Publications


CFHTLenS: Weak lensing constraints on the ellipticity of galaxy-scale matter haloes and the galaxy-halo misalignment

Monthly Notices of the Royal Astronomical Society 454 (2015) 1432-1452

T Schrabback, S Hilbert, H Hoekstra, P Simon, E van Uitert, T Erben, C Heymans, H Hildebrandt, TD Kitching, Y Mellier, L Miller, L Van Waerbeke, P Bett, J Coupon, L Fu, MJ Hudson, B Joachimi, M Kilbinger, K Kuijken

© 2015 The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society. We present weak lensing constraints on the ellipticity of galaxy-scale matter haloes and the galaxy-halo misalignment. Using data from the Canada-France-Hawaii Telescope Lensing Survey (CFHTLenS), we measure the weighted-average ratio of the aligned projected ellipticity components of galaxy matter haloes and their embedded galaxies, fh, split by galaxy type. We then compare our observations to measurements taken from the Millennium Simulation, assuming different models of galaxy-halo misalignment. Using the Millennium Simulation, we verify that the statistical estimator used removes contamination from cosmic shear. We also detect an additional signal in the simulation, which we interpret as the impact of intrinsic shape-shear alignments between the lenses and their large-scale structure environment. These alignments are likely to have caused some of the previous observational constraints on fh to be biased high. From CFHTLenS, we find fh = -0.04 ± 0.25 for early-type galaxies, which is consistent with current models for the galaxy-halo misalignment predicting fh ≃ 0.20. For late-type galaxies we measure fh = 0.69-0.36+0.37 from CFHTLenS. This can be compared to the simulated results which yield fh ≃ 0.02 for misaligned late-type models.


Corrigendum to "spectroscopic needs for imaging dark energy experiments" [Astropart. Phys. 63 (2015) 81-100]

Astroparticle Physics 65 (2015) 112-113

JA Newman, A Abate, FB Abdalla, S Allam, SW Allen, R Ansari, S Bailey, WA Barkhouse, TC Beers, MR Blanton, M Brodwin, JR Brownstein, RJ Brunner, MC Kind, JL Cervantes-Cota, E Cheu, NE Chisari, M Colless, J Comparat, J Coupon, CE Cunha, A De La Macorra, IP Dell'Antonio, BL Frye, EJ Gawiser, N Gehrels, K Grady, A Hagen, PB Hall, AP Hearin, H Hildebrandt, CM Hirata, S Ho, K Honscheid, D Huterer, Ž Ivezić, JP Kneib, JW Kruk, O Lahav, R Mandelbaum, JL Marshall, DJ Matthews, B Ménard, R Miquel, M Moniez, HW Moos, J Moustakas, AD Myers, C Papovich, JA Peacock, C Park, M Rahman, J Rhodes, JS Ricol, I Sadeh, A Slozar, SJ Schmidt, DK Stern, JA Tyson, A Von Der Linden, RH Wechsler, WM Wood-Vasey, AR Zentner


Contamination of early-type galaxy alignments to galaxy lensing-CMB lensing cross-correlation

Monthly Notices of the Royal Astronomical Society 453 (2015) 682-689

NE Chisari, J Dunkley, L Miller, R Allison

© 2015 The Authors. Galaxy shapes are subject to distortions due to the tidal field of the Universe. The crosscorrelation of galaxy lensing with the lensing of the cosmic microwave background (CMB) cannot easily be separated from the cross-correlation of galaxy intrinsic shapes with CMB lensing. Previous work suggested that the intrinsic alignment contamination can be 15 per cent of this cross-spectrum for the CFHT Stripe 82 (CS82) and Atacama Cosmology Telescope surveys. Here we re-examine these estimates using up-to-date observational constraints of intrinsic alignments at a redshift more similar to that of CS82 galaxies. We find an ≈ 10 per cent contamination of the cross-spectrum from red galaxies, with ≈ 3 per cent uncertainty due to uncertainties in the redshift distribution of source galaxies and the modelling of the spectral energy distribution. Blue galaxies are consistent with being unaligned, but could contaminate the cross-spectrum by an additional 9.5 per cent within current 95 per cent confidence levels. While our fiducial estimate of alignment contamination is similar to previous work, our work suggests that the relevance of alignments for CMB lensing-galaxy lensing cross-correlation remains largely unconstrained. Little information is currently available about alignments at z > 1.2. We consider the upper limiting case where all z > 1.2 galaxies are aligned with the same strength as low-redshift luminous red galaxies, finding as much as ≈ 60 per cent contamination.


