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Cluster Lensing: Peering into the Past,
Planning for the Future
Talk Abstracts

Listing of Talk Abstracts

Observational Constraints on Astrophysical Biases in Strong Lensing Selected Galaxy Clusters
Dr.  Matthew Bayliss (Harvard/CfA)
Galaxy clusters that act as strong gravitational lenses are among the rarest objects in the Universe, such that a small fraction of the total galaxy cluster population dominates the global cluster-scale strong lensing cross section. Simulations point toward a host of astrophysical biases that could cause galaxy clusters selected for the presence of bright giant arcs to systematical differ from clusters selected on the other bases, such as mass-proxy observables. I will present results from several recent and forthcoming papers with empirical evidence for/against several specific astrophysical biases in a strong lensing selected sample of galaxy clusters. Specifically, we find no evidence for increased incidence of cool core activity in strong lensing clusters, arguing against the ability of baryonic cooling in evolved clusters to systematically increase the total matter concentrations in their cores. I will also present a direct measurement indicating that orientation bias is a significant driver of large strong lensing cross sections using stacked dynamical observations of strong lensing clusters.
A New Differential Approach to establish the Star Formation Rate Density at z~9 from Searches for Galaxies Behind Lensing Clusters
Dr.  Rychard Bouwens (Leiden Observatory)
A powerful approach to finding galaxies in the early universe is to take advantage of the large high-magnification search volume behind galaxy clusters. Already this approach has been responsible for identifying tantalizing, yet largely robust z~10-11 galaxies in the 25-cluster CLASH program. However, it has been unclear whether these discoveries can be simply translated into a derived SFR density, due to concerns that the selection volume behind lensing clusters may be significantly model dependent (and thus possibly highly uncertain). Fortunately, it is possible to largely circumvent these uncertainties, using a new differential approach presented in Bouwens et al. 2012. With this approach, we simply derive the evolution by comparing the number of candidates found in our highest redshift sample, i.e., z~9-11, with the number of candidates found in a slightly lower redshift sample, i.e., at z~8 and then correcting for the difference in selection volumes. The ratio of selection volumes is almost entirely model independent, due to the very small differences (i.e., 1%) between the DLS/DS factors at z~8 and z~9-11. We apply this approach to a 4-object z~9 sample we have identified over the CLASH clusters and find that the SFR density at z~9 is just ~0.22_{-0.15}^{+0.30)x the value at z~8. Combining this measurement with z~8 LF determinations from the field, we derive an absolute SFR density at z~9. Our z~9 result is consistent with earlier estimates from Zheng et al. 2012 and Coe et al. 2013. Our result is also similar to simultaneous findings over the HUDF from Ellis et al. 2013 and Oesch et al. 2013, but suffers from significantly smaller large-scale structure uncertainties.  
Cluster Lenses as Cosmic Telescopes
Marusa Bradac (UC Davis)
The advent of Wide Field Camera 3 (WFC3) on HST enabled us to detect galaxies as far as z~11. They are likely beacons of the epoch of reionization, which marked the end of the so-called “Dark Ages” and signified the transformation of the universe from opaque to transparent. Clusters of galaxies, when used as cosmic telescopes, can greatly simplify the task of studying and finding these and lower-z galaxies. With a massive cluster one can gain several magnitudes of magnification over a typical observing field, enabling imaging and spectroscopic studies of intrinsically lower-luminosity galaxies than would otherwise be observable, even with the largest telescopes. In this talk I will present the current state of the field and briefly discuss our future goals with among others MOSFIRE@Keck and Spitzer.
CLASH: Magnified High-Redshift (z >~ 6) Galaxies
Dr.  Larry Bradley (STScI)
Gravitational lensing by massive galaxy clusters has been highlighted as an efficient approach to detect and study the properties of faint high-redshift galaxies. We present the status of our search for high-redshift (z>~6) galaxies from the ongoing CLASH (Cluster Lensing And Supernova survey with Hubble) observations of galaxy clusters. Thanks to the magnification provided by the foreground clusters, some of these galaxy candidates are among the most distant ever seen. Our large sample of magnified z>~6 Lyman Break Galaxies is also unique in that we have observations in seven ACS/WFC optical bands and all five WFC3/IR broad-band filters, enabling very accurate photometric redshift selections. We will discuss our recent results, including the observed surface densities of z>~6 galaxies as compared to comparably deep and ultra-deep HST blank-field surveys.  
