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Cluster Lensing: Peering into the Past,
Planning for the Future
Poster Presentations

Listing of Poster Abstracts

Uncovering The Hidden Treasures: Searching for Strong Gravitational Arcs in HLA/HSC
Dr.  Sahar Allam (STScI)
The Hubble Legacy Archive (HLA; Jenkner et al. 2006), and The Hubble Source Catalog (HSC; Whitmore et al. 2013) represent a new resource for Strong Gravitational Arcs searches. We present preliminary results of a pilot study to search for strong gravitational lenses around clusters throughout a subset of the HLA/HSC database.  
WFC3 Grism Survey of the HST Frontier Fields
Dr.  Hakim Atek (EPFL)
The HST frontier fields will enable the exploration of the faintest galaxy population in the distant universe, possibly responsible for the cosmic reionization. I will show why complementary WFC3 grism observations are essential to achieve this goal. Such spectroscopic data will broaden the science case, and enhance the legacy of the frontier fields. This observational dataset will remain unmatched until the JWST era.
Simulating Wide-Field Slitless Spectroscopy with JWST/NIRISS
Dr.  Van Dixon (STScI)
The Near Infrared Imager and Slitless Spectrograph (NIRISS) aboard the James Webb Space Telescope (JWST) will offer wide-field slitless spectroscopy (WFSS) with a resolving power R = 150 over the wavelength range 0.8 to 2.25 microns. In this band, NIRISS will be sensitive to Lyman-alpha emission from galaxies with redshifts 6 < z < 17. To explore its ability to observe such high-redshift galaxies, we have modeled a NIRISS observation of the massive galaxy cluster MACS J0647+7015. Using published images, photometry, and redshifts from the CLASH survey, we constructed a series of simulated direct and dispersed images in the six filters used for WFSS with NIRISS. To each image were added 180 high-redshift galaxies distributed uniformly in space, redshift, and magnitude. Using Source Extractor, we identified 7200 galaxies in the F200W direct image, including 165 of the high-redshift sources (the remainder were lost to bright foreground objects). From this catalog, we selected 1000 objects, including all 165 of the high-redshift sources. We performed photometry of these 1000 sources in each direct image and extracted their spectra (using the aXe software package) from each dispersed image. A subset of our team was given these data and asked to identify the high-redshift galaxies. We will present the results of this analysis and discuss their implications for the ability of NIRISS to detect and parameterize high-redshift galaxies in crowded fields. NIRISS is provided to the JWST project by the Canadian Space Agency under the leadership of Rene Doyon of the University de Montreal. The prime contractor is COM DEV Canada.  
Image deflections from density fluctuations along the line of sight
Dr.  Ole Host (Dark Cosmology Centre)
The observed light from a distant galaxy can be strongly deflected by a galaxy cluster but it will also pick up additional small deflections from the inhomogeneous matter distribution all along the line of sight. This additional cosmic weak lensing is possibly the strongest systematic in strong lensing modelling of galaxy clusters. We present a calculation of the image position covariance caused by cosmic weak lensing, based on the non-linear matter power spectrum. We discuss how the strength of the systematic varies with source redshift and image separation on the sky, and illustrate the implementation in strong lensing analyses.
Cluster Lensed Supernovae from the Ground and Space
Saurabh Jha (Rutgers University)
Supernovae observed behind galaxy clusters provide an opportunity to directly measure the absolute lensing magnification with a single object, breaking the mass sheet degeneracy and probing cluster mass models at a unique scale. I present initial results from two current observational campaigns to find and study such events: 1) from HST as part of the CLASH MCT survey and 2) from the ground with CLULESS: the CLUster and LEnsed Supernova Survey. In the near future, a few such supernovae could also be discovered by the HST Frontier Fields campaign, while from the ground, LSST promises to deliver hundreds of measurably magnified type-Ia supernovae behind clusters. Finally, I will present the serendipitous discovery in CLULESS of one of the largest, nearly complete, Einstein rings known.
Modeling the cluster lens of SDSS J1110+6459 with the Hubble Space Telescope
Traci Johnson (University of Michigan)
Our collaboration has been awarded 107 orbits of HST time for imaging 73 lensed arcs at z~1-3 in 37 clusters selected from the SDSS. We present the lens model for one of the first clusters in our survey, SDSS J1110+6459, with a bright, triply-imaged arc at z=2.418. The cluster was modeled using the publicly available LENSTOOL software, which utilizes a Bayesian MCMC to constrain the parameter space of the lensing potentials. Two sets of secondary arcs of unknown redshift have been identified and used as additional constraints for the model, with redshifts left as free parameters for the model. The lensing system is best modeled by a highly-elongated mass distribution, composed of several cluster-sized and galaxy-sized halos, centered on the brightest cluster galaxy, producing magnifications of 10 to 20 across the middle image of the primary arc. We present a source reconstruction of the lensed galaxy and measurements of the physical sizes of individual star-forming clumps within the galaxy. With similar analyses to be completed on our entire HST sample, we will begin to understand star formation at the fundamental physical scale across the entire peak of star formation history in the Universe.