Gravitational lensing analysis of the Kilo-Degree Survey

Monthly Notices of the Royal Astronomical Society 454 (2015) 3500-3532

K Kuijken, C Heymans, H Hildebrandt, R Nakajima, T Erben, JTA De Jong, M Viola, A Choi, H Hoekstra, L Miller, E Van Uitert, A Amon, C Blake, M Brouwer, A Buddendiek, IF Conti, M Eriksen, A Grado, J Harnois-Déraps, E Helmich, R Herbonnet, N Irisarri, T Kitching, D Klaes, F La Barbera, N Napolitano, M Radovich, P Schneider, C Sifón, G Sikkema, P Simon, A Tudorica, E Valentijn, GV Kleijn, L Van Waerbeke

© 2015 The Authors. Published by Oxford University Press on behalf of the Royal Astronomical Society. The Kilo-Degree Survey (KiDS) is a multi-band imaging survey designed for cosmological studies from weak lensing and photometric redshifts. It uses the European Southern Observatory VLT Survey Telescope with its wide-field camera OmegaCAM. KiDS images are taken in four filters similar to the Sloan Digital Sky Survey ugri bands. The best seeing time is reserved for deep r-band observations. The median 5σ limiting AB magnitude is 24.9 and the median seeing is below 0.7 arcsec. Initial KiDS observations have concentrated on the Galaxy and Mass Assembly (GAMA) regions near the celestial equator, where extensive, highly complete redshift catalogues are available. A total of 109 survey tiles, 1 square degree each, form the basis of the first set of lensing analyses of halo properties of GAMA galaxies. Nine galaxies per square arcminute enter the lensing analysis, for an effective inverse shear variance of 69 arcmin-2. Accounting for the shape measurement weight, the median redshift of the sources is 0.53. KiDS data processing follows two parallel tracks, one optimized for weak lensing measurement and one for accurate matched-aperture photometry (for photometric redshifts). This technical paper describes the lensing and photometric redshift measurements (including a detailed description of the Gaussian aperture and photometry pipeline), summarizes the data quality and presents extensive tests for systematic errors that might affect the lensing analyses. We also provide first demonstrations of the suitability of the data for cosmological measurements, and describe our blinding procedure for preventing confirmation bias in the scientific analyses. The KiDS catalogues presented in this paper are released to the community through http://kids.strw.leidenuniv.nl.


Intrinsic alignments of group and cluster galaxies in photometric surveys

Monthly Notices of the Royal Astronomical Society 445 (2014) 726-748

NE Chisari, R Mandelbaum, MA Strauss, EM Huff, NA Bahcall

© 2014 The Authors. Intrinsic alignments of galaxies have been shown to contaminate weak gravitational lensing observables on linear scales, r < 10 h-1 Mpc, but studies of alignments in the non-linear regime have thus far been inconclusive. We present an estimator for extracting the intrinsic alignment signal of galaxies around stacked clusters of galaxies from multiband imaging data. Our estimator removes the contamination caused by galaxies that are gravitationally lensed by the clusters and scattered in redshift space due to photometric redshift uncertainties. It uses posterior probability distributions for the redshifts of the galaxies in the sample and it is easily extended to obtain the weak gravitational lensing signal while removing the intrinsic alignment contamination.We apply this algorithm to groups and clusters of galaxies identified in the Sloan Digital Sky Survey 'Stripe 82' co-added imaging data over~150 deg2.We find that the intrinsic alignment signal around stacked clusters in the redshift range 0.1 > z > 0.4 is consistent with zero. In terms of the tidal alignment model of Catelan et al., we set joint constraints on the strength of the alignment and the bias of the lensing groups and clusters on scales between 0.1 and 10h-1 Mpc, bLC1pcrit = -2+14-14 × 10-4. This constrains the contamination fraction of alignment to lensing signal to the range between [ - 18, 23] per cent below scales of 1 h-1 Mpc at 95 per cent confidence level, and this result depends on our photometric redshift quality and selection criteria used to identify background galaxies. Our results are robust to the choice of photometric band in which the shapes are measured (i and r) and to centring on the Brightest Cluster Galaxy or on the geometrical centre of the clusters.


Can weak lensing surveys confirm BICEP2?

Physical Review D - Particles, Fields, Gravitation and Cosmology 90 (2014)

NE Chisari, C Dvorkin, F Schmidt

The detection of B-modes in the cosmic microwave background (CMB) polarization by the BICEP2 experiment, if interpreted as evidence for a primordial gravitational wave background, has enormous ramifications for cosmology and physics. It is crucial to test this hypothesis with independent measurements. A gravitational wave background leads to B-modes in galaxy shape correlations (shear) both through lensing and tidal alignment effects. Since the systematics and foregrounds of galaxy shapes and CMB polarization are entirely different, a detection of a cross correlation between the two observables would provide conclusive proof for the existence of a primordial gravitational wave background. We find that upcoming weak lensing surveys will be able to detect the cross correlation between B-modes of the CMB and galaxy shapes. However, this detection is not sufficient to confirm or falsify the hypothesis of a primordial origin for CMB B-mode polarization. © 2014 American Physical Society.