Cluster Based Weak Lensing Tomography
Prof.  Douglas Clowe (Ohio University)
I will present results from a weak lensing tomography measurement around 10 high-redshift clusters of galaxies and discuss the cosmological constraints which can be obtained. Cluster based weak lensing tomography is a geometric only measurement of cosmological parameters, and thus sensitive to many fewer cosmological parameters than is cosmic shear, and is insensitive to cosmic variance. I will also discuss the potential systematic errors in this technique, how it can help improve tomographic cosmic shear measurements, and some likely applications for it in the future beyond cosmological parameter estimation.
The Next Frontiers of Gravitational Lensing
Dr.  Dan Coe (STScI)
Strong gravitational lensing by massive galaxy clusters has yielded precise dark matter maps and magnified images of some of the most distant galaxies known. The observations have tested our theories of structure formation and placed new constraints on reionization. In the coming years, deep space-based imaging obtained with the Hubble Frontier Fields and Spitzer programs including SURF'S Up will reveal galaxies which are intrinsically fainter and more distant than any observed before. The large numbers of galaxies strongly lensed by each cluster will enable detailed dark matter mapping, precise magnification estimates for the lensed galaxies, and constraints on cosmology. Joint analyses combining strong + weak lensing, kinematics, X-ray, and SZ observations are required to properly model triaxial halos and to cross-calibrate mass observables used in large surveys. Our challenge for the future is to refine our analysis tools to make optimal use of the data to come.
Galaxy Clusters in the Era of Survey Science
Dr.  Sanghamitra Deb (Argonne National Laboratory)
The coming decade is going to be very exciting for gravitational lensing science with large surveys like the Dark Energy Survey (DES), the Subaru Hyper-suprime Cam (HSC) , the Large Synaptic Survey Telescope (LSST) and the space based Euclid mission returning data with unprecedented precision. Lensing directly probes the distribution of matter through the bending of light due to gravity. Massive galaxy clusters produce lensing of background sources on all scales ranging from weak distortions, change of number density due to magnification to multiple images near their cores. Lensing masses are important in calibrating the mass-observable (X-ray temperature or SZ scaling) relation. I will highlight the study of number density changes due the magnification of background sources as a means of measuring masses of clusters. I will pay particular attention to the systematics such as clustering of background galaxies, shadowing by cluster members and others involved in magnification measurements. Additionally I will also highlight an ongoing cluster lensing study involving a sample of 20 supermassive clusters where we are using the information from shear and magnification to calibrate the normalization and scatter in the mass-temperature relationship of clusters.  
Mapping the mass -- successes and challenges
Dr.  Ian Dell'Antonio (Brown University)
The measurement of clusters of galaxies via gravitational lensing (both weak and strong) has seen great progress in the past 25 years, both in terms of analysis techniques and in terms of the data the techniques are applied to. I will highlight notable advances we have made along the way and highlight still-unsolved questions in our quest to measure the distribution of mass within clusters.
Overview of Lensing Cluster Surveys: Past, Present, and Future
Dr.  Eiichi Egami (University of Arizona)
The use of massive galaxy clusters as powerful cosmic telescopes has been established, and a number of large lensing cluster surveys are being conducted with HST, Spitzer, and Herschel as well as with other powerful observing facilities on the ground. I will present an overview of such lensing cluster surveys in the past and present with their scientific highlights, and will discuss the prospect for the future, especially in regard to the newly initiated HST Frontier Field program.
Rare Views of High Redshift Galaxies and Lensing Clusters
Dr.  Brenda Frye (University of Arizona)
We present results of the first space-based census of emission line galaxies (ELGs) in the field of a massive lensing cluster (Abell 1689; z=0.187). Forty-three ELGs are identified to a flux of i_775=27.3 via slitless grism spectroscopy. One low-mass strongly-lensed galaxy at z=0.7895 is caught at the onset of a major starburst. We break the continuous line-emitting region of this giant arc into seven ~1kpc bins (intrinsic size) and measure a variety of metallicity dependent line ratios which are traced back onto the source plane. At still higher redshift, we present the spectrum of a bright galaxy at z = 4.9 in 14 h of integration on VLT FORS2. Although fainter than a QSO, the absence of a strong central ultraviolet flux source in this star forming galaxy enables a measurement of the H I flux transmission in the IGM in the vicinity of a high redshift object. We find that the effective H I optical depth is remarkably high within a large 14 Mpc (physical) region surrounding the galaxy compared to that seen towards QSOs at similar redshifts. Evidently, this high-redshift galaxy is located in a region of space where the amount of H I is much larger than that seen at similar epochs in the diffuse IGM. Investigation of this and other candidate objects are under investigation as the possible progenitors of massive galaxy clusters.  