Strong gravitational lens modeling software parameters are sensitive to small changes in redshift
Alan Lefor (Tohoku University)
We undertook this study to compare the effect of changes in redshift on results for four different strong gravitational lens software modeling codes (Lenstool/Glafic, two parametric codes, and GRALE/Pixelens, two non-parametric codes) in their analysis of mock data as well as the giant gravitational quasar SDSSJ1004+4112, with variations in zlens and zsource. System geometries were selected to demonstrate the effect on calculated parameters, and may not have physical significance. Cosmology was set at Omega_m=0.26 and Omega_Lambda=0.74. Using a point mass with four symmetrical images (an Einstein cross), 10 models were tested with each code, varying the system geometry, zlens and zsource. The effects of system geometry (Dds/Ds) on time delay, H0, Einstein radius and mass were compared. Models of SDSSJ1004+4112, were also evaluated testing 10 geometries with each code. The effects of small changes in redshift were evaluated with all four of the codes tested, with both models. In some models, the changes were greatest when the change in Dds/Ds was greatest, but this was not seen uniformly. These data demonstrate the effect of changes in redshift on parameters calculated by each of the codes, and not necessarily between codes due to variability in the models. These results show that even small changes in redshift can significantly affect calculated parameters, and are model dependent. This effect on calculated parameters suggests the need for accurate redshift data to maximize the data from lens models.
Measuring halo truncation in clusters using gravitational flexion
Ms.  Rebecca Levinson (MIT)
The mass of a cluster has to come from somewhere. Theory predicts that this mass is tidally stripped from the outer parts of non-BCG cluster members, resulting in galaxies whose mass profiles are truncated at small radii. This stripping and resultant halo truncation would be most severe for the inner most cluster members. While numerical simulations confirm this theoretical assertion, simulations are a far cry from empirical evidence. We have a program underway from the ground to use gravitational flexions to measure the predicted radially variant halo truncation of galaxies in clusters. A similar analysis is proposed for HST Cycle 21 by Ben Cain. Flexions are the third derivatives of the lensing potential, which in conjunction with shear make galaxies appear arced in lensed images. While the magnitude of the flexion effect is weaker than shear, the intrinsic scatter of flexions in galaxies make gravitational flexion a much more sensitive tool than shear for probing halo truncation in galaxies.  
Magnified SNe behind CLASH clusters
Dr.  Jakob Nordin (UC Berkeley/LBL)
We present Type Ia supernovae (SNe Ia) detected behind clusters observed as part of the CLASH MCT program. As standard candles, their observed luminosities can be used to probe more than the expansion rate. We compare the magnification predicted from supernova light curves with that from magnification maps. While current samples are small, this method puts constraints on lensing models and their assumptions.  
Spectroscopic follow-up of lensed sources to determine physical conditions
Dr.  Jane Rigby (NASA Goddard)
One of the many advantages of targeting lensing clusters is the ability to target highly magnified lensed galaxies for spectroscopic follow-up. With only one to a few hours of spectroscopic integration on bright-lensed targets, the resulting spectra are of sufficient quality to measure the density, ionization conditions, metallicity, abundance pattern, and initial mass function of the star-forming regions. Without lensing, such spectra would require tens to hundreds of hours of integration per target. I will summarize our work to date to characterize the evolving relationship between stellar mass, metallicity, and star formation rate at 1
Quantifying the Evolution of the Specific Star Formation Rate from z~3 to z~8 using Gravitationally Lensed Galaxies from CLASH
Miss  Renske Smit (Leiden University)
By using strong lensing clusters as cosmic telescopes we can probe deep enough to study the build-up of galaxies out to very high redshifts. An important tracer of the efficiency of galaxy build-up with time is the global specific Star Formation Rate (sSFR), or the amount of star formation per unit mass. In recent years it has become apparent that a discrepancy exists in the evolution of the sSFR between simulations and observations. Specifically, at redshift z~3-8 both semi-analytic models as well as hydrodynamical simulations predict a strong rise in sSFR with redshift while most observations show the well-known specific star formation rate plateau. A potential problem in the measurement of galaxy masses at redshift z>~4 is the contamination of strong nebular emission lines in the 3.6 and 4.5 mum Spitzer/IRAC bands that probe the rest-frame optical stellar continuum. To more accurately characterize the masses and other properties of high-redshift galaxies, high signal-to-noise IRAC imaging in combination with good redshift determinations are required. We use highly magnified galaxies found behind 12 strong lensing clusters in the CLASH survey to estimate the median sSFR and the equivalent widths of OIII and other relevant nebular emission lines out to z~8. We find evidence for a moderately rising specific star formation rate from redshift z~3 to z~8.