CFHTLenS: Cosmological constraints from a combination of cosmic shear two-point and three-point correlations

Monthly Notices of the Royal Astronomical Society 441 (2014) 2725-2743

L Fu, M Kilbinger, T Erben, C Heymans, H Hildebrandt, H Hoekstra, TD Kitching, Y Mellier, L Miller, E Semboloni, P Simon, L Van Waerbeke, J Coupon, J Harnois-Déraps, MJ Hudson, K Kuijken, B Rowe, T Schrabback, S Vafaei, M Velander

Higher order, non-Gaussian aspects of the large-scale structure carry valuable information on structure formation and cosmology, which is complementary to second-order statistics. In this work, we measure second- and third-order weak-lensing aperture-mass moments from the Canada-France-Hawaii Lensing Survey (CFHTLenS) and combine those with cosmic microwave background (CMB) anisotropy probes. The third moment is measured with a significance of 2σ. The combined constraint on Σ8 = σ8(Ωm/0.27)α is improved by 10 per cent, in comparison to the second-order only, and the allowed ranges for Ωm and σ8 are substantially reduced. Including general triangles of the lensing bispectrum yields tighter constraints compared to probing mainly equilateral triangles. Second- and third-order CFHTLenS lensing measurements improve Planck CMB constraints on Ωm and σ8 by 26 per cent for flat Λ cold dark matter. For a model with free curvature, the joint CFHTLenS-Planck result is Ωm = 0.28 ± 0.02 (68 per cent confidence), which is an improvement of 43 per cent compared to Planck alone. We test how our results are potentially subject to three astrophysical sources of contamination: source-lens clustering, the intrinsic alignment of galaxy shapes, and baryonic effects. We explore future limitations of the cosmological use of third-order weak lensing, such as the non-linear model and the Gaussianity of the likelihood function. © 2014 The Authors. Published by Oxford University Press on behalf of the Royal Astronomical Society.


Spectroscopic needs for imaging dark energy experiments

Astroparticle Physics (2014)

JA Newman, A Abate, FB Abdalla, SS Allam, SW Allen, R Ansari, SJ Bailey, WA Barkhouse, TC Beers, MR Blanton, M Brodwin, JR Brownstein, RJ Brunner, M Carrasco Kind, JL Cervantes-Cota, E Cheu, NE Chisari, M Colless, J Comparat, J Coupon, CE Cunha, A De La Macorra, IP Dell'Antonio, BL Frye, EJ Gawiser, NA Gehrels, KJ Grady, A Hagen, PB Hall, AP Hearin, HM Hildebrandt, C Hirata, S Ho, K Honscheiď, D Huterer, ZE Ivezić, JPJP Kneib, JW Kruk, O Lahav, R Mandelbaum, JL Marshall, DJ Matthews, BJ Ménard, R Miquel, M Moniez, HW Moos, J Moustakas, AD Myers, CJ Papovich, JA Peacock, C Park, MA Rahman, JD Rhodes, JS Ricol, IH Sadeh, A Slozar, SJ Schmidt, DK Stern, J Anthony Tyson, A Von Der Linden, RH Wechsler, WM Wood-Vasey, AR Zentner

