even after fully marginalizing over the galaxy evolution parameters. Thus, the statistics of lensed quasars robustly confirm the accelerated cosmic expansion.

We study the mass distribution of a sample of 28 galaxy clusters using strong and weak lensing observations. The clusters are selected via their strong lensing properties as part of the Sloan Giant Arcs Survey (SGAS) from the Sloan Digital Sky Survey (SDSS). Mass modelling of the strong lensing information from the giant arcs is combined with weak lensing measurements from deep Subaru/Suprime-cam images to primarily obtain robust constraints on the concentration parameter and the shape of the mass distribution. We find that the concentration c(vir) is a steep function of the mass, c(vir) alpha M-vir(-0.59 +/- 0.12), with the value roughly consistent with the lensing-bias-corrected theoretical expectation for high-mass (similar to 10(15) h(-1) M circle dot) clusters. However, the observationally inferred concentration parameters appear to be much higher at lower masses (similar to 10(14) h(-1) M circle dot), possibly a consequence of the modification to the inner density profiles provided by baryon cooling. The steep massconcentration relation is also supported from direct stacking analysis of the tangential shear profiles. In addition, we explore the 2D shape of the projected mass distribution by stacking weak lensing shear maps of individual clusters with prior information on the position angle from strong lens modelling, and find significant evidence for a large mean ellipticity with the best-fitting value of < e >= 0.47 +/- 0.06 for the mass distribution of the stacked sample. We find that the luminous cluster member galaxy distribution traces the overall mass distribution very well, although the distribution of fainter cluster galaxies appears to be more extended than the total mass.

Gravitational lens systems containing lensed quasars are important as cosmological probes, as diagnostics of structural properties of the lensing galaxies and as tools to study the quasars themselves. The largest lensed quasar sample is the Sloan Digital Sky Survey (SDSS) Quasar Lens Search (SQLS), drawn from the SDSS. We attempt to extend this survey using observations of lens candidates selected from a combination of the quasar sample from the SDSS and the UKIRT Infrared Deep Sky Survey (UKIDSS). This adds somewhat higher image quality together with a wider range of wavelength for the selection process. In previous pilot surveys we observed five objects, finding two lenses; here we present further observations of 20 objects in which we find four lenses, of which two are independently discovered in SQLS. Following earlier work on the combination of these two surveys, we have refined our method and find that use of a colour-separation diagnostic, where we select for separations between components which appear to decrease in wavelength, is an efficient method to find lensed quasars and may be useful in ongoing and future large-scale strong lensing surveys with instruments such as Pan-STARRS and LSST. The new lenses have mostly high flux ratios, with faint secondaries buried in the lensing galaxy and typically 610 times less bright than the primary. Our survey brings the total number of lenses discovered in the SDSS quasar sample to 46, plus 13 lenses already known. This is likely to be up to 6070 per cent of the total number of lensed quasars; we briefly discuss strategies by which the rest might be found.

Hyper Suprime-Cam (HSC) is an 870 Mega pixel prime focus camera for the 8.2 m Subaru telescope. The wide field corrector delivers sharp image of 0.25 arc-sec FWHM in r-band over the entire 1.5 degree (in diameter) field of view. The collimation of the camera with respect to the optical axis of the primary mirror is realized by hexapod actuators whose mechanical accuracy is few microns. As a result, we expect to have seeing limited image most of the time. Expected median seeing is 0.67 arc-sec FWHM in i-band. The sensor is a p-ch fully depleted CCD of 200 micron thickness (2048 x 4096 15 mu m square pixel) and we employ 116 of them to pave the 50 cm focal plane. Minimum interval between exposures is roughly 30 seconds including reading out arrays, transferring data to the control computer and saving them to the hard drive. HSC uniquely features the combination of large primary mirror, wide field of view, sharp image and high sensitivity especially in red. This enables accurate shape measurement of faint galaxies which is critical for planned weak lensing survey to probe the nature of dark energy. The system is being assembled now and will see the first light in August 2012.

We perform high-resolution ray-tracing simulations to investigate probability distribution functions (PDFs) of lensing convergence, shear, and magnification on distant sources up to the redshift of z(s) = 20. We pay particular attention to the shot noise effect in N-body simulations by explicitly showing how it affects the variance of the convergence. We show that the convergence and magnification PDFs are closely related to each other via the approximate relation mu = (1 - kappa)(-2), which can reproduce the behavior of PDFs surprisingly well up to the high magnification tail. The mean convergence measured in the source plane is found to be systematically negative, rather than zero as often assumed, and is correlated with the convergence variance. We provide simple analytical formulae for the PDFs, which reproduce simulated PDFs reasonably well for a wide range of redshifts and smoothing sizes. As explicit applications of our ray-tracing simulations, we examine the strong-lensing probability and the magnification effects on the luminosity functions of distant galaxies and quasars.

The quasar SDSS J133401.39+331534.3 at z = 2.426 is found to be a two-image gravitationally lensed quasar with an image separation of 0 ''.833. The object is first identified as a lensed quasar candidate in the Sloan Digital Sky Survey Quasar Lens Search, and then confirmed as a lensed system from follow-up observations at the Subaru and University of Hawaii 2.2 m telescopes. We estimate the redshift of the lensing galaxy to be 0.557 based on absorption lines in the quasar spectra as well as the color of the galaxy. In particular, we observe the system with the Subaru Telescope AO188 adaptive optics with a laser guide star, in order to derive accurate astrometry, which well demonstrates the usefulness of the laser guide star adaptive optics imaging for studying strong lens systems. Our mass modeling with improved astrometry implies that a nearby bright galaxy similar to 4 '' apart from the lensing galaxy is likely to affect the lens potential.