Preparing for the Future: Samples, Statistics and Telescopes
Prof.  Mike Gladders (The University of Chicago)
The coming decade promises to be exciting times for cluster strong lensing, with the burgeoning of lens samples, observational resources to study them, and simulations to contextualize them. I will highlight some recent progress on cluster lens samples using the SDSS, and project forward from there to the DES and LSST. Critical to exploiting these sample will be the coming generation of telescopes: I will use recent results from HST, Magellan, Keck and other facilities to look forward to JWST and GMT. Time permitting I will also say a few words about simulations.  
Why not use Flexion?
Prof.  Dave Goldberg (Drexel University)
Over the last decade, Flexion -- the "banananess" introduced through 2nd order lensing -- has become a much-discussed, but little-used tool for gravitational lensing reconstructions of clusters. Though upwards of a dozen groups have produced simulated Flexion fields, only a handful have used Flexion to constrain cluster substructure. In this talk, I will review the basics of Flexion, give an overview of the work to date, provide a discussion of why so much of the work is theoretical, and a discussion of the complications of Flexion measurement. Most importantly, I will conclude with what needs to be done to effectively exploit and measure this signal in a cluster environment.  
Probing the Small-Scale Primordial Power Spectrum with Lensed High-z Galaxies
Prof.  Zoltan Haiman (Columbia University)
Cluster lensing allows the detection of ultra-faint, ultra-high redshift galaxies. These lensed deep fields also probe very small comoving volumes, and even a few galaxies found in such small volumes implies a very high space density, corresponding to those of low-mass dark matter halos. This implies significant primordial power on small scales, allowing these observations to test popular alternatives to standard cold dark matter (CDM) models, which are designed to reduce the power on scales below 0.1-1 Mpc. As an example, I will discuss constraints on warm dark matter (WDM) models from low-mass halos uncovered by existing high-redshift cluster lensing observations, as well as improvements on these constraints that will be possible from the forthcoming Frontier Fields.  
The Mass Distribution in Galaxy Clusters
Mr.  Stefan Hilbert (Max Planck Institute for Astrophysics)
I discuss how strong and weak lensing can give insight into the mass distribution in galaxy clusters, in particular into the overall mass profile, the mass around cluster member galaxies, and the amount of substructure. I discuss uncertainties in our expectations from theory as well as observational uncertainties.  
Arc statistics - Is it really a problem for LambdaCDM?
Dr.  Assaf Horesh (Caltech)
Gravitational lensing is frequently used to map the evolution of cluster mass profiles, ellipticities, and substructure. One approach is to perform detailed modeling of individual clusters using strong and weak lensing, but this is generally suited only to deep data for individual clusters that exhibit numerous lensed images, and the results may not be representative of the majority of clusters. A complementary approach is to measure the statistics of lensed arcs in large samples of clusters. Over the past decade, a debate has gone on about the nature of cluster samples selected by different methods, and about whether or not real clusters are highly more efficient producers of arcs than expected from theory. I will present an analysis of the arc statistics in a sample of ~100 galaxy clusters observed with the HST/ACS. X-ray selected clusters are much more efficient lenses than optically-selected clusters of similar optical luminosity, showing that optical selection yields lower-mass, perhaps marginally bound, structures. I will also present our past and in-progress comparison of observed arc statistics with simulations that aim to include all the theoretical and observational aspects of the problem.