Ongoing and near-future imaging-based dark energy experiments are critically dependent upon photometric redshifts (a.k.a. photo-z's): i.e., estimates of the redshifts of objects based only on flux information obtained through broad filters. Higher-quality, lower-scatter photo-z's will result in smaller random errors on cosmological parameters; while systematic errors in photometric redshift estimates, if not constrained, may dominate all other uncertainties from these experiments. The desired optimization and calibration is dependent upon spectroscopic measurements for secure redshift information; this is the key application of galaxy spectroscopy for imaging-based dark energy experiments. Hence, to achieve their full potential, imaging-based experiments will require large sets of objects with spectroscopically-determined redshifts, for two purposes: • Training: Objects with known redshift are needed to map out the relationship between object color and z (or, equivalently, to determine empirically-calibrated templates describing the rest-frame spectra of the full range of galaxies, which may be used to predict the color-z relation). The ultimate goal of training is to minimize each moment of the distribution of differences between photometric redshift estimates and the true redshifts of objects, making the relationship between them as tight as possible. The larger and more complete our "training set" of spectroscopic redshifts is, the smaller the RMS photo-z errors should be, increasing the constraining power of imaging experiments. Requirements: Spectroscopic redshift measurements for ∼30,000 objects over >∼15 widely-separated regions, each at least ∼20 arcmin in diameter, and reaching the faintest objects used in a given experiment, will likely be necessary if photometric redshifts are to be trained and calibrated with conventional techniques. Larger, more complete samples (i.e., with longer exposure times) can improve photo-z algorithms and reduce scatter further, enhancing the science return from planned experiments greatly (increasing the Dark Energy Task Force figure of merit by up to ∼50%). Options: This spectroscopy will most efficiently be done by covering as much of the optical and near-infrared spectrum as possible at modestly high spectral resolution (λ/Δλ > ∼3000), while maximizing the telescope collecting area, field of view on the sky, and multiplexing of simultaneous spectra. The most efficient instrument for this would likely be either the proposed GMACS/MANIFEST spectrograph for the Giant Magellan Telescope or the OPTIMOS spectrograph for the European Extremely Large Telescope, depending on actual properties when built. The PFS spectrograph at Subaru would be next best and available considerably earlier, c. 2018; the proposed ngCFHT and SSST telescopes would have similar capabilities but start later. Other key options, in order of increasing total time required, are the WFOS spectrograph at TMT, MOONS at the VLT, and DESI at the Mayall 4 m telescope (or the similar 4MOST and WEAVE projects); of these, only DESI, MOONS, and PFS are expected to be available before 2020. Table 2-3 of this white paper summarizes the observation time required at each facility for strawman training samples. To attain secure redshift measurements for a high fraction of targeted objects and cover the full redshift span of future experiments, additional near-infrared spectroscopy will also be required; this is best done from space, particularly with WFIRST-2.4 and JWST. Calibration: The first several moments of redshift distributions (the mean, RMS redshift dispersion, etc.), must be known to high accuracy for cosmological constraints not to be systematics-dominated (equivalently, the moments of the distribution of differences between photometric and true redshifts could be determined instead). The ultimate goal of calibration is to characterize these moments for every subsample used in analyses - i.e., to minimize the uncertainty in their mean redshift, RMS dispersion, etc. - rather than to make the moments themselves small. Calibration may be done with the same spectroscopic dataset used for training if that dataset is extremely high in redshift completeness (i.e., no populations of galaxies to be used in analyses are systematically missed). Accurate photo-z calibration is necessary for all imaging experiments. Requirements: If extremely low levels of systematic incompleteness (<∼0.1%) are attained in training samples, the same datasets described above should be sufficient for calibration. However, existing deep spectroscopic surveys have failed to yield secure redshifts for 30-60% of targets, so that would require very large improvements over past experience. This incompleteness would be a limiting factor for training, but catastrophic for calibration. If <∼0.1% incompleteness is not attainable, the best known option for calibration of photometric redshifts is to utilize cross-correlation statistics in some form. The most direct method for this uses cross-correlations between positions on the sky of bright objects of known spectroscopic redshift with the sample of objects that we wish to calibrate the redshift distribution for, measured as a function of spectroscopic z. For such a calibration, redshifts of ∼100,000 objects over at least several hundred square degrees, spanning the full redshift range of the samples used for dark energy, would be necessary. Options: The proposed BAO experiment eBOSS would provide sufficient spectroscopy for basic calibrations, particularly for ongoing and near-future imaging experiments. The planned DESI experiment would provide excellent calibration with redundant cross-checks, but will start after the conclusion of some imaging projects. An extension of DESI to the Southern hemisphere would provide the best possible calibration from cross-correlation methods for DES and LSST. We thus anticipate that our two primary needs for spectroscopy - training and calibration of photometric redshifts - will require two separate solutions. For ongoing and future projects to reach their full potential, new spectroscopic samples of faint objects will be needed for training; those new samples may be suitable for calibration, but the latter possibility is uncertain. In contrast, wide-area samples of bright objects are poorly suited for training, but can provide high-precision calibrations via cross-correlation techniques. Additional training/calibration redshifts and/or host galaxy spectroscopy would enhance the use of supernovae and galaxy clusters for cosmology. We also summarize additional work on photometric redshift techniques that will be needed to prepare for data from ongoing and future dark energy experiments. © 2014 Elsevier B.V. All rights reserved.


Updates from Astrobites: The Astro-ph Reader's Digest

(2014)

B Montet, N Chisari, J Donaldson, C Dressing, M Drout, C Faesi, J Fuchs, S Kohler, E Lovegrove, E Mills, E Nesvold, E Newton, A Olmstead, J Vasel, L Weiss, Astrobites Team

Astrobites (http://astrobites.com) is a daily blog aimed at undergraduates interested in astrophysical research and written by a team of graduate students located at diverse institutions across the United States. Primarily, we present journal articles recently posted to astro-ph in a brief format that is accessible to anyone with a general background in the physical sciences, including readers who are not yet familiar with the astrophysical literature. Special posts offer career guidance for undergraduates (e.g. applying for an NSF graduate fellowship) and describe personal experiences (e.g. attending an astronomy summer school). We present recent readership statistics and potential methods for incorporating Astrobites into the classroom. We also discuss the Astrobites format across multiple social media platforms, including the newly launched Astroplots, and highlight our recent work organizing the annual "Communicating Science" workshop for graduate students.