The distribution function of Einstein rings
Mr.  Jesús Vega (Universidad Autónoma de Madrid)
Strong lensing is one of the most direct probes of both the mass distribution in galaxy clusters and of the cosmological abundance of clusters. Indeed, not only lensing is used to constrain the density profiles and the masses of clusters, but the distribution of (large) Einstein radii depends on the cosmological distribution function of masses of clusters. We make comprehensive predictions of the Einstein radius distribution function through a combination of numerical methods based on detailed ray-tracing in state-of-the-art MUSIC simulated clusters and a semi-analytical method to infer the evolution with redshift of the distribution. The variance in the predictions arises from a number of reasons: the presence of sub-structures, orientation, concentration, unrelaxed configurations, which are difficult to assess analytically and can only be studied through massive Monte Carlo simulations. We produce the distribution function resulting from a billion simulated clusters and present a comparison with the 10,000+ SDSS clusters whose Einstein radii distribution was inferred by Zitrin et al. 2011.  
Comparison of X-ray, Weak Lensing, Dynamics, and Star Formation in Abell 2465
Dr.  Gary Wegner (Dartmouth College)
A comparison of the distribution of mass from Chandra X-ray data, dynamics, and weak lensing will be made with the locations of star forming galaxies in the merging cluster Abell 2465.  
A New Approach to Identifying Optimal Mass Configurations for Lensing High-Redshift Galaxies
Mr.  Kenneth Wong (University of Arizona)
We investigate whether lines of sight containing multiple cluster-scale halos are the best cosmic telescopes for lensing high-redshift (z~10) sources into detectability. We find that multiple projected halos are can result in improvement in the detection of faint, high-z sources compared to single halos of equivalent total mass due to the interactions among the lensing potentials when the projected halos overlap. Using integrated LRG luminosity density as a tracer of mass, we have identified the 200 lines of sight in the SDSS that are likely to contain the largest total integrated masses. These fields contain a diversity of single massive clusters and chance alignments of multiple halos in projection, and are likely to be among the best gravitational lenses known. Our ongoing galaxy spectroscopy in the first two of these fields reveals the presence of large total masses (> 3 x 10^15 M_sun) along the line of sight and multiple cluster-scale structures. Archival Subaru imaging in these fields reveals multiple lensed arcs, confirming the lensing strength of these mass distributions.
Cluster Lensing As The Cosmic Telescope to Resolve The Chemical Evolution of Galaxies
Dr.  Tiantian Yuan (RSAA Australian National University)
Taking advantages of the multiple magnified sources in strong lensing clusters, we have started in 2009 the Lensed Emission-Line Galaxy Metallicity Survey (LEGMS). Our first results of ~ 20 lensed galaxies have shown the powerful ability of lensing to probe the lower mass end of high-z galaxies, as well as to spatially resolve the mass components (bulges, HII regions) of galaxies. This poster shows some of our most recent results and how our results on chemical evolution of galaxies depends on the accuracy of the total magnification of each galaxy. In the spatially resolve cases, the problem of robustly co-adding the IFU data of different multiple images of the same galaxy poses a great challenge to our current method of source-plane reconstruction.
Star formation efficiencies and extinction properties of strongly-lensed L* galaxies at z~2
Dr.  Michel Zamojski (Geneva Observatory)
We present an analysis of a sample of z~1.5-3 strongly lensed star-forming galaxies observed from the optical to the FIR/sub-mm and in CO, covering their stellar, dust and molecular components. The observations were carried out with HST, Spitzer, Herschel, and the IRAM/PdBI and allow us to detect individual galaxies in the IR, below the usual depths of deep blank field studies, well into the LIRG regime and even below for some objects, thus probing an IR-luminosity range representative of the general population of star-forming galaxies at those redshifts. We present empirical results about the IR and UV properties, dust extinction, gas mass, star formation efficiencies and star formation law, and compare this to both local and more luminous high-z galaxies.