Gravitational Lensing Analysis of the 12 'high-redshift' MACS Galaxy Clusters
Dr.  Mathilde Jauzac (Astrophysics & Cosmology Research Unit)
In Jauzac et al. (2012), we presented the weak gravitational lensing analysis of MACSJ0717.5+3745, an X-ray luminous cluster, at a redshift of z ~0.55. This cluster is part of the high-z MACS subsample, 12 galaxy clusters et z > 0.5. Two of these clusters are part of the HST Frontier Field program. By making a weak gravitational lensing analysis of MACSJ0717 and its outskirts, we were able to report the weak-lensing detection of a large-scale filament which funnels the matter into the core of the cluster. Our analysis is based on a mosaic of 18 HST/ACS maps, i.e. an area of ~10x20 arcmin2. To test the consistency of our weak lensign analysis, we first compared our results with the strong lensing analysis of Limousin et al. (2012, A&A) of the cluster core. The weak and strong lensing density profiles of the cluster core showed a really good agreement. In terms of mass integrated in a radius of 500 kpc (given the same center), the strong lensing gives 1.060.03 1e15 Msun, while the weak lensing gives 1.040.08 1e15 Msun. The excellent agreement between both values and density profiles confirmed the strength of our weak lensing analysis. Using the mass modeling method presented in Jauzac et al. (2012), we extend the study to the whole high-z sample. Thanks to the multi-wavelength dataset available for these clusters, we are obtaining strongly constrained mass models for all of them. The multi-wavelength analysis of massive galaxy clusters is one of the most efficient way to study the different distant galaxy populations observed thanks to the natural amplification provided by these massive objects, but it also provides a unique way to constrain the cluster mass distribution, and the nature of dark matter. Thanks to an accurate modeling of the cluster mass distributions, we derive as a function of redshift (from 0.5 to 0.7) the mass profiles, and the total mass, and compare them with the baryonic matter distribution. These measurements give information on the evolution of the mass assembly in galaxy clusters. Therefore, the direct comparison of these results with simulations will put constraints on models of cluster evolution.  
Entropy-regularized Strong-lensing Mass Reconstruction of Abell 1689
Dr.  James Jee (UC Davis)
Parameterized mass reconstruction of strong lensing clusters is subject to degeneracy. In many cases, it is possible to obtain multiple models that can explain the distributions of the same multiple images. Since cluster magnification must be known as accurate as possible in order to interpret detections of high-redshift galaxies, it is important to explore all possible mass configurations and rule out unphysical cases. I suggest an entropy-regularized strong-lensing mass reconstruction method as a tool to resolve the problem. The method has been applied to Abell 1689. I will present and discuss the result of this well-studied cluster obtained from the new approach.  
Weak Lensing Galaxy Cluster Field Reconstruction
Dr.  Eric Jullo (Laboratoire d'Astrophysique de Marseille)
In this talk, I will compare three methods to reconstruct galaxy cluster density fields with weak lensing data. The first method called FLens integrates an inpainting concept to invert the shear field with possible gaps, and a multi-scale entropy denoising procedure to remove the noise contained in the final reconstruction, that arises mostly from the random intrinsic shape of the galaxies. The second and third methods are based on a model of the density field made of a multi-scale grid of radial basis functions. In one case, the model parameters are computed with a linear inversion involving a singular value decomposition. In the other case, they are estimated using a Bayesian MCMC optimization implemented in the lensing software Lenstool. I will show reconstruction results on simulated an real data of the filament in the field of the galaxy cluster MACS J0717+3745.  
Are the Einstein radii of the high z X-ray galaxy clusters consistent with LCDM?
Dr.  Madhura Killedar (University of Trieste)
The discrepancy between the observed and predicted strong lensing properties of galaxy clusters present a source of tension for LCDM cosmology. The X-ray selected >0.5 MACS cluster sample has posed such a challenge due to large measured Einstein radii; however, more complete treatment of baryonic processes in simulations, self-consistent cluster selection and recent updates to mass models care of the ongoing CLASH project may reduce the tension. We study the relationship between Einstein radii, X-ray luminosities and inferred masses, comparing those found for the high-z MACS sample with predictions at z=0.5 and z=0.6 from hydrodynamic simulations that include --- along with other baryonic processes --- star formation, chemical evolution, supernovae and AGN feedback. Furthermore, we analyse cluster selection criteria (X-ray flux, mass and/or dynamical state) and the large (mock) sample-to-sample variation due to triaxiality, and conclude with a view of what is required from upcoming cluster data and simulations.  
