arXiv subjects

Jean-Paul Kneib

Publications and source records attributed to Jean-Paul Kneib.

At least 235 records · Page 13Linked to original sources

arXiv2009

Handbook for the GREAT08 Challenge: An image analysis competition for cosmological lensing

The GRavitational lEnsing Accuracy Testing 2008 (GREAT08) Challenge focuses on a problem that is of crucial importance for future observations in cosmology. The shapes of distant galaxies can be used to determine the properties of dark energy and the nature of gravity, because light from those galaxies is bent by gravity from the intervening dark matter. The observed galaxy images appear distorted, although only slightly, and their shapes must be precisely disentangled from the effects of pixelisation, convolution and noise. The worldwide gravitational lensing community has made significant progress in techniques to measure these distortions via the Shear TEsting Program (STEP). Via STEP, we have run challenges within our own community, and come to recognise that this particular image analysis problem is ideally matched to experts in statistical inference, inverse problems and computational learning. Thus, in order to continue the progress seen in recent years, we are seeking an infusion of new ideas from these communities. This document details the GREAT08 Challenge for potential participants. Please visit http://www.great08challenge.info for the latest information.

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arXiv2009

Detection of PAH and Far-Infrared Emission from the Cosmic Eye: Probing the Dust and Star Formation of Lyman Break Galaxies

We report the results of a Spitzer infrared study of the Cosmic Eye, a strongly lensed, L*_UV Lyman Break Galaxy (LBG) at z=3.074. We obtained Spitzer IRS spectroscopy as well as MIPS 24 and 70 micron photometry. The Eye is detected with high significance at both 24 and 70 microns and, when including a flux limit at 3.5 mm, we estimate an infrared luminosity of L_IR = 8.3 (+4.7-4.4) x10^11 L_sun assuming a magnification of 28+-3. This L_IR is eight times lower than that predicted from the rest-frame UV properties assuming a Calzetti reddening law. This has also been observed in other young LBGs, and indicates that the dust reddening law may be steeper in these galaxies. The mid-IR spectrum shows strong PAH emission at 6.2 and 7.7 microns, with equivalent widths near the maximum values observed in star-forming galaxies at any redshift. The L_PAH-to-L