Group Meeting Papers

http://arxiv.org/abs/1111.4434

Authors: H. Hildebrandt, T. Erben, K. Kuijken, L. van Waerbeke, C. Heymans, J. Coupon, J. Benjamin, C. Bonnett, L. Fu, H. Hoekstra, T. D. Kitching, Y. Mellier, L. Miller, M. Velander, M. J. Hudson, B. T. P. Rowe, T. Schrabback, E. Semboloni, N. Benitez

Abstract: Here we present the results of various approaches to measure accurate colours and photometric redshifts (photo-zs) from wide-field imaging data. We use data from the Canada-France-Hawaii-Telescope Legacy Survey (CFHTLS) which have been reprocessed by the CFHT Lensing Survey (CFHTLenS) team in order to carry out a number of weak gravitational lensing studies. An emphasis is put on the correction of systematic effects in the photo-zs arising from the different Point Spread Functions (PSF) in the five optical bands. Different ways of correcting these effects are discussed and the resulting photo-z accuracies are quantified by comparing the photo-zs to large spectroscopic redshift (spec-z) data sets. Careful homogenisation of the PSF between bands leads to increased overall accuracy of photo-zs. The gain is particularly pronounced at fainter magnitudes where galaxies are smaller and flux measurements are affected more by PSF-effects. We discuss ways of defining more secure subsamples of galaxies as well as a shape- and colour-based star-galaxy separation method, and we present redshift distributions for different magnitude limits. We also study possible re-calibrations of the photometric zeropoints (ZPs) with the help of galaxies with known spec-zs. We find that if PSF-effects are properly taken into account, a re-calibration of the ZPs becomes much less important suggesting that previous such re-calibrations described in the literature could in fact be mostly corrections for PSF-effects rather than corrections for real inaccuracies in the ZPs. The implications of this finding for future surveys like KiDS, DES, LSST, or Euclid are mixed. On the one hand, ZP re-calibrations with spec-zs might not be as accurate as previously thought. On the other hand, careful PSF homogenisation might provide a way out and yield accurate, homogeneous photometry without the need for full spectroscopic coverage. This is the first paper in a series describing the technical aspects of CFHTLenS.

Authors: Alessandro Sonnenfeld, Tommaso Treu, Raphael Gavazzi, Philip J. Marshall, Matthew W. Auger, Sherry H. Suyu, Leon V. E. Koopmans, Adam S. Bolton

Abstract: Stars and dark matter account for most of the mass of early-type galaxies, but uncertainties in the stellar population and the dark matter profile make it challenging to distinguish between the two components. Nevertheless, precise observations of stellar and dark matter are extremely valuable for testing the many models of structure formation and evolution. We present a measurement of the stellar mass and inner slope of the dark matter halo of a massive early-type galaxy at $z=0.222$. The galaxy is the foreground deflector of the double Einstein ring gravitational lens system SDSSJ0946+1006, also known as the "Jackpot". By combining the tools of lensing and dynamics, we first constrain the mean slope of the total mass density profile ($\rho_{\rm{tot}}\propto r^{-\gamma'}$) within the radius of the outer ring to be $\gamma' = 1.98\pm0.02\pm0.01$. Then we obtain a bulge-halo decomposition, assuming a power-law form for the dark matter halo. Our analysis yields $\gamma_{\rm{DM}} = 1.7\pm0.2$ for the inner slope of the dark matter profile, in agreement with theoretical findings on the distribution of dark matter in ellipticals, and a stellar mass from lensing and dynamics $M_*^{\rm{LD}} = 5.5_{-1.3}^{+0.4}\times10^{11}M_\Sun$. By comparing this measurement with stellar masses inferred from stellar population synthesis fitting we find that a Salpeter IMF provides a good description of the stellar population of the lens while a Chabrier IMF is ruled out at the 95% confidence level. Our data suggest that growth by accretion of small systems from a compact red nugget is a plausible formation scenario for this object.

Comments: By combining lensing and dynamics and redshift information of this 2 Einstein ring system the authors measure the total matter slope of this galaxy. Comparing with SPS models, they also exclude a Chabrier IMF on a 95% level basis.

http://arxiv.org/abs/1110.2854

Authors: M. Sereno (POLITO), A. Zitrin (TAU)

Abstract: 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.

Comments: When accounting for halo asphericity and orientation with a Bayesian analysis, the tension between 3d mass-concentration-relations from clusters and lensing measurements of 2d projected concentrations is resolved in this cluster sample.

http://arxiv.org/abs/1111.1070

Abstract: We describe a new method for measuring galaxy magnification due to weak gravitational lensing. Our method makes use of a tight scaling relation between galaxy properties that are modified by gravitational lensing, such as apparent size, and other properties that are not, such as surface brightness. In particular, we use a version of the well-known fundamental plane relation for early type
galaxies. This modified "photometric fundamental plane" replaces velocity dispersions with photometric galaxy properties, thus obviating the need for spectroscopic data. We present the first detection of magnification using this method by applying it to photometric catalogs from the Sloan Digital Sky Survey. This analysis shows that the derived magnification signal is comparable to that available from conventional methods using gravitational shear. We suppress the dominant sources of systematic error and discuss modest improvements that may allow this method to equal or even surpass the signal-to-noise achievable with shear. Moreover, some of the dominant sources of systematic error are substantially different from those of shear-based techniques. Thus, combining the two techniques addresses the major weaknesses of each and provides a substantial improvement over either method used in isolation. With this new technique, magnification becomes a necessary measurement tool for the coming era of large ground-based surveys intending to measure gravitational lensing.

Comment: By using tight correlations between properties that change and properties that do not change through lensing, they can suppress the intrinsic noise of magnification measurements, which is pretty neat.