Weak gravitational lensing with the square kilometre array

Proceedings of Science 2014-January (2014)

ML Brown, DJ Bacon, S Camera, I Harrison, B Joachimi, RB Metcalf, A Pourtsidou, K Takahashi, JA Zuntz, FB Abdalla, S Bridle, M Jarvis, TD Kitching, L Miller, P Patel

© Copyright owned by the author(s). We investigate the capabilities of various stages of the SKA to perform world-leading weak gravitational lensing surveys. We outline a way forward to develop the tools needed for pursuing weak lensing in the radio band. We identify the key analysis challenges and the key pathfinder experiments that will allow us to address them in the run up to the SKA. We identify and summarize the unique and potentially very powerful aspects of radio weak lensing surveys, facilitated by the SKA, that can solve major challenges in the field of weak lensing. These include the use of polarization and rotational velocity information to control intrinsic alignments, and the new area of weak lensing using intensity mapping experiments. We show how the SKA lensing surveys will both complement and enhance corresponding efforts in the optical wavebands through cross-correlation techniques and by way of extending the reach of weak lensing to high redshift.


3D cosmic shear: Cosmology from CFHTLenS

Monthly Notices of the Royal Astronomical Society 442 (2014) 1326-1349

TD Kitching, AF Heavens, J Alsing, T Erben, C Heymans, H Hildebrandt, H Hoekstra, A Jaffe, A Kiessling, Y Mellier, L Miller, L van Waerbeke, J Benjamin, J Coupon, L Fu, MJ Hudson, M Kilbinger, K Kuijken, BTP Rowe, T Schrabback, E Semboloni, M Velander

This paper presents the first application of 3D cosmic shear to a wide-field weak lensing survey. 3D cosmic shear is a technique that analyses weak lensing in three dimensions using a spherical harmonic approach, and does not bin data in the redshift direction. This is applied to CFHTLenS, a 154 square degree imaging survey with a median redshift of 0.7 and an effective number density of 11 galaxies per square arcminute usable for weak lensing. To account for survey masks we apply a 3D pseudo-C ℓ approach on weak lensing data, and to avoid uncertainties in the highly non-linear regime, we separately analyse radial wavenumbers k ≤ 1.5 and 5.0 h Mpc -1 , and angular wavenumbers ℓ ≈ 400-5000. We show how one can recover 2D and tomographic power spectra from the full 3D cosmic shear power spectra and present a measurement of the 2D cosmic shear power spectrum, and measurements of a set of 2-bin and 6-bin cosmic shear tomographic power spectra; in doing so we find that using the 3D power in the calculation of such 2D and tomographic power spectra from data naturally accounts for a minimum scale in the matter power spectrum. We use 3D cosmic shear to constrain cosmologies with parametersω M ,ω B , σ 8 , h, n s , w 0 and w a . For a non-evolving dark energy equation of state, and assuming a flat cosmology, lensing combined with Wilkinson Microwave Anisotropy Probe 7 results in h= 0.78 ± 0.12, ΩM = 0.252 ± 0.079, σ 8 = 0.88 ± 0.23 and w=-1.16 ± 0.38 using only scales k ≤ 1.5 h Mpc -1 . We also present results of lensing combined with first year Planck results, where we find no tension with the results from this analysis, but we also find no significant improvement over the Planck results alone. We find evidence of a suppression of power compared to Lambda cold dark matter (LCDM) on small scales 1.5 < k ≤ 5.0 h Mpc -1 in the lensing data, which is consistent with predictions of the effect of baryonic feedback on the matter power spectrum. © 2014 The Authors. Published by Oxford University Press on behalf of the Royal Astronomical Society.


An improved model of charge transfer inefficiency and correction algorithm for the Hubble Space Telescope

Monthly Notices of the Royal Astronomical Society 439 (2014) 887-907

R Massey, T Schrabback, O Cordes, O Marggraf, H Israel, L Miller, D Hall, M Cropper, T Prod'homme, SM Niemi

Charge-coupled device (CCD) detectors, widely used to obtain digital imaging, can be damaged by high energy radiation. Degraded images appear blurred, because of an effect known as Charge Transfer Inefficiency (CTI), which trails bright objects as the image is read out. It is often possible to correct most of the trailing during post-processing, by moving flux back to where it belongs. We compare several popular algorithms for this: quantifying the effect of their physical assumptions and tradeoffs between speed and accuracy. We combine their best elements to construct a more accurate model of damaged CCDs in the Hubble Space Telescope's Advanced Camera for Surveys/Wide Field Channel, and update it using data up to early 2013. Our algorithm now corrects 98 per cent of CTI trailing in science exposures, a substantial improvement over previous work. Further progress will be fundamentally limited by the presence of read noise. Read noise is added after charge transfer so does not get trailed-but it is incorrectly untrailed during post-processing. © 2014 The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society.