We have measured the Sunyaev-Zel'dovich (SZ) effect for a sample of 10 strong lensing selected galaxy clusters using the Sunyaev-Zel'dovich Array (SZA). The SZA is sensitive to structures on spatial scales of a few arcminutes, while the strong lensing mass modeling constrains the mass at small scales (typically <30 ''). Combining the two provides information about the projected concentrations of the strong lensing clusters. The Einstein radii we measure are twice as large as expected given the masses inferred from SZ scaling relations. A Monte Carlo simulation indicates that a sample randomly drawn from the expected distribution would have a larger median Einstein radius than the observed clusters about 3% of the time. The implied overconcentration has been noted in previous studies and persists for this sample, even when we take into account that we are selecting large Einstein radius systems, suggesting that the theoretical models still do not fully describe the observed properties of strong lensing clusters.

Using a large set of ray tracing in N-body simulations, we examine lensing profiles around massive dark haloes in detail, with a particular emphasis on the profile at around the virial radii. We compare radial convergence profiles, which are measured accurately in the raytracing simulations by stacking many dark haloes, with our simple analytic model predictions. Our analytic models consist of a main halo, which is modelled by the Navarro-Frenk-White (NFW) density profile with three different forms of the truncation, plus the correlated matter (two-halo term) around the main halo. We find that the smoothly truncated NFW profile best reproduces the simulated lensing profiles, out to more than 10 times the virial radius. We then use this analytic model to investigate potential biases in cluster weak lensing studies in which a single, untruncated NFW component is usually assumed in interpreting observed signals. We find that cluster masses, inferred by fitting reduced tangential shear profiles with the NFW profile, tend to be underestimated by similar to 5-10 per cent if fitting is performed out to similar to 10-30 arcmin. In contrast, the concentration parameter is overestimated typically by similar to 20 per cent for the same fitting range. We also investigate biases in computing the signal-to-noise ratio of weak lensing mass peaks, finding them to be similar to 4 per cent for significant mass peaks. In the appendices, we provide useful formulae for the smoothly truncated NFW profile.

We present results from a spectroscopic program targeting 26 strong-lensing cluster cores that were visually identified in the Sloan Digital Sky Survey (SDSS) and the Second Red-Sequence Cluster Survey (RCS-2). The 26 galaxy cluster lenses span a redshift range of 0.2 < z < 0.65, and our spectroscopy reveals 69 unique background sources with redshifts as high as z = 5.200. We also identify redshifts for 262 cluster member galaxies and measure the velocity dispersions and dynamical masses for 18 clusters where we have redshifts for N >= 10 cluster member galaxies. We account for the expected biases in dynamical masses of strong-lensing-selected clusters as predicted by results from numerical simulations and discuss possible sources of bias in our observations. The median dynamical mass of the 18 clusters with N >= 10 spectroscopic cluster members is M-Vir = 7.84 x 10(14) M-circle dot h(0.7)(-1), which is somewhat higher than predictions for strong-lensing-selected clusters in simulations. The disagreement is not significant considering the large uncertainty in our dynamical data, systematic uncertainties in the velocity dispersion calibration, and limitations of the theoretical modeling. Nevertheless our study represents an important first step toward characterizing large samples of clusters that are identified in a systematic way as systems exhibiting dramatic strong-lensing features.

Using a high-resolution radio image, we successfully resolve the two-fold image components B and C of the quasar lens system SDSS J1029+2623. The flux anomalies associated with these two components in the optical regime persist, albeit less strongly, in our radio observations suggesting that the cluster must be modeled by something more than a single central potential. We argue that placing substructure close to one of the components can account for a flux anomaly with negligible changes in the component positions. Our best-fit model has a substructure mass of similar to 10(9) M-circle dot up to the mass-sheet degeneracy, located roughly 0 ''.1 west and 0 ''.1 north of component B. We demonstrate that a positional offset between the centers of the source components can explain the differences between the optical and radio flux ratios.

We measure the redshift distribution of a sample of 28 giant arcs discovered as a part of the Sloan Giant Arcs Survey. Gemini/GMOS-North spectroscopy provides precise redshifts for 24 arcs, and "redshift desert" constrains for the remaining 4 arcs. This is a direct measurement of the redshift distribution of a uniformly selected sample of bright giant arcs, which is an observable that can be used to inform efforts to predict giant arc statistics. Our primary giant arc sample has a median redshift z = 1.821 and nearly two-thirds of the arcs, 64%, are sources at z greater than or similar to 1.4, indicating that the population of background sources that are strongly lensed into bright giant arcs resides primarily at high redshift. We also analyze the distribution of redshifts for 19 secondary strongly lensed background sources that are not visually apparent in Sloan Digital Sky Survey imaging, but were identified in deeper follow-up imaging of the lensing cluster fields. Our redshift sample for the secondary sources is not spectroscopically complete, but combining it with our primary giant arc sample suggests that a large fraction of all background galaxies that are strongly lensed by foreground clusters reside at z greater than or similar to 1.4. Kolmogorov-Smirnov tests indicate that our well-selected, spectroscopically complete primary giant arc redshift sample can be reproduced with a model distribution that is constructed from a combination of results from studies of strong-lensing clusters in numerical simulations and observational constraints on the galaxy luminosity function.

We develop a new method of combining cluster observables (number counts and cluster-cluster correlation functions) and stacked weak lensing signals of background galaxy shapes, both of which are available in a wide-field optical imaging survey. Assuming that the clusters have secure redshift estimates, we show that the joint experiment enables a self-calibration of important systematic errors inherent in these measurements, including the source redshift uncertainty and the cluster mass-observable relation, by adopting a single population of background source galaxies for the lensing analysis. The single source galaxy population allows us to use the relative strengths of the stacked lensing signals at different cluster redshifts for calibrating the source redshift uncertainty, which in turn leads to accurate measurements of the mean cluster mass in each redshift and mass bin. In addition, our formulation of the stacked lensing signals in Fourier space simplifies the Fisher matrix calculations, as well as the marginalization over the cluster off-centering effect which is one of the most significant uncertainties in the stacked lensing analysis. We show that upcoming wide-field surveys covering more than a few thousand square degrees yield stringent constraints on cosmological parameters including dark energy parameters, without any priors on nuisance parameters that model systematic uncertainties. Specifically, the stacked lensing information improves the dark energy figure of merit by a factor of 4, compared to that from the cluster observables alone. The primordial non-Gaussianity parameter can also be constrained with a level of sigma(f(NL)) similar to 10. In this method, the mean source redshift is well calibrated to an accuracy of 0.1 in redshift, and the mean cluster mass in each bin to 5-10% accuracies, which demonstrates the success of the self-calibration of systematic uncertainties from the joint experiment.

