Strong Lensing Analysis of the Powerful Lensing Cluster MACS J2135.2-0102 (zz=0.33)

We present a light-traces-mass (LTM) strong-lensing model of the massive lensing cluster MACS J2135.2-0102 ($z$=0.33; hereafter MACS2135), known in part for hosting the Cosmic Eye galaxy lens. MACS2135 is also known to multiply-lens a $z=$2.3 sub-mm galaxy near the Brightest Cluster Galaxy (BCG), as well as a prominent, triply-imaged system at a large radius of $\sim$37" south of the BCG. We use the latest available Hubble imaging to construct an accurate lensing model for this cluster, identifying six new multiply-imaged systems with the guidance of our LTM method, so that we have roughly quadrupled the number of lensing constraints. We determine that MACS2135 is amongst the top lensing clusters known, comparable in size to the Hubble Frontier Fields. For a source at $z_{s}=2.32$, we find an effective Einstein radius of $\theta_{e}=27\pm3$", enclosing $1.12 \pm0.16 \times10^{14}$ $M_{\odot}$. We make our lens model, including mass and magnification maps, publicly available, in anticipation of searches for high-$z$ galaxies with the James Webb Space Telescope for which this cluster is a compelling target.Comment: 7 pages, 2 figures (3 subfigures in total), 1 table; Published in ApJ; V2: accepted versio

Frontier Fields: High-Redshift Predictions and Early Results

The Frontier Fields program is obtaining deep Hubble and Spitzer Space Telescope images of new "blank" fields and nearby fields gravitationally lensed by massive galaxy clusters. The Hubble images of the lensed fields are revealing nJy sources (AB mag > 31), the faintest galaxies yet observed. In this paper, we present high-redshift (z > 6) number count predictions for the full program and candidates in three of the first Hubble Frontier Fields images. The full program will transform our understanding of galaxy evolution in the first 600 million years (z > 9). Where previous programs yielded perhaps a dozen z > 9 candidates, the Frontier Fields may yield ~70 (~6 per field). We base this estimate on an extrapolation of luminosity functions observed between 4 < z < 8 and gravitational lensing models submitted by the community. However, in the first two deep infrared Hubble images obtained to date, we find z ~ 8 candidates but no strong candidates at z > 9. This might suggest a deficit of faint z > 9 galaxies as also reported in the Ultra Deep Field (even while excesses of brighter z > 9 galaxies were reported in shallower fields). At these redshifts, cosmic variance (field-to-field variation) is expected to be significant (greater than +/-50%) and include clustering of early galaxies formed in overdensities. The full Frontier Fields program will significantly mitigate this uncertainty by observing six independent sightlines each with a lensing cluster and nearby blank field.Comment: Submitted for publication in the Astrophysical Journal. 15 pages, 17 figure

Miscentring in Galaxy Clusters: Dark Matter to Brightest Cluster Galaxy Offsets in 10,000 SDSS Clusters

We characterise the typical offset between the Dark Matter (DM) projected centre and the Brightest Cluster Galaxy (BCG) in 10,000 SDSS clusters. To place constraints on the centre of DM, we use an automated strong-lensing 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 2D 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, \Delta, 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^{+0.003}_{-0.004}, and \sigma=0.501\pm0.004 (95% confidence levels), or =0.564\pm0.005, and \sigma=0.475\pm0.007. 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 ~10% 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 help characterise the merger, relaxation and evolution history of clusters, in future studies. [ABRIDGED]Comment: 15 pages, 11 figures; MNRAS in press; V3 includes minor text update

Triaxial strong-lensing analysis of the z > 0.5 MACS clusters: the mass-concentration relation

The high concentrations derived for several strong-lensing clusters present a major inconsistency between theoretical LambdaCDM expectations and measurements. Triaxiality and orientation biases might be at the origin of this disagreement, as clusters elongated along the line-of-sight would have a relatively higher projected mass density, boosting the resulting lensing properties. Analyses of statistical samples can probe further these effects and crucially reduce biases. In this work we perform a fully triaxial strong-lensing analysis of the 12 MACS clusters at z > 0.5, a complete X-ray selected sample, and fully account for the impact of the intrinsic 3D shapes on their strong lensing properties. We first construct strong-lensing mass models for each cluster based on multiple-images, and fit projected ellipsoidal Navarro-Frenk-White halos with arbitrary orientations to each mass distribution. We then invert the measured surface mass densities using Bayesian statistics. Although the Einstein radii of this sample are significantly larger than predicted by LambdaCDM, here we find that the mass-concentration relation is in full agreement with results from N-body simulations. The z > 0.5 MACS clusters suffer from a moderate form of orientation bias as may be expected for X-ray selected samples. Being mostly unrelaxed, at a relatively high redshift, with high X-ray luminosity and noticeable substructures, these clusters may lie outside the standard concentration-Einstein radius relation. Our results remark the importance of triaxiality and properly selected samples for understanding galaxy clusters properties, and suggest that higher-z, unrelaxed low-concentration clusters form a different class of prominent strong gravitational lenses. Arc redshift confirmation and weak lensing data in the outer region are needed to further refine our analysis.Comment: 12 pages, 12 figures; in press on MNRA