Lensing signatures of cluster structure and physics
Prof.  Lindsay King (University of Texas at Dallas)
We consider the impact of the physical properties of clusters on their weak and strong lensing signatures. In particular, we summarise a mock weak lensing analysis of ~3000 cluster mass halos from the Millennium simulations, and assess how factors such as triaxiality and mass substructure affect mass models. We also outline how cluster baryonic physics is manifest in the production of giant arcs.  
Modelling Cluster Substructure
Dr.  Richard Massey (Durham University)
Direct evidence for the existence of dark matter and measurements of its interaction cross-section have been provided by the physical offset between dark matter, galaxies and and intra-cluster gas in merging systems like the Bullet Cluster. I will present n-body simulations of both standard and self-interacting dark matter demonstrating that the offset is also present in the infalling substructure of minor mergers. In such low-mass systems the gravitational lensing signal comes primarily from weak lensing. A fundamental step in determining such an offset in sub-structure is the ability to accurately measure the positions of dark matter sub-peaks. Using simulated Hubble Space Telescope observations, we make a first assessment of the precision and accuracy with which we can measure infalling groups using weak gravitational lensing. We demonstrate that using an existing and well-used mass reconstruction algorithm can measure the positions of 2.5 x 10^ 13Msun substructures with a bias of less than 0.3". In this regime, our analysis suggests the precision is sufficient to detect (at 3-sigma statistical significance) the expected mean offset between dark matter and baryonic gas in infalling groups from a sample of ~50 massive clusters.  
The extreme case of MACSJ0717 from recent CLASH lensing work
Dr.  Elinor Medezinski (Johns Hopkins University)
The galaxy cluster MACSJ0717.5+3745 (z=0.55) is the largest known cosmic lens, with complex internal structures. I present a complete weak and strong-lensing analysis of its mass properties from wide-field Subaru/Suprime-Cam and high-resolution Hubble Space Telescope (HST) observations taken as part of CLASH. A unique combination of consistent weak-lensing shear, magnification and strong-lensing allows an accurate radial mass profile of the cluster and its surrounding large-scale structure to be reliably derived out to 5 Mpc/h. The central non-parametric lensing map reveals the internal dark matter substructure is overall consistent with the light. The wide-field weak-lensing mass map reveals a clear filamentary structure traced by distinct mass halos. The total mass of the cluster as determined by several methods is Mvir=(2.8+/-0.4)*10^{15} M_sun, making it the most massive cluster known beyond z>0.5, albeit still marginally within LCDM realm.
Mass and light distributions of massive galaxy clusters with DECam
Dr.  Peter Melchior (The Ohio State University)
DECam is the newly commissioned imager of the Dark Energy Survey with a very large Field-of-View of more than 3 square degrees. During its Science Verification phase, we targeted four massive galaxy clusters visible from CTIO to measure their weak-lensing effect. I will show some of the first science-quality images from DECam and present the photometry and shape analysis that yielded mass and light maps of each system. Because of the large FoV, these maps allow us to determine the connection between mass and light on unprecedented scales.  
Simulations and Lens Modeling Uncertainties
Dr.  Massimo Meneghetti (INAF - Osservatorio Astronomico di Bologna)
I will discuss, with the help of mock observations of numerically simulated lenses, how the current lensing based methods are able to constrain the mass distribution of the lenses. I will discuss both the applications of strong and weak lensing methods, illustrating their possible biases, and discussing how these might be avoided.  
Non-parametric, full-scale cluster reconstructions for CLASH
Dr.  Julian Merten (JPL / Caltech)
There are several ways to reconstruct the mass distribution in galaxy clusters from lensing constraints. While strong lensing features, like multiple images or giant arcs, only appear near the core of the total mass distribution, weak lensing probes the full field-of-view but on lower spatial resolution. The consistent combination of both regimes allows the reliable reconstruction of all important cluster properties like total mass, density profile, sub-structure content and general morphology. While many reconstruction approaches using strong lensing constraints assume a parametric form for the cluster's underlying density profile, weak lensing generally does not need this assumption. In my talk I will present SaWLens, one of the methods used to reconstruct the mass distribution of the clusters in the Cluster Lensing and Supernova Survey with Hubble (CLASH). SaWLens is a fully non-parametric method combining weak and strong lensing constraints on an adaptively refined grid. I present the great effort we invested to assess the systematics of such a method and to justify the reliability of its results. We used numerical simulations of clusters and created realistic, simulated CCD images with the help of a ray-tracing realization of a full lensing scenario. The Mock data we created included all sorts of realistic noise contributions and posed the ideal test bed for our reconstruction algorithms. After extensive test runs we applied our methods to real data and I will present our results for the reconstructions of the CLASH clusters, showing a significant relaxation of earlier tension in the mass-concentration relation. More results from the study of merging clusters of galaxies and future prospects for such analysis methods will end my presentation.  