We present the second report of our systematic search for strongly lensed quasars from the data of the Sloan Digital Sky Survey (SDSS). From extensive follow-up observations of 136 candidate objects, we find 36 lenses in the full sample of 77,429 spectroscopically confirmed quasars in the SDSS Data Release 5. We then define a complete sample of 19 lenses, including 11 from our previous search in the SDSS Data Release 3, from the sample of 36,287 quasars with i < 19.1 in the redshift range 0.6 < z < 2.2, where we require the lenses to have image separations of 1 '' < theta < 20 '' and i-band magnitude differences between the two images smaller than 1.25 mag. Among the 19 lensed quasars, three have quadruple-image configurations, while the remaining 16 show double images. This lens sample constrains the cosmological constant to be Omega(A) = 0.84(-0.08)(+ 0.06) (stat.)(-0.07)(+0.09) (syst.) assuming a flat universe, which is in good agreement with other cosmological observations. We also report the discoveries of seven binary quasars with separations ranging from 1.'' 1 to 16.'' 6, which are identified in the course of our lens survey. This study concludes the construction of our statistical lens sample in the full SDSS-I data set.

We report the discovery of a gravitationally lensed quasar identified serendipitously in the Sloan Digital Sky Survey (SDSS). The object, SDSS J094604.90+183541.8, was initially targeted for spectroscopy as a luminous red galaxy, but the SDSS spectrum has the features of both a z = 0.388 galaxy and a z = 4.8 quasar. We have obtained additional imaging that resolves the system into two quasar images separated by 3.'' 06 and a bright galaxy that is strongly blended with one of the quasar images. We confirm spectroscopically that the two quasar images represent a single-lensed source at z = 4.8 with a total magnification of 3.2, and we derive a model for the lensing galaxy. This is the highest redshift lensed quasar currently known. We examine the issues surrounding the selection of such an unusual object from existing data and briefly discuss implications for lensed quasar surveys.

We present a strong lens analysis of SDSS J1004+4112, a unique quasar lens produced by a massive cluster of galaxies at z = 0.68, using newly developed software for gravitational lensing. We find that our parametric mass model well reproduces all observations, including the positions of quasar images as well as those of multiply imaged galaxies with measured spectroscopic redshifts, time delays between quasar images, and the positions of faint central images. The predicted large total magnification of mu similar to 70 suggests that the lens system is indeed a useful site for studying the fine structure of a distant quasar and its host galaxy. The dark halo component is found to be unimodal, centered on the brightest cluster galaxy and the Chandra X-ray surface brightness profile. In addition, the orientation of the halo component is quite consistent with those of the brightest cluster galaxy and member galaxy distribution, implying that the lensing cluster is a relaxed system. The radial profile of the best-fit mass model is in good agreement with a mass profile inferred from the X-ray observation. While the inner radial slope of the dark halo component is consistent with being -1, a clear dependence of the predicted A-D time delay on the slope indicates that an additional time-delay measurement will improve constraints on the mass model.

Cadenced optical imaging surveys in the next decade will be capable of detecting time-varying galaxy-scale strong gravitational lenses in large numbers, increasing the size of the statistically well-defined samples of multiply imaged quasars by two orders of magnitude, and discovering the first strongly lensed supernovae. We carry out a detailed calculation of the likely yields of several planned surveys, using realistic distributions for the lens and source properties and taking magnification bias and image configuration detectability into account. We find that upcoming wide-field synoptic surveys should detect several thousand lensed quasars. In particular, the Large Synoptic Survey Telescope (LSST) should find more than some 8000 lensed quasars, some 3000 of which will have well-measured time delays. The LSST should also find some 130 lensed supernovae during the 10-yr survey duration, which is compared with similar to 15 lensed supernovae predicted to be found by a deep, space-based supernova survey done by the Joint Dark Energy Mission. We compute the quad fraction in each survey, predicting it to be similar to 15 per cent for the lensed quasars and similar to 30 per cent for the lensed supernovae. Generating a mock catalogue of around 1500 well-observed double-image lenses, as could be derived from the LSST survey, we compute the available precision on the Hubble constant and the dark energy equation parameters for the time-delay distance experiment (assuming priors from Planck): the predicted marginalized 68 per cent confidence intervals are Sigma(w(0)) = 0.15, Sigma(w(a)) = 0.41 and Sigma(h) = 0.017. While this is encouraging in the sense that these uncertainties are only 50 per cent larger than those predicted for a space-based Type Ia supernova sample, we show how the dark energy figure of merit degrades with decreasing knowledge of the lens mass distribution.