The dark matter filament between Abell 222/223

Proceedings of the International Astronomical Union 11 (2014) 193-198

JP Dietrich, N Werner, D Clowe, A Finoguenov, T Kitching, L Miller, A Simionescu

© International Astronomical Union 2016. Weak lensing detections and measurements of filaments have been elusive for a long time. The reason is that the low density contrast of filaments generally pushes the weak lensing signal to unobservably low scales. To nevertheless map the dark matter in filaments exquisite data and unusual systems are necessary. SuprimeCam observations of the supercluster system Abell 222/223 provided the required combination of excellent seeing images and a fortuitous alignment of the filament with the line-of-sight. This boosted the lensing signal to a detectable level and led to the first weak lensing mass measurement of a large-scale structure filament. The filament connecting Abell 222 and Abell 223 is now the only one traced by the galaxy distribution, dark matter, and X-ray emission from the hottest phase of the warm-hot intergalactic medium. The combination of these data allows us to put the first constraints on the hot gas fraction in filaments.


Weak gravitational lensing with the Square Kilometre Array

Proceedings of Science 9-13-June-2014 (2014)

ML Brown, DJ Bacon, S Camera, I Harrison, B Joachimi, RB Metcalf, A Pourtsidou, K Takahashi, JA Zuntz, FB Abdalla, S Bridle, M Jarvis, TD Kitching, L Miller, P Patel

© Copyright owned by the author(s) under the terms of the Creative Commons Attribution-NonCommercial-ShareAlike Licence. We investigate the capabilities of various stages of the SKA to perform world-leading weak gravitational lensing surveys. We outline a way forward to develop the tools needed for pursuing weak lensing in the radio band. We identify the key analysis challenges and the key pathfinder experiments that will allow us to address them in the run up to the SKA. We identify and summarize the unique and potentially very powerful aspects of radio weak lensing surveys, facilitated by the SKA, that can solve major challenges in the field of weak lensing. These include the use of polarization and rotational velocity information to control intrinsic alignments, and the new area of weak lensing using intensity mapping experiments. We show how the SKA lensing surveys will both complement and enhance corresponding efforts in the optical wavebands through cross-correlation techniques and by way of extending the reach of weak lensing to high redshift.


CFHTLenS: The relation between galaxy dark matter haloes and baryons from weak gravitational lensing

Monthly Notices of the Royal Astronomical Society 437 (2014) 2111-2136

M Velander, E Van Uitert, H Hoekstra, J Coupon, T Erben, C Heymans, H Hildebrandt, TD Kitching, Y Mellier, L Miller, L Van Waerbeke, C Bonnett, L Fu, S Giodini, MJ Hudson, K Kuijken, B Rowe, T Schrabback, E Semboloni

We present a study of the relation between dark matter halo mass and the baryonic content of their host galaxies, quantified through galaxy luminosity and stellar mass. Our investigation uses 154 deg2 of Canada-France-Hawaii Telescope Lensing Survey (CFHTLenS) lensing and photometric data, obtained from the CFHT Legacy Survey. To interpret the weak lensing signal around our galaxies, we employ a galaxy-galaxy lensing halo model which allows us to constrain the halo mass and the satellite fraction. Our analysis is limited to lenses at redshifts between 0.2 and 0.4, split into a red and a blue sample. We express the relationship between dark matter halo mass and baryonic observable as a power lawwith pivot points of 1011 h -270 L and 2 × 1011 h -270 M for luminosity and stellar mass, respectively. For the luminosity-halo mass relation, we find a slope of 1.32 ± 0.06 and a normalization of 1.19+0.06 -0.07 × 1013 h -170 M for red galaxies, while for blue galaxies the best-fitting slope is 1.09+0.20-0.13 and the normalization is 0.18+0.04 -0.05 × 1013 h -170 M. Similarly, we find a best-fitting slope of 1.36+0.06-0.07 and a normalization of 1.43+0.11-0.08 × 1013 h -170 M for the stellar mass-halo mass relation of red galaxies, while for blue galaxies the corresponding values are 0.98+0.08-0.07 and 0.84+0.20-0.16 × 1013 h -170 M. All numbers convey the 68 per cent confidence limit. For red lenses, the fraction which are satellites inside a larger halo tends to decrease with luminosity and stellar mass, with the sample being nearly all satellites for a stellar mass of 2 × 109 h -270 M. The satellite fractions are generally close to zero for blue lenses, irrespective of luminosity or stellar mass. This, together with the shallower relation between halo mass and baryonic tracer, is a direct confirmation from galaxy-galaxy lensing that blue galaxies reside in less clustered environments than red galaxies.We also find that the halo model, while matching the lensing signal around red lenses well, is prone to overpredicting the large-scale signal for faint and less massive blue lenses. This could be a further indication that these galaxies tend to be more isolated than assumed. © 2013 The Authors. Published by Oxford University Press on behalf of the Royal Astronomical Society.