Quantifying the relative power of galaxy clusters vs field surveys for probing the highest redshift galaxies
Dr.  Leonidas Moustakas (JPL/Caltech)
The magnified view that galaxy clusters offer of the high redshift universe comes at the expense of a dilution of the volume per unit area surveyed. Using gravitational lens models of CLASH clusters and analytic approximations to these, I present the relative survey efficiency of field versus cluster observations for identifying the faintest galaxies at redshifts z~6-12, and quantitative estimates of the uncertainty in recovering accurate measurements of the luminosity function.
Cosmology with Lensing Clusters: Dawn of a New Era
Prof.  Priyamvada Natarajan (Yale University)
Gravitational lensing by clusters provides a unique laboratory for exploring and constraining the properties of dark matter as well as dark energy. Strong lensing clusters, can be used to feasibly place constraints on the dark energy equation of state. This is achieved by using multiple images of sources at several distinct redshift planes behind a cluster. First results of this technique applied to Abell 1689 are promising. With the availability of data from the proposed HST FFI clusters, robust cosmological constraints comparable to and competitive with other more established methods can be derived.  
The Dark Matter Profile in the Cores of Galaxy Clusters
Andrew Newman (Caltech)
The inner dark matter (DM) density slope is an important test of our understanding of DM and its interplay with baryons during galaxy formation. Two of the main challenges in comparing to numerical simulations are measuring the mass profile over a wide radial range, and separating the contribution of stellar mass in the BCG, which is significant in the cluster core. I will show how combining strong and weak lensing with resolved stellar kinematic measures in the BCGs of relaxed clusters addresses both of these. In a sample of 7 relaxed clusters, we found that the slope of the total density profile follows predictions from CDM-only simulations down to surprisingly small radii within the BCG, whereas when the DM is isolated, its density slope is systematically shallower. I will briefly discuss physical scenarios that may explain these findings along with possible routes to further progress.
The shape of cluster-scale dark matter halos
Prof.  Masamune Oguri (Kavli IPMU, University of Tokyo)
The dark matter distribution in clusters is important as a fundamental test of the standard cold dark matter model and also for the use of the abundance and clustering of clusters as a cosmological probe. I will present recent measurements of the dark matter distirbution using both strong and weak lensing. While the dark matter distribution on average agrees very well with the standard model prediction, sometimes lensing reveals highly complex structure of clusters, which could pose a challenge to use clusters for precision cosmology.  
Hubble Space Telescope search for highz z~7 galaxies through Galaxy Clusters
Dr.  Danuta Paraficz (Laboratoire d’Astrophysique de Marseille)
Massive clusters, acting as powerful cosmic lenses, can constrain the abundance and properties of low-luminosity star-forming sources beyond z~7; such sources are thought to be responsible for ending cosmic reionization. The large magnification possible in the critical regions of well-constrained clusters brings sources into view that lie at or beyond the limits of conventional exposures such as the UDF. We have shown that the combination of HST and Spitzer is particularly effective in delivering the physical properties of these distant sources, constraining their mass, age and past star formation history. Indirectly, we therefore gain a valuable glimpse to yet earlier epochs. We present the results of a systematic search through 4 lensing clusters with ACS/F814W and WFC3/[F110W+F160W] (in conjunction with existing deep IRAC data). Based on the identification of about 50 lensed galaxies at 6.5
First results from GLASS: A new non-parameteric strong lensing tool
Prof.  Justin Read (University of Surrey )
I present results from a new non-parametric lensing tool: GLASS which builds on an earlier code PixeLens. Like PixeLens, GLASS uses a grid of mass pixels to model the lens, searching through many models consistent with the data to marginalise over model uncertainties. However, GLASS improves on PixeLens in a number of important respects: (i) it uses a significantly improved sampling strategy that is both faster and results in significantly smaller errors; (ii) it is built on a fully modular framework that allows us to use, for example, different basis functions; (iii) non-linear constraints can be applied in post-processing (for example, filters to remove models with spurious additional images or models inconsistent with stellar dynamics data are already implemented); (iv) the stellar mass (if known) can be applied as a prior; and (v) adaptive resolution can be used in regions of interest. I show a number of tests of the code applied to dynamically realistic mock N-body data and discuss what quality of data are most important for measuring the circularly averaged mass profile of a lens and its shape, and for probing cosmology. Finally, I present some preliminary results from GLASS applied to a large ensemble of galaxy and cluster lenses.  