We present new measurements of dark matter distributions in 25 X-ray luminous clusters by making a full use of the two-dimensional (2D) weak-lensing signals obtained from high-quality Subaru/Suprime-Cam imaging data. Our approach to directly compare the measured lensing shear pattern with elliptical model predictions allows us to extract new information on the mass distributions of individual clusters, such as the halo ellipticity and mass centroid. We find that these parameters on the cluster shape are little degenerate with cluster mass and concentration parameters. By combining the 2D fitting results for a subsample of 18 clusters, the elliptical shape of dark matter haloes is detected at 7 sigma significance level. The mean ellipticity is found to be < e > = < 1 - b/a > = 0.46 +/- 0.04 (1 sigma), which is in excellent agreement with a theoretical prediction based on the standard collisionless cold dark matter model. The mass centroid can be constrained with a typical accuracy of similar to 20 arcsec (similar to 50 h(-1) kpc) in radius for each cluster. The mass centroid position fairly well matches the position of the brightest cluster galaxy, with some clusters showing significant offsets. Thus, the 2D shear fitting method enables us to assess one of the most important systematic errors inherent in the stacked cluster weak-lensing technique, the mass centroid uncertainty. In addition, the shape of the dark mass distribution is found to be only weakly correlated with that of the member galaxy distribution or the brightest cluster galaxy. We carefully examine possible sources of systematic errors in our measurements including the effect of substructures, the cosmic shear contamination, fitting regions and the dilution effect, and find none of them to be significant. Our results demonstrate the power of high-quality imaging data for exploring the detailed spatial distribution of dark matter, which should improve the ability of future surveys to conduct cluster cosmology experiments.

Weak lensing work can be badly compromised by unlensed foreground and cluster members which dilute the true lensing signal. We show how the lensing amplitude in multicolour space can be harnessed to securely separate cluster members from the foreground and background populations for three massive clusters, A1703 (z = 0.258), A370 (z = 0.375) and RXJ1347-11 (z = 0.451) imaged with Subaru. The luminosity functions of these clusters when corrected for dilution show similar faint-end slopes, alpha similar or equal to - 1.0, with no marked faint-end upturn to our limit of M(R) similar or equal to - 15.0, and only a mild radial gradient. In each case, the radial profile of the mass-to-light ratio (M/L) peaks at intermediate radius, similar or equal to 0.2r(vir), at a level of 300-500(M/L(R))(circle dot), and then falls steadily towards similar to 100(M/L(R))(circle dot) at the virial radius, similar to the mean field level. This behaviour is likely due to the relative paucity of central late-type galaxies, whereas for the E/S0 sequence only a mild radial decline in M/L is found for each cluster. We discuss this behaviour in the context of detailed simulations where predictions for tidal stripping may now be tested accurately with observations.

We report the discovery and confirmation of eight new two-image lensed quasars by the Sloan Digital Sky Survey (SDSS) Quasar Lens Search. The lenses are SDSS J0904+1512 (image separation theta = 1 ''.13, source redshift z(s) = 1.826), SDSS J1054+2733 (theta = 1 ''.27, z(s) = 1.452), SDSS J1055+4628 (theta = 1 ''.15, z(s) = 1.249), SDSS J1131+1915 (theta = 1 ''.46, z(s) = 2.915), SDSS J1304+2001 (theta = 1 ''.87, z(s) = 2.175), SDSS J1349+1227 (theta = 3 ''.00, z(s) = 1.722), SDSS J1455+1447 (theta = 1 ''.73, z(s) = 1.424), and SDSS J1620+1203 (theta = 2 ''.77, z(s) = 1.158). Three of them, SDSS J1055+4628, SDSS J1455+1447, and SDSS J1620+1203, satisfy the criteria for constructing our statistical sample for studying the cosmological model. Based on galactic absorption lines of the lens galaxies, we also derive lens redshifts of z(l) = 0.398 and z(l) = 0.513 for SDSS J1620+1203 and the previously discovered lens SDSS J0746+4403, respectively.

We present observations of a new double-image gravitational lens system, ULAS J082016.1+081216, of image separation 2.3 arcsec and high (similar to 6) flux ratio. The system is selected from the Sloan Digital Sky Survey (SDSS) spectroscopic quasar list using new high-quality images from the UKIRT (United Kingdom Infrared Telescope) Deep Sky Survey (UKIDSS). The lensed quasar has a source redshift of 2.024, and we identify the lens galaxy as a faint red object of redshift 0.803 +/- 0.001. Three other objects from the UKIDSS survey, selected in the same way, were found not to be lens systems. Together with the earlier lens found using this method, the SDSS-UKIDSS lenses have the potential to significantly increase the number of quasar lenses found in SDSS, to extend the survey to higher flux ratios and lower separations, and to give greater completeness which is important for statistical purposes.

We report on the discovery of a very bright z = 2.00 star-forming galaxy that is strongly lensed by a foreground z = 0.422 luminous red galaxy (LRG), SDSS J120602.09+514229.5. This system, nicknamed the "Clone," was found in a systematic search for bright arcs lensed by LRGs and brightest cluster galaxies in the Sloan Digital Sky Survey Data Release 5 sample. Follow-up observations on the Subaru 8.2 m telescope on Mauna Kea and the Astrophysical Research Consortium 3.5 m telescope at Apache Point Observatory confirmed the lensing nature of this system. A simple lens model for the system, assuming a singular isothermal ellipsoid mass distribution, yields an Einstein radius of theta(Ein) = 3.82 +/- 0.03 or 14.8 +/- 0.1 h(-1) kpc at the lens redshift. The total projected mass enclosed within the Einstein radius is 2.10 +/- 0.03 x 10(12) h(-1) M-circle dot, and the magnification factor for the source galaxy is 27 +/- 1. Combining the lens model with our gVriz photometry, we find a (unlensed) star formation rate (SFR) for the source galaxy of 32 h(-1) M-circle dot yr(-1), adopting a fiducial constant SFR model with an age of 100 Myr and E(B - V) = 0.25. With an apparent magnitude of r = 19.8, this system is among the very brightest lensed z >= 2 galaxies, and provides an excellent opportunity to pursue detailed studies of the physical properties of an individual high-redshift star-forming galaxy.

