ACS observations of massive lensing clusters permit an order of magnitude increase in the numbers of multiply-lensed background galaxies identified behind a given cluster. We have developed a code to take the pixels belonging to any given image and generate counter-images with full resolution, so that multiple systems are convincingly and exhaustively identified. Over 130 images of 35 multiply lensed galaxies are found behind A1689, including many radial arcs and also tiny counter-images projected on the center of mass. The derived mass profile is found to flatten steadily towards the center, like and NFW profile, with a mean slope $d{\log{\Sigma}}/d{\log{r}}\approx-0.55\pm0.1$, over the range $r<250$ kpc/h, which is somewhat steeper than predicted for such a massive halo. We also clearly see the expected geometric increase of bend angles with redshift, however, given the low redshift of A1689, $z=0.18$, the dependence on cosmological parameters is weak, but using higher redshift clusters from our GTO program we may derive a more competitive constraint.To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html

We present results from weak lensing analyses of 20 high-redshift clusters in the ESO Distant Cluster Survey (EDisCS) from both deep optical VLT and HST/ACS imaging. We find that the EDisCS sample is composed primarily of clusters that are less massive than those in current X-ray selected samples at similar redshifts, but that all of the fields are likely to contain massive clusters rather than superpositions low mass groups. We find good agreement between the weak lensing measurements in the VLT and ACS images. Finally we determine the noise level in the shear measurements for the VLT fields as a function of exposure time and seeing and demonstrate that future ground-based surveys which plan to perform deep optical imaging for use in weak lensing measurements must achieve point-spread functions smaller than a median of $0\farcs 6$ FWHM.To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html

We present a new model of the lensing cluster of galaxies MS2137-23. By incorporating strong and weak lensing data in a fully elliptical lens modeling, we show that the total density profile must be close to the predictions of numerical simulations with an inner slope well consistent with a NFW profile. The model that best fits the lensing constraints is used to infer to line-of-sight velocity distribution (LOSVD) of stars in the central cD galaxy. This distribution is found to be far from Maxwellian. The important non-Gaussian tails produce a significant low-bias when measuring the velocity dispersion by assuming Gaussian absorption lines. In the case of MS2137, most of the information comes from multiples arcs. Internal kinematics of stars only help modeling the innermost kiloparsecs. However, in clusters with fewer constraints (no radial arcs for instance), a detailed modeling of both strong lensing and internal kinematics is crucial.To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html

We discuss two methods for constraining the equation of state of dark energy using strong gravitational lensing by galaxy clusters. In the so called “arc statistics” approach, we compare the cluster efficiency for producing giant arcs in several dark-energy cosmologies and in the “standard” $\Lambda$CDM and OCDM models. We find that the expected abundance of gravitational arcs depends on the equation of state of dark energy and reflects the dependence of halo concentrations on cosmology. In agreement with results in previous works, the lensing cross section is very sensitive to dynamical processes occurring in the lenses. Then we use gravitational arcs for tracing the position of the lens critical curves and we measure their scaling with the source redshift in a variety of cosmological models. We find that there is a degeneracy between several lens properties and the equation of state of dark energy which can be broken only after an extremely precise modeling of the lens. Instead of using this “golden lens” approach, we check whether combining the information from a statistical sample of clusters we can distinguish among the various cosmologies. We test the method on a sample of numerically simulated clusters and we reproduce the results expected from the analytic models.To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html

We discuss how strong lensing by galaxy clusters may be used to study the properties of dark matter halos and the expansion history of the universe. First, we show how the characteristics of $\Lambda$CDM clusters determine their lensing properties, and show how these properties are manifested in some of the new giant arcs discovered behind SDSS clusters. Next, we compare the statistics of strong lensing by clusters expected in the $\Lambda$CDM model to the observed statistics of giant arcs and wide-separation quasars. Finally, we discuss the cosmographic uses of clusters with multiple arcs, pointing out several sources of noise which can produce $\gtrsim100$ % errors in derived cosmological parameters.To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html

While there is convincing evidence that the central regions ($r \ll R_e$) of early-type galaxies are dominated by stars and that the outer regions ($r \gg R_e $) are dominated by dark matter, the structure of early-type galaxies in the transition region (a few effective radii $R_e$) between the stars and the dark matter is unclear both locally and in gravitational lenses. Understanding the structure of galaxies in this transition region is a prerequisite for understanding dark matter halos and how they relate to the luminous galaxy. Potentially the best probe of this region is the sample of $\sim 80$ strong gravitational lenses. I review the determination of mass distributions using gravitational lenses using image positions, statistics, stellar dynamics, time delays and microlensing. While the present situation is confusing, there is little doubt that the existing problems can be resolved by further observations.To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html