Brightest cluster galaxies at the present epoch

Astrophysical Journal 797 (2014)

TR Lauer, M Postman, MA Strauss, GJ Graves, NE Chisari

© 2014. The American Astronomical Society. All rights reserved. We have obtained photometry and spectroscopy of 433 z ≤ 0.08 brightest cluster galaxies (BCGs) in a full-sky survey of Abell clusters to construct a BCG sample suitable for probing deviations from the local Hubble flow. The BCG Hubble diagram over 0 < z < 0.08 is consistent to within 2% of the Hubble relation specified by a Ωm = 0.3, Λ = 0.7 cosmology. This sample allows us to explore the structural and photometric properties of BCGs at the present epoch, their location in their hosting galaxy clusters, and the effects of the cluster environment on their structure and evolution. We revisit the Lm-α relation for BCGs, which uses α, the log-slope of the BCG photometric curve of growth, to predict the metric luminosity in an aperture with 14.3 kpc radius, Lm, for use as a distance indicator. Residuals in the relation are 0.27 mag rms. We measure central stellar velocity dispersions, σ, of the BCGs, finding the Faber-Jackson relation to flatten as the metric aperture grows to include an increasing fraction of the total BCG luminosity. A three-parameter "metric plane" relation using α and σ together gives the best prediction of Lm, with 0.21 mag residuals. The distribution of projected spatial offsets, rx of BCGs from the X-ray-defined cluster center is a steep γ = -2.33 power law over 1 < rx < 103 kpc. The median offset is ∼ 10 kpc, but ∼ 15% of the BCGs have rx > 100 kpc. The absolute cluster-dispersion normalized BCG peculiar velocity |ΔV1|/σc follows an exponential distribution with scale length 0.39±0.03. Both Lm and α increase with σc. The á parameter is further moderated by both the spatial and velocity offset from the cluster center, with larger α correlated with the proximity of the BCG to the cluster mean velocity or potential center. At the same time, position in the cluster has little effect on Lm. Likewise, residuals from the metric plane show no correlation with either the spatial or velocity offset from the cluster center. The luminosity difference between the BCG and second-ranked galaxy, M2, increases as the peculiar velocity of the BCG within the cluster decreases. Further, when M2 is a close luminosity "rival" of the BCG, the galaxy that is closest to either the velocity or X-ray center of the cluster is most likely to have the larger α. We conclude that the inner portions of the BCGs are formed outside the cluster, but interactions in the heart of the galaxy cluster grow and extend the envelopes of the BCGs.


Cosmological information in the intrinsic alignments of luminous red galaxies

Journal of Cosmology and Astroparticle Physics 2013 (2013)

N Elisa Chisari, C Dvorkin

The intrinsic alignments of galaxies are usually regarded as a contaminant to weak gravitational lensing observables. The alignment of Luminous Red Galaxies, detected unambiguously in observations from the Sloan Digital Sky Survey, can be reproduced by the linear tidal alignment model of Catelan, Kamionkowski &amp; Blandford (2001) on large scales. In this work, we explore the cosmological information encoded in the intrinsic alignments of red galaxies. We make forecasts for the ability of current and future spectroscopic surveys to constrain local primordial non-Gaussianity and Baryon Acoustic Oscillations (BAO) in the cross-correlation function of intrinsic alignments and the galaxy density field. For the Baryon Oscillation Spectroscopic Survey, we find that the BAO signal in the intrinsic alignments is marginally significant with a signal-to-noise ratio of 1.8 and 2.2 with the current LOWZ and CMASS samples of galaxies, respectively, and increasing to 2.3 and 2.7 once the survey is completed. For the Dark Energy Spectroscopic Instrument and for a spectroscopic survey following the EUCLID redshift selection function, we find signal-to-noise ratios of 12 and 15, respectively. Local type primordial non-Gaussianity, parametrized by fNL = 10, is only marginally significant in the intrinsic alignments signal with signal-to-noise ratios < 2 for the three surveys considered.© 2013 IOP Publishing Ltd and Sissa Medialab srl.