We derive radial mass profiles of four strong lensing selected clusters which show prominent giant arcs (Abell 1703, SDSS J1446+3032, SDSS J1531+3414, and SDSS J2111-0115), by combining detailed strong lens modeling with weak lensing shear measured from deep Subaru Suprime-cam images. Weak lensing signals are detected at high significance for all four clusters, whose redshifts range from z = 0.28 to 0.64. We demonstrate that adding strong lensing information with known arc redshifts significantly improves constraints on the mass density profile, compared with those obtained from weak lensing alone. While the mass profiles are well fitted by the universal form predicted in N-body simulations of the Lambda-dominated cold dark matter model, all four clusters appear to be slightly more centrally concentrated (the concentration parameters c(vir) similar to 8) than theoretical predictions, even after accounting for the bias toward higher concentrations inherent in lensing-selected samples. Our results are consistent with previous studies which similarly detected a concentration excess, and increase the total number of clusters studied with the combined strong and weak lensing technique to 10. Combining our sample with previous work, we find that clusters with larger Einstein radii are more anomalously concentrated. We also present a detailed model of the lensing cluster Abell 1703 with constraints from multiple image families, and find the dark matter inner density profile to be cuspy with the slope consistent with -1, in agreement with expectations.

Difference imaging provides a new way to discover gravitationally lensed quasars because few nonlensed sources will show spatially extended, time variable flux. We test the method on the fields of lens candidates in the Sloan Digital Sky Survey (SDSS) Supernova Survey region from the SDSS Quasar Lens Search (SQLS) and one serendipitously discovered lensed quasar. Starting from 20,536 sources, including 49 SDSS quasars, 32 candidate lenses/lensed images, and one known lensed quasar, we find that 174 sources including 35 SDSS quasars, 16 candidate lenses/lensed images, and the known lensed quasar are nonperiodic variable sources. We can measure the spatial structure of the variable flux for 119 of these variable sources and identify only eight as candidate extended variables, including the known lensed quasar. Only the known lensed quasar appears as a close pair of sources on the difference images. Inspection of the remaining seven suggests they are false positives, and only two were spectroscopically identified quasars. One of the lens candidates from the SQLS survives our cuts, but only as a single image instead of a pair. This indicates a false positive rate of order similar to 1/4000 for the method, or given our effective survey area of order 0.82 deg(2), similar to 5 per deg(2) in the SDSS Supernova Survey. The fraction of quasars not found to be variable and the false positive rate would both fall if we had analyzed the full, later data releases for the SDSS fields. While application of the method to the SDSS is limited by the resolution, depth, and sampling of the survey, several future surveys such as Pan-STARRS, LSST, and SNAP will significantly improve on these limitations.

We report the discovery of five gravitationally lensed quasars from the Sloan Digital Sky Survey (SDSS). All five systems are selected as two-image lensed quasar candidates from a sample of high-redshift (z > 2.2) SDSS quasars. We confirmed their lensing nature with additional imaging and spectroscopic observations. The new systems are SDSS J0819+5356 (source redshift z(s) = 2.237, lens redshift z(l) = 0.294, and image separation theta = 4 ''.04), SDSS J1254+2235 (z(s) = 3.626, theta = 1 ''.56), SDSS J1258+1657 (z(s) = 2.702, theta = 1 ''.28), SDSS J1339+1310 (z(s) = 2.243, theta = 1 ''.69), and SDSS J1400+3134 (z(s) = 3.317, theta = 1 ''.74). We estimate the lens redshifts of the latter four systems to be z(l) = 0.2-0.8 from the colors and magnitudes of the lensing galaxies. We find that the image configurations of all systems are well reproduced by standard mass models. Although these lenses will not be included in our statistical sample of z(s) < 2.2 lenses, they expand the number of lensed quasars which can be used for high-redshift galaxy and quasar studies.

We show that the ability to probe primordial non-Gaussianity with cluster counts is drastically improved by adding the excess variance of counts which contains information on the clustering. The conflicting dependences of changing the mass threshold and including primordial non-Gaussianity on the mass function and biasing indicate that the self-calibrated cluster counts break the degeneracy between primordial non-Gaussianity and the observable-mass relation. Based on the Fisher matrix analysis, we show that the count variance improves constraints on f(NL) by more than an order of magnitude. It exhibits little degeneracy with dark energy equation of state. We forecast that upcoming Hyper Suprime-Cam survey and Dark Energy Survey will constrain primordial non-Gaussianity at the level sigma(f(NL))similar to 8, which is competitive with constraints from next-generation cosmic microwave background experiments.

Gravitational lenses on galaxy scales are plausibly modelled as having ellipsoidal symmetry and a universal dark matter density profile, with a Sersic profile to describe the distribution of baryonic matter. Predicting all lensing effects requires knowledge of the total lens potential: in this work we give analytic forms for that of the above hybrid model. Emphasising that complex lens potentials can be constructed from simpler components in linear combination, we provide a recipe for attaining elliptical symmetry in either projected mass or lens potential. We also provide analytic formulae for the lens potentials of Sersic profiles for integer and half-integer index. We then present formulae describing the gravitational lensing effects due to smoothly-truncated universal density profiles in cold dark matter model. For our isolated haloes the density profile falls off as radius to the minus fifth or seventh power beyond the tidal radius, functional forms that allow all orders of lens potential derivatives to be calculated analytically, while ensuring a non-divergent total mass. We show how the observables predicted by this profile differ from that of the original infinite-mass NFW profile. Expressions for the gravitational flexion are highlighted. We show how decreasing the tidal radius allows stripped haloes to be modelled, providing a framework for a fuller investigation of dark matter substructure in galaxies and clusters. Finally we remark on the need for finite mass halo profiles when doing cosmological ray-tracing simulations, and the need for readily-calculable higher order derivatives of the lens potential when studying catastrophes in strong lenses.