Observations of multiple-image gravitational lens systems suggest that the projected mass distributions of galaxy haloes may contain substantial inhomogeneities. The fraction of the halo mass in dense substructure is still highly uncertain, but could be as large as a few percent. While halo substructure is seen in numerical simulations of CDM haloes, little of this substructure survives in the innermost regions of haloes, and thus the observational claims for substructure at small projected radii are slightly surprising. There is evidence, however, that even the highest-resolution simulations published to-date are still limited by numerical effects that heat and disrupt substructure artificially in high-density regions. By comparing numerical and semi-analytic (SA) models of halo substructure, we show that current simulations probably underestimate the mass fraction in substructure at small projected radii, by a factor of at least 2–3. We discuss the prospects for using lensing observations as a fundamental test of the nature of dark matter.To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html

We use $\Lambda$CDM numerical simulations to model the density profiles and substructure populations in a set of sixteen dark matter halos with resolutions of up to seven million particles within the virial radius. These simulations allow us to follow robustly the formation and evolution of the central cusp over a large mass range of 10$^{11}$ to 10$^{14}$$\msun$ down to approximately 0.5% of the virial radius, and from redshift 5 to the present. The cusp of the density profile is set at redshifts of two or greater and remains remarkably stable to the present time, when considered in non-comoving coordinates.

We fit our halos to a 2 parameter profile where the steepness of the asymptotic cusp is given by $\gamma$, and its radial extent is described by the concentration, $c_{\gamma}$. In our simulations, we find $\gamma$ = 1.4 - 0.08Log($M/M_*$) for halos of 0.01$M_*$ to 1000$M_*$, with a large scatter of $\Delta\gamma \sim \pm 0.3$; and $c_{\gamma} = 8(M/M_*)^{-0.15}$ with a large $M/M_*$ dependent scatter roughly equal to $\pm c_{\gamma}$. Our redshift zero halos have inner slope parameters ranging approximately from r$^{-1}$ to r$^{-1.5}$, with a median of roughly r$^{-1.3}$. This two parameter profile fit works well for all halo types present in our simulations, whether or not they show evidence of a steep asymptotic cusp.

The substructure population is independent of host halo mass and redshift with halo to halo scatter in the substructure velocity distribution function of a factor of roughly two to four. The radial distribution of substructure halos (subhalos) is consistent with the mass profile over the radial range where the possibility of artificial numerical disruption of subhalos can be most reliably excluded, r$\simgt$0.3 r$_{vir}$, although a weakly shallower subhalo profile is favored by the data. We discuss the implications that our results have on gravitational lensing studies of halo structure.To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html

The shape and size of a galaxy-mass dark matter halo can only be measured using a luminous tracer, such as stars, gas or photons. In the last case, gravitational lensing of extended background sources provides many paths for photons to sample the gravitational potential of the lensing galaxy. Multiple independent light paths provide tighter constraints on the mass profile of the lens galaxy than is possible from single path sampled by the lensed images of unresolved sources.

Using software specifically developed to model resolved images, we analyse the optical and radio Einstein ring systems 0047-2808 and MG1549+3047. For 0047-2808, elliptical pseudo-isothermal models (e.g. SIE, PIEP) are able to reproduce the lensed image well, with a surface mass power-law $\Sigma \propto r^{-\gamma}$, best fitted by $\gamma = 1.07 \pm 0.03$, which is slightly steeper than isothermal. Using a stars-plus-halo model, we find the projected halo is rounder than the visible galaxy (axis ratio $0.82 \pm 0.01$) and aligned closely with the stars (offset $8^{\circ} \pm 1$). Preliminary results for MG1549+3047 also rule out the constant M/L model. A stars + PIEP model suggests a very round (axis ratio $\ge 0.98$) halo.To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html

Galaxy-galaxy lensing (GGL) measures the 2-point cross-correlation between galaxies and mass in the universe. In this work we seek to generalise this effect by considering the third-order correlations between galaxies and mass: galaxy-galaxy-galaxy lensing. We introduce the higher-order cross-correlation functions and discuss their physical interpretation in terms of the halo model. We then demonstrate that measuring the lensing shear of background galaxies about foreground galaxy pairs, which we term galaxy-galaxy-galaxy lensing, provides a measure of the third-order cross functions. We note that current surveys in which detections of the cosmic shear 3-point correlation function have been reported should also be able to detect galaxy-galaxy-galaxy lensing. Indeed, we suggest that this effect may already have been detected in recent studies of GGL.To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html

We study the dark matter halos of galaxies with galaxy-galaxy lensing using the COMBO-17 survey. This survey offers an unprecedented data set for studying lens galaxies at $z=0.2-0.7$ including redshift information and spectral classification from 17 optical filters for objects brighter than $R=24$. So far, redshifts and classification for the lens galaxies have mainly been available for local surveys like the Sloan Digital Sky Survey (SDSS). Further, redshifts for the source galaxies have typically not been available at all but had to be estimated from redshift probability distribution which – for faint surveys – even had to be extrapolated.