CFHTLenS: Mapping the large-scale structure with gravitational lensing

Monthly Notices of the Royal Astronomical Society 433 (2013) 3373-3388

L Van Waerbeke, J Benjamin, T Erben, C Heymans, H Hildebrandt, H Hoekstra, TD Kitching, Y Mellier, L Miller, J Coupon, J Harnois-Déraps, L Fu, M Hudson, M Kilbinger, K Kuijken, B Rowe, T Schrabback, E Semboloni, S Vafaei, E van Uitert, M Velander

We present a quantitative analysis of the largest contiguous maps of projected mass density obtained from gravitational lensing shear. We use data from the 154 deg2 covered by the Canada-France-Hawaii Telescope Lensing Survey (CFHTLenS). Our study is the first attempt to quantitatively characterize the scientific value of lensing maps, which could serve in the future as a complementary approach to the study of the dark universe with gravitational lensing. We show that mass maps contain unique cosmological information beyond that of traditional two-point statistical analysis techniques. Using a series of numerical simulations, we first show how, reproducing the CFHTLenS observing conditions, gravitational lensing inversion provides a reliable estimate of the projected matter distribution of large-scale structure. We validate our analysis by quantifying the robustness of the maps with various statistical estimators. We then apply the same process to the CFHTLenS data. We find that the two-point correlation function of the projected mass is consistent with the cosmological analysis performed on the shear correlation function discussed in the CFHTLenS companion papers. The maps also lead to a significant measurement of the third-order moment of the projected mass, which is in agreement with analytic predictions, and to a marginal detection of the fourth-order moment. Tests for residual systematics are found to be consistent with zero for the statistical estimators we used. A new approach for the comparison of the reconstructed mass map to that predicted from the galaxy distribution reveals the existence of giant voids in the dark matter maps as large as 3° on the sky. Our analysis shows that lensing mass maps are not only consistent with the results obtained by the traditional shear approach, but they also appear promising for new techniques such as peak statistics and the morphological analysis of the projected dark matter distribution. © 2013 The Authors. Published by Oxford University Press on behalf of the Royal Astronomical Society.


Probing the accelerating Universe with radio weak lensing in the JVLA Sky Survey

(2013)

ML Brown, FB Abdalla, A Amara, DJ Bacon, RA Battye, RJ Beswick, M Birkinshaw, V Böhm, S Bridle, IWA Browne, CM Casey, C Demetroullas, TE lin, PG Ferreira, ST Garrington, KJB Grainge, ME Gray, CA Hales, I Harrison, AF Heavens, C Heymans, CL Hung, NJ Jackson, MJ Jarvis

We outline the prospects for performing pioneering radio weak gravitational lensing analyses using observations from a potential forthcoming JVLA Sky Survey program. A large-scale survey with the JVLA can offer interesting and unique opportunities for performing weak lensing studies in the radio band, a field which has until now been the preserve of optical telescopes. In particular, the JVLA has the capacity for large, deep radio surveys with relatively high angular resolution, which are the key characteristics required for a successful weak lensing study. We highlight the potential advantages and unique aspects of performing weak lensing in the radio band. In particular, the inclusion of continuum polarisation information can greatly reduce noise in weak lensing reconstructions and can also remove the effects of intrinsic galaxy alignments, the key astrophysical systematic effect that limits weak lensing at all wavelengths. We identify a VLASS "deep fields" program (total area ~10-20 square degs), to be conducted at L-band and with high-resolution (A-array configuration), as the optimal survey strategy from the point of view of weak lensing science. Such a survey will build on the unique strengths of the JVLA and will remain unsurpassed in terms of its combination of resolution and sensitivity until the advent of the Square Kilometre Array. We identify the best fields on the JVLA-accessible sky from the point of view of overlapping with existing deep optical and near infra-red data which will provide crucial redshift information and facilitate a host of additional compelling multi-wavelength science.


On the shear estimation bias induced by the spatial variation of colour across galaxy profiles

Monthly Notices of the Royal Astronomical Society 432 (2013) 2385-2401

E Semboloni, H Hoekstra, Z Huang, VF Cardone, M Cropper, B Joachimi, T Kitching, K Kuijken, M Lombardi, R Maoli, Y Mellier, L Miller, J Rhodes, R Scaramella, T Schrabback, M Velander

The spatial variation of the colour of a galaxy may introduce a bias in the measurement of its shape if the point spread function (PSF) profile depends on wavelength. We study how this bias depends on the properties of the PSF and the galaxies themselves. The bias depends on the scales used to estimate the shape, which may be used to optimize methods to reduce the bias. Here, we develop a general approach to quantify the bias. Although applicable to any weak lensing survey, we focus on the implications for the ESA Euclid mission. Based on our study of synthetic galaxies, we find that the bias is a few times 10-3 for a typical galaxy observed by Euclid. Consequently, it cannot be neglected and needs to be accounted for. We demonstrate how one can do so using spatially resolved observations of galaxies in two filters. We show that Hubble Space Telescope (HST) observations in the F606W and F814W filters allow us to model and reduce the bias by an order of magnitude, sufficient to meet Euclid's scientific requirements. The precision of the correction is ultimately determined by the number of galaxies for which spatially resolved observations in at least two filters are available. We use results from the Millennium simulation to demonstrate that archival HST data will be sufficient for the tomographic cosmic shear analysis with the Euclid data set. © 2013 The Author. Published by Oxford University Press on behalf of the Royal Astronomical Society.

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