The Einstein radius plays a central role in lens studies as it characterizes the strength of gravitational lensing. In particular, the distribution of Einstein radii near the upper cut-off should probe the probability distribution of the largest mass concentrations in the universe. Adopting a triaxial halo model, we compute expected distributions of large Einstein radii. To assess the cosmic variance, we generate a number of Monte Carlo realizations of all-sky catalogues of massive clusters. We find that the expected largest Einstein radius in the universe is sensitive to parameters characterizing the cosmological model, especially sigma(8): for a source redshift of unity, they are 42(-7)(+9), 35(-6)(+8) and 54(-7)(+12) arcsec (errors denote 1 sigma cosmic variance), assuming best-fitting cosmological parameters of the Wilkinson Microwave Anisotropy Probe five-year (WMAP5), three-year (WMAP3) and one-year (WMAP1) data, respectively. These values are broadly consistent with current observations given their incompleteness. The mass of the largest lens cluster can be as small as similar to 10(15) M-circle dot. For the same source redshift, we expect in all sky similar to 35 (WMAP5), similar to 15 (WMAP3) and similar to 150 (WMAP1) clusters that have Einstein radii larger than 20 arcsec. For a larger source redshift of 7, the largest Einstein radii grow approximately twice as large. Whilst the values of the largest Einstein radii are almost unaffected by the level of the primordial non-Gaussianity currently of interest, the measurement of the abundance of moderately large lens clusters should probe non-Gaussianity competitively with cosmic microwave background experiments, but only if other cosmological parameters are well measured. These semi-analytic predictions are based on a rather simple representation of clusters, and hence calibrating them with N-body simulations will help to improve the accuracy. We also find that these 'superlens' clusters constitute a highly biased population. For instance, a substantial fraction of these superlens clusters have major axes preferentially aligned with the line-of-sight. As a consequence, the projected mass distributions of the clusters are rounder by an ellipticity of similar to 0.2 and have similar to 40-60 per cent larger concentrations compared with typical clusters with similar redshifts and masses. We argue that the large concentration measured in A1689 is consistent with our model prediction at the 1.2 sigma level. A combined analysis of several clusters will be needed to see whether or not the observed concentrations conflict with predictions of the flat Lambda-dominated cold dark matter model.

We present the discovery of a new quadruply lensed quasar. The lens system, SDSS J1330+1810 at z(s) = 1.393, was identified as a lens candidate from the spectroscopic sample of the Sloan Digital Sky Survey. Optical and near-infrared images clearly show four quasar images with a maximum image separation of 1.76 arcsec, as well as a bright lensing galaxy. We measure a redshift of the lensing galaxy of z(1) = 0.373 from absorption features in the spectrum. We find a foreground group of galaxies at z = 0.31, centred at similar to 120 arcsec southwest of the lens system. Simple mass models fit the data quite well, including the flux ratios between images, although the lens galaxy appears to be similar to 1 mag brighter than expected by the Faber-Jackson relation. Our mass modelling suggests that shear from nearby structure is affecting the lens potential.

We present the results of deep spectroscopy for the central region of the Cluster lens SDSS J1004+4112 with the Subaru Telescope. A secure detection of an emission line of the faint blue stellar object (component E) near the center of the brightest cluster galaxy (G I) confirms that it is the central fifth image of the lensed quasar system. In addition, we measured the stellar velocity dispersion of G1 to be sigma(*) = 352 +/- 13 km s(-1). We combined these results to obtain constraints on the mass M-BH of the putative black hole (BH) at the center of the inactive galaxy G1, and hence on the M-BH-sigma(*) relation at the lens redshift z(1) = 0.68. From detailed mass modeling, we placed an upper limit on the black hole mass, M-BH < 2.1 x 10(10)M(circle dot) at the 1 sigma level (< 3.1 x 10(10)M(circle dot) at 3 sigma), which is consistent with black hole masses expected from the local and redshift-evolved MBH-or,, relations, MBH similar to 10(9)-10(10)M(circle dot).

We derive lens distortion and magnification profiles of four well-known clusters observed with Subaru. Each cluster is very well fitted by the general form predicted for cold dark matter (CDM) dominated halos, with good consistency found between the independent distortion and magnification measurements. The inferred level of mass concentration is surprisingly high, 8 < cvir < 15 (< c(vir)> = 10.39 +/- 0.91), compared to the relatively shallow profiles predicted by the Lambda CDM model, c(vir) = 5.06 +/- 1.10 (for < M(vir)> = 1.25 x 10(15) M(circle dot) h(-1)). This represents a 4 sigma discrepancy, and includes the relatively modest effects of projection bias and profile evolution derived from N-body simulations, which oppose each other with little residual effect. In the context of CDM-based cosmologies, this discrepancy implies that clusters collapse earlier (z >= 1) than predicted (z < 0.5), when the universe was correspondingly denser.

We present the first result of a survey for strong galaxy-galaxy lenses in Sloan Digital Sky Survey (SDSS) images. SDSS J082728.70+223256.4 was selected as a lensing candidate using selection criteria based on the color and positions of objects in the SDSS photometric catalog. Follow-up imaging and spectroscopy showed this object to be a lensing system. The lensing galaxy is elliptical at z = 0.349 in a galaxy cluster. The lensed galaxy has the spectrum of a post-starburst galaxy at z = 0.766. The lensing galaxy has an estimated mass of similar to 1.2 x 10(12)M(circle dot), and the corresponding mass-to-light ratio in the B-band is similar to 26M(circle dot)/L-circle dot inside 1.1 effective radii of the lensing galaxy. Our study shows how catalogs drawn from multi-band surveys can be used to find strong galaxy-galaxy lenses having multiple lens images. Our strong lensing candidate selection based on photometry-only catalogs will be useful in future multi-band imaging surveys such as SNAP and LSST.