To study the dark matter halos we parametrize the lens galaxies as singular isothermal spheres (SIS) or by Navarro-Frenk-White (NFW) profiles. In both cases we find a dependence of the velocity dispersion or virial radius, respectively, on lens luminosity and colour. For the SIS model, we are able to reproduce the Tully-Fisher/Faber-Jackson relation on a scale of $150h^{-1}~\mathrm{kpc}$. For the NFW profile we also calculate virial masses, mass-to-light ratios and rotation velocities.

Finally, we investigate differences between the three survey fields used here.To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html

The Hubble Deep Field (North) and the flanking fields are used investigate the occurrence of multiple weak lensing deflections along the line of sight in relatively deep imaging data ($z_{\rm lens} \sim 0.6$, $z_{\rm source} \sim 1.2$). Ray tracing simulations of galaxy–galaxy lensing in the HDF-North show that proper inclusion of multiple weak deflections is important for a correct prediction of the net shear for most sources, and for a given source redshift the number of multiple weak deflections is largely insensitive to the cosmography. The effects of multiple weak deflections on the magnitude of the weak lensing signal are, of course, strong functions of the adopted halo parameters. Independent of the halo parameters, however, the closest lens to a source (in projection on the sky) is not the strongest lens in the case of more than 50% of the sources which acquire a net shear of $\gamma \lo 0.01$. In addition, multiple weak deflections result in a tangential shear about the lens centers that is greater than the tangential shear that would occur if source galaxies were lensed solely by the closest lens. Further, multiple weak deflections give rise to correlated image ellipticities and account for a substantial amount of the total cosmic shear signal on small angular scales in $\Lambda$CDM and open CDM models.To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html

The gravitational lensing constraints on the small mass end of the $\Lambda$CDM mass function are discussed. Here a conservative approach is taken where the most difficult to explain image flux anomalies in strong lenses are emphasized. Numerical simulations are performed to compare predictions for the $\Lambda$CDM small scale mass function with the observed flux ratios. It is found that the cusp caustic lens anomalies and the disagreements between monochromatic flux ratios and simple lens models can be explained without any substructure in the primary lenses' dark matter halos. Extragalactic $\Lambda$CDM halos are enough to naturally explain these cases. This does not mean that substructure within the host lens is not contributing. In fact, it could be dominating the lensing. It should also be noted that this extragalactic population has not been directly observed either.

Spectroscopic gravitational lensing provides more information on the nature of these substructures. In the one relevant case in which this technique has been used so far, observations of Q2237+0305, there is evidence that there are more small mass halos ($\sim 10^6\msun$) than is expected in the $\Lambda$CDM model.To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html

Present day estimates of the Hubble constant based on Cepheids and on the cosmic microwave background radiation are uncertain by roughly 10% (on the conservative assumption that the universe may not be perfectly flat). Gravitational lens time delay measurements can produce estimates that are less uncertain, but only if a variety of major difficulties are overcome. These include a paucity of constraints on the lensing potential, the degeneracies associated with mass sheets and the central concentration of the lensing galaxy, multiple lenses, microlensing by stars, and the small variability amplitude typical of most quasars. To date only one lens meets all of these challenges. Several suffer only from the central concentration degeneracy, which may be lifted if one is willing to assume that systems with time delays are either like better constrained systems with non-variable sources, or alternatively, like nearby galaxies.To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html

We describe a new project aiming at measuring time delays for most known lensed quasars, from optical light curves obtained with five (almost) dedicated 1-2 m telescopes in the Northern and Southern hemispheres. The goal is to evaluate the Hubble constant H$_0$ with a precision below 2%. We present here numerical simulations in order to define the optimal temporal sampling in our observations as a function of typical quasar variations, object visibility, and for a given accuracy on the individual photometric points. It is also emphasized that the ongoing effort to obtain deep imaging using both space and ground based facilities must be continued, as illustrated by the comparison of HST and VLT near-IR images of the “cloverleaf”: H 1413+117.To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html