Towards high accuracy mass distributions of cluster cores
Dr.  Johan Richard (Centre de Recherche Astrophysique de Lyon)
Since the first discoveries of giant arcs and weak shear, the two regimes of lensing: strong and weak, have been used to probe the mass distribution of galaxy clusters. I will present new results for the modelling of X-ray selected cluster samples, focusing on the mass reconstructions of cluster cores where clusters are also widely used for their "natural telescope" capabilities. With the large amount of upcoming datasets (imaging and spectroscopy) accompanying the new Hubble Frontiers Field Initiative, it is key to review the methodology and the limiting systematics in cluster lensing mass reconstructions: - accurate identification of multiple systems used as constraints - redshift determination - priors on the mass distribution of the clusters - combination of multiple probes (X-ray, light distribution, SZ) to recover the effect of geometrical projections. These define the roadmap to ensure high accuracy mass distributions, allowing the community to these massive clusters as cosmic telescopes.
First results from the CLASH-VLT project
Dr.  Piero  Rosati (ESO)
To complement the very rich CLASH HST data set, we have undertaken a large spectroscopic campaign with VLT/VIMOS of the 14 CLASH clusters in the southern hemisphere. The goal of this program is to obtain ~500 spectroscopic members per cluster and redshifts of relatively bright lensed galaxies out to z~7. I will focus on first results which specifically arise from the HST-VLT synergy, in combination with Subaru panoramic imaging and Chandra observations. In particular, the high-quality of this combined data set allows us to probe matter density profiles with unprecedented accuracy and negligible systematics over 3 decades in radius from a combination of three independent probes: lensing, galaxy dynamics, and X-ray.
Galaxy halo truncation in MACSJ1206 from Strong Lensing Modelling
Dr.  Stella Seitz (Universitaetssternwarte Muenchen / University Observatory Munich)
We use the 16-filter HST-CLASH data of MACSJ1206 to identify 12 multiply imaged systems. We separately describe the mass distribution associated with the cluster members and the larger scale dark matter of the cluster and obtain a combined mass distribution reproducing the multiple image positions very accurately. We refine the model to then also precisely match the light distribution of the extended gravitational arc and its counterimage in 3 filters simultaneously. We finally use the lens modelling to constrain the velocity dispersion and cutoff scale of the cluster member galaxy halos. We predict the correct Faber Jackson relation at the cluster redshift and demonstrate that galaxy haloes are substantially cut off in dense cluster environments.
Galaxy clusters as cosmic telescopes: first results from the large HST program
Dr.  Keren Sharon (University of Michigan)
I will present first results from our large survey of strong-lensing galaxy clusters, that are being observed by HST in Cycle 20 (GO13003, PI Gladders). The 37 cluster fields are selected from the Sloan Giant Arcs Survey (SGAS) to be lensing background galaxies at z~2 into highly magnified, bright giant arcs. The principal goal of the program is to combine the exquisite image quality of HST with the magnification boost due to lensing, in order to measure the sizes, luminosities, star formation rates, and stellar populations of individual star forming clumps in these galaxies, to study star formation at its fundamental scale over the entire peak of the star formation history in the Universe. Some preliminary results that will be presented here for the first time include lens models and source reconstructions for a portion of the sample that has already been observed. Strong lensing models are valuable tools to constrain the distribution of total projected mass at the clusters cores. In combination with observations in other wavelengths and/or scales, strong lensing masses can be used to constrain the inner slope of the halo mass distribution.