We report the discovery of a new gravitational lens system. This object, ULAS J234311.93-005034.0, is the first to be selected by using the new UKIRT Infrared Deep Sky Survey (UKIDSS), together with the Sloan Digital Sky Survey (SDSS). The ULAS J234311.93-005034.0 system contains a quasar at redshift 0.788 which is doubly imaged, with separation 1.4 arcsec. The two quasar images have the same redshift and similar, though not identical, spectra. The lensing galaxy is detected by subtracting point spread functions from R-band images taken with the Keck telescope. The lensing galaxy can also be detected by subtracting the spectra of the A and B images, since more of the galaxy light is likely to be present in the latter. No redshift is determined from the galaxy, although the shape of its spectrum suggests a redshift of about 0.3. The object's lens status is secure, due to the identification of two objects with the same redshift together with a lensing galaxy. Our imaging suggests that the lens is found in a cluster environment, in which candidate arc-like structures, that require confirmation, are visible in the vicinity. Further discoveries of lenses from the UKIDSS survey are likely as part of this programme, due to the depth of UKIDSS and its generally good seeing conditions.

We identify a third image in the unique quasar lens SDSS J1029+2623, the second known quasar lens produced by a massive cluster of galaxies. The spectrum of the third image shows similar emission and absorption features but has a redder continuum than the other two images, which can be explained by differential extinction or microlensing. We also identify several lensed arcs. Our observations suggest a complicated structure of the lens cluster at. We argue that the three lensed images are produced by a naked cusp on the basis of successful z approximate to 0.6 mass models, the distribution of cluster member galaxies, and the shapes and locations of the lensed arcs. Lensing by a naked cusp is quite rare among galaxy-scale lenses but is predicted to be common among large-separation lensed quasars. Thus the discovery can be viewed as support for an important theoretical prediction of the standard cold dark matter model.

We (s) overdot (w) overdot report on the first results of an imaging survey to detect strong gravitational lensing targeting the richest clusters selected from the photometric data of the Sloan Digital Sky Survey ( SDSS) with follow-up deep imaging observations from the Wisconsin - Indiana - Yale NOAO ( WIYN) 3.5 m telescope and the University of Hawaii 88 inch telescope ( UH88). The clusters are selected from an area of 8000 deg(2) using the red cluster sequence technique and span the redshift range 0.1 less than or similar to z less than or similar to 0.6, corresponding to a comoving cosmological volume of similar to 2 Gpc(3). Our imaging survey thus targets a volume more than an order of magnitude larger than any previous search. A total of 240 clusters were imaged of which 141 had sub-arcsecond image quality. Our survey has uncovered 16 new lensing clusters with definite giant arcs, an additional 12 systems for which the lensing interpretation is very likely, and 9 possible lenses which contain shorter arclets or candidate arcs which are less certain and will require further observations to confirm their lensing origin. Among these new systems are several of the most dramatic examples of strong gravitational lensing ever discovered, with multiple bright arcs at large angular separation. These will likely become "poster-child" gravitational lenses similar to Abell 1689 and CL0024+1654. The new lenses discovered in this survey will enable future systematic studies of the statistics of strong lensing and their implications for cosmology and our structure formation paradigm.

We report the first results of our systematic search for strongly lensed quasars using the spectroscopically confirmed quasars in the Sloan Digital Sky Survey (SDSS). Among 46,420 quasars from the SDSS Data Release 3 (similar to 4188 deg(2)), we select a subsample of 22,683 quasars that are located at redshifts between 0.6 and 2.2 and are brighter than the Galactic extinction-corrected i-band magnitude of 19.1. We identify 220 lens candidates from the quasar subsample, for which we conduct extensive and systematic follow-up observations in optical and near-infrared wavebands, in order to construct a complete lensed quasar sample at image separations between 1 '' and 20 '' and flux ratios of faint to bright lensed images larger than 10(-0.5). We construct a statistical sample of 11 lensed quasars. Ten of these are galaxy-scale lenses with small image separations (similar to 1 ''-2 '') and one is a large separation (15 '') system which is produced by a massive cluster of galaxies, representing the first statistical sample of lensed quasars including both galaxy- and cluster-scale lenses. The Data Release 3 spectroscopic quasars contain an additional 11 lensed quasars outside the statistical sample.

We present cosmological results from the statistics of lensed quasars in the Sloan Digital Sky Survey (SDSS) Quasar Lens Search. By taking proper account of the selection function, we compute the expected number of quasars lensed by early-type galaxies and their image separation distribution assuming a flat universe, which is then compared with seven lenses found in the SDSS Data Release 3 to derive constraints on dark energy under strictly controlled criteria. For a cosmological constant model (w = - 1) we obtain Omega = 0.74(-0.15)(+0.11) (stat.)(-0.06)(+0.13) (syst.). Allowing w to be a free parameter we find Omega(M) = 0.26(-0.06)(+0.07) (stat.)(-0.05)(+0.03) (syst.) and w = - 1.1 +/- 0.6(stat.)(-0.5)(+0.3) (syst.) when combined with the constraint from the measurement of baryon acoustic oscillations in the SDSS luminous red galaxy sample. Our results are in good agreement with earlier lensing constraints obtained using radio lenses, and provide additional confirmation of the presence of dark energy consistent with a cosmological constant, derived independently of type Ia supernovae.

We present the discovery of four gravitationally lensed quasars selected from the spectroscopic quasar catalog of the Sloan Digital Sky Survey (SDSS). We describe imaging and spectroscopic follow-up observations that support the lensing interpretation of the following four quasars: SDSS J0832+ 0404 (image separation theta = 1.'' 98, source redshift z(s) = 1.115, lens redshift z(l) = 0.659); SDSS J1216+ 3529 (theta = 1.'' 49, z(s) = 2.012); SDSS J1322+1052 (theta = 2.'' 00, z(s) = 1.716); and SDSS J1524+4409 (theta = 1.'' 67, z(s) = 1.210, z(l) = 0.320). Each system has two lensed images. We find that the fainter image component of SDSS J0832+ 0404 is significantly redder than the brighter component, perhaps because of differential reddening by the lensing galaxy. The lens potential of SDSS J1216+ 3529 might be complicated by the presence of a secondary galaxy near the main lensing galaxy.

Most of the optical gravitational lenses recently discovered in the Sloan Digital Sky Survey Quasar Lens Search (SQLS) have two-images rather than four-images, in marked contrast to radio lenses for which the fraction of four-image lenses (quad fraction) is quite high. We revisit the quad fraction among optical lenses by taking the selection function of the SQLS into account. We find that the current observed quad fraction in the SQLS is indeed lower than, but consistent with, the prediction of our theoretical model. The low quad fraction among optical lenses, together with the high quad fraction among radio lenses, implies that the quasar optical luminosity function has a relatively shallow faint end slope.

We report the discovery of a new gravitationally lensed quasar, SDSS J131339.98+515128.3, at a redshift of 1.875 with an image separation of 1.24 arcsec. The lensing galaxy is clearly detected in visible light follow-up observations. We also identify three absorption-line doublets in the spectra of the lensed quasar images, from which we measure the lens redshift to be 0.194. Like several other known lenses, the lensed quasar images have different continuum slopes. This difference is probably the result of reddening and microlensing in the lensing galaxy. The lensed quasar was selected by correlating Sloan Digital Sky Survey spectroscopic quasars with Two Micron All Sky Survey (2MASS) sources and choosing quasars that show near-infrared (near-IR) excess. The near-IR excess can originate, for example, from the contribution of the lensing galaxy at near-IR wavelengths. We show that the near-IR excess technique is indeed an efficient method to identify lensed systems from a large sample of quasars.

We report the discovery of a quadruply imaged quasar, SDSS J125107.57+ 293540.5, selected from the Sloan Digital Sky Survey. Follow- up imaging reveals that the system consists of four blue pointlike components in a typical cusp lens geometry surrounding a central red galaxy. The source redshift is 0.802, and the lens redshift is 0.410. The maximum image separation between the lensed components is 1.79 ''. While the image configuration is well reproduced by standard mass models with reasonable parameter values, the flux ratios predicted by these models differ from the observed ratios in all bands. This is suggestive of small- scale structure in this lens, although a definitive identification of the anomaly requires more accurate photometry and astrometry.

The 28 cataclysmic variables (CVs) found in 2005 in the Sloan Digital Sky Survey are presented with their coordinates, magnitudes, and spectra. Five of these systems are previously known CVs (HH Cnc, SX LMi, QZ Ser, AP CrB, and HS 1016+3412), and the rest are new discoveries. Additional spectroscopic, photometric, and/or polarimetric observations of 10 systems were carried out, resulting in estimates of the orbital periods for seven of the new binaries. The 23 new CVs include one eclipsing system, one new Polar, and five systems whose spectra clearly reveal atmospheric absorption lines from the underlying white dwarf.

We use high-resolution N-body numerical simulations to study the number of predicted large-separation multiply imaged systems produced by clusters of galaxies in the Sloan Digital Sky Survey (SDSS) photometric and spectroscopic quasar samples. We incorporate the condensation of baryons at the centre of clusters by (artificially) adding a brightest cluster galaxy (BCG) as a truncated isothermal sphere. We make predictions in two flat cosmological models: a Λ cold dark matter (ΛCDM) model with a matter density and a model favoured by the Wilkinson Microwave Anisotropy Probe (WMAP) three-year data with and (WMAP3). We found that the predicted multiply imaged quasars with separation >10 arcsec is about 6.2 and 2.6 for the SDSS photometric (with an effective area of 8000 deg2) and spectroscopic (with an effective area of 5000 deg2) quasar samples, respectively, in the Λ CDM0 model; the predicted numbers of large-separation lensed quasars agree well with the observations. These numbers are reduced by a factor of 7 or more in the WMAP3 model, and are consistent with the data at ≲8 per cent level. The predicted cluster lens redshift peaks around redshift 0.5, and 90 per cent are between 0.2 and 1. The ratio of systems with at least four image systems and those with is about 1/3.5 for both the Λ CDM0 and the WMAP3 models, and for both the photometric and the spectroscopic quasar samples. We find that the BCG creates a central circular region, comparable to the Einstein ring of the BCG, where the central image disappears in the usual three- and five-image configurations. If we include four image systems as an extreme case of five-image systems (with an infinitely demagnified central image), we find that 68 per cent of the central images are fainter by a factor of 100 than the brightest image and about 80 per cent are within 1.5 arcsec of the BCG. © 2007 RAS.

Time delays between lensed multiple images have been known to provide an interesting probe of the Hubble constant, but such an application is often limited by degeneracies with the shape of lens potentials. We propose a new statistical approach to examine the dependence of time delays on the complexity of lens potentials, such as higher order perturbations, nonisothermality, and substructures. Specifically, we introduce a dimensionless reduced time delay and explore its behavior analytically and numerically as a function of the image configuration, which is characterized by the asymmetry and opening angle of the image pair. In particular, we derive a realistic conditional probability distribution for a given image configuration from Monte Carlo simulations. We find that the probability distribution is sensitive to the image configuration such that more symmetric and/or smaller opening-angle image pairs are more easily affected by perturbations on the primary lens potential. On average time delays of double lenses are less scattered than those of quadruple lenses. Furthermore, the realistic conditional distribution allows a new statistical method to constrain the Hubble constant from observed time delays. We find that 16 published time delay quasars constrain H-0 to be 70 +/- 6 km s(-1) Mpc(-1), where the value and its error are estimated using jackknife resampling. Systematic errors coming from the heterogeneous nature of the quasar sample and the uncertainty of the input distribution of lens potentials can be larger than the statistical error. After including rough estimates of important systematic errors, we find H-0 = 68 +/- 6(stat.) +/- 8( syst:) km s(-1) Mpc(-1). The reasonable agreement of the value of the Hubble constant with other estimates indicates the usefulness of our new approach as a cosmological and astrophysical probe, particularly in the era of large-scale synoptic surveys.