We present $VR$ magnification ratios of the double quasar Q0957+561AB, which are inferred from frames taken with the 2.56m Nordic Optical Telescope. From two different photometric techniques ($pho2comC$ and $psfphot$) and a reasonable range for the time delay in the system (415–430 days), we find optical continuum ratios depending on the wavelength. These chromatic ratios are consistent with either differential extinction in a dust system or gravitational microlensing in the deflector. Although the dusty scenario is only viable for a compact dust cloud in the line of sight to the A component, the possible values for the differential extinction and the ratio of total to selective extinction in the $V$ band are surprisingly reasonable. To decide on the true origin of the anomalous ratios (extinction, microlensing or a mixed scenario), we are carrying out new monitoring campaigns and planning detailed observations from the best ground–based and space telescopes.To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html

Compact groups of galaxies recently have been discovered in association with several strong gravitational lens systems. These groups provide additional convergence to the lensing potential and thus affect the value of $H_0$ derived from the systems. Lens system time delays are now being measured with uncertainties of only a few percent or better. Additionally, vast improvements are being made in incorporating observational constraints such as Einstein ring structures and stellar velocity dispersions into the lens models. These advances are reducing the uncertainties on $H_0$ to levels at which the the effects of associated galaxy groups may contribute significantly to the overall error budget. We describe a dedicated multiwavelength program, using Keck, HST, and Chandra, to find such groups and measure their properties. We present, as a case study, results obtained from observations of the CLASS lens system B1608+656 and discuss the implications for the value of $H_0$ derived from this system.To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html

Gravitational microlensing is a powerful tool to detect compact matter on mass scales ranging from roughly $10^{-6} M_\odot$ to $10^6 M_\odot$ . Of particular importance is the fact that lensing is sensitive to both luminous and dark matter. There are two practically important regimes of microlensing: cosmological and Galactic. The former deals with the effect of stellar mass objects at cosmological distances on even more distant quasars, the latter treats stellar mass objects in the Milky Way or its Halo on stars in the Magellanic Clouds, the Galactic Bulge or M31. Microlensing has been detected in about ten quasars, roughly a dozen microlensing events towards stars in the Magellanic Clouds are on record, more than 1000 events have been measured towards stars in the Bulge. The large majority of microlensing events towards quasars and towards stars in the Magellanic Clouds or the Bulge can be explained by ordinary stellar mass objects. A small fraction of the Galactic microlensing events ($< 5 - 10$ %) could potentially be due to dark matter objects, including stellar mass Black Holes. Current and planned experiments will clarify the question whether any dark matter objects are necessary at all to explain microlensing events.To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html

We report on a number of ongoing experiments to obtain spectrophotometric data of lensed quasars. We find that spectral differences are a widespread phenomenon, suggesting that microlensing is very common. We discuss some aspects of how to distinguish microlensing from other sources of spectral differences. Extinction by intervening dust can have similar broad band signatures, but can be distinguished or even separated from microlensing using spectrophotometry. We present recent results on extinction and microlensing in a $z=0.93$ lensing galaxy. We also present a case study of the issue of discrepant flux ratios in a quadruple QSO, where we find that microlensing probably plays a role. We finally discuss the aspect of microlensing-induced emission line profile variations.To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html

The nature and the location of the lenses discovered in the microlensing surveys done so far towards the LMC remain unclear.

This contribution is comprised of two distinct parts. In the first part, motivated by these questions, we compute the optical depth for the different intervening populations and the number of expected events for self-lensing, using a recently drawn coherent picture of the geometrical structure and dynamics of the LMC disk. By comparing the theoretical quantities with the values of the observed events it is possible to put some constraints on the location and the nature of the machos. Clearly, given the large uncertainties and the few events at our disposal it is not yet possible to draw sharp conclusions, nevertheless we find that up to 3-4 macho events might be due to lenses in LMC, which are most probably low mass stars, but that hardly all events can be due to self-lensing. A plausible solution is that the events observed so far are due to lenses belonging to different intervening populations: low mass stars in the LMC, in the thick disk, in the spheroid and some true machos in the halo of the Milky Way and the LMC itself. We report also on recent results of microlensing searches in direction of the M31 galaxy, by using the pixel method. The present analysis still does not allow yet to draw sharp conclusions on the macho content of the M31 galaxy.

In the second part (section 5), a preliminary account of the final results from the EROS-2 programme is presented. Based on the analysis of 33 million LMC and SMC stars followed during 6.7 years, strict limits on the macho content of the galactic halo are presented; they cover the range of macho masses between 0.0001 and 100 solar mass. The limits are better than 20% (resp. 5%) of the standard halo for masses between 0.0002 and 10 (resp. 0.001 to 0.1) solar mass. This is presently the data set with the largest sensitivity to halo machos.To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html