Ultra-Faint Lensed Galaxies Behind Abell 1689: Luminosity Functions and Properties
Prof.  Brian Siana (UC Riverside)
We are conducting a 60 orbit WFC3/UVIS ultraviolet imaging campaign of the lensing cluster Abell 1689. With the first 34 orbits we have identified more than 60 star-forming z~2 galaxies in a single pointing. With accurate models of the magnification of the background source plane, we are able to reconstruct an accurate luminosity function more than 100x fainter than previous determinations. The faint end slope is quite steep and the result is effectively unchanged when considering realistic uncertainties in magnifications and distance moduli. We emphasize that we would likely have found more galaxies at this redshift with the same exposure time in the field, but using the cluster lens is valuable for studying this inaccessible faint population for the first time. This new population of galaxies significantly increases the estimated star formation density at this important epoch. We will also present estimates of the dust extinction, stellar masses and ages.
Cosmography with cluster and group strong lenses
Prof.  Tommaso Treu (UCSB)
Cluster and group acting as strong gravitational lenses are powerful porbes of the composition and geometry of the universe. I will briefly review recent results based on the used on multiple sets of multiple images and on detailed modeling of time-delay systems. I will show that they provide a cost effective means to complement other probes like the cosmic microwave background of supernovae Ia.  
Grale Reconstruction of Clusters -- A New look at Substructure
Prof.  Liliya Williams (University of Minnesota)
I will present strong lensing mass reconstruction of three massive clusters, Abell 1689, 2218 and 3827, using Grale. Grale is a free-form, adaptive grid, genetic algorithm based lensing mass reconstruction method. All three clusters have statistically significant deviations between their light and mass distributions. While the origin of these deviations is still uncertain, at least in the case of A3827 it is unlikely to be due to superimposed line of sight structures, and is most likely due to the complex distribution of dark matter within the cluster core.
Finding the First Galaxies with a New Kind of Gravitational Lens Telescope
Prof.  Ann Zabludoff (University of Arizona)
Detecting very high-redshift (z ~ 10) galaxies is key to understanding the first generation of stars, how the first galaxies formed, and to what extent their UV emission is responsible for reionizing the neutral hydrogen in the early Universe. Observing these sources is extremely challenging, and huge investments of HST time are being devoted to this goal. Gravitational lensing by foreground galaxy clusters can help, but only if the volume surveyed for high-z sources remains large despite the magnification. Thus, the best gravitational lens telescopes may be heretofore unexplored lines-of-sight whose integrated mass is large and optimally distributed to produce the highest etendue, ie, the largest possible area in the source plane with high magnification. We use both toy models and the Millennium simulations to characterize the best beams. Lines-of-sight containing multiple, projected cluster-scale halos are less common, but more likely than single-cluster lenses to produce high etendue. We have now identified 200 promising lines-of-sight in the SDSS DR9. Galaxy spectroscopy and ground-based imaging confirm that they are powerful gravitational lens telescopes, capable of probing further down the luminosity function of the earliest galaxies.  
Multi-band Study of Most Distant Galaxies
Dr.  Wei  Zheng (Johns Hopkins University)
The upcoming HST Frontier Fields will discover dozens of galaxy candidates at redshift z>9, but additional data at wavelengths longer than 1.6 micron are needed to confirm and characterize them. Thanks to gravitational lensing, ground-based K-band and Spitzer/IRAC images will reach a search limit that is considerably fainter that in the UDF and enable us to rule out low-redshift interlopers and estimate the age of continuum shape of high-redshift galaxies. These complimentary data will also be useful for studying the galaxies at redshift 4
A Complete Light-Traces-Mass Approach for Lens Modeling
Dr.  Adi Zitrin (ITA/ZAH, Heidelberg University)
I will review our parametric mass modeling method (Zitrin et al. 2009) which takes full advantage of the light-traces-mass approach, for both the galaxies, and the dark matter distributions. This very simple assumption, via the procedure I will outline in the talk, is accurate enough to readily identify and reproduce the multiple images across the cluster field, so that the mass model is iteratively refined. I will show some examples, recent results, review some systematics and differences from other common parameterizations, and talk about using this method in an automated manner for the analysis of large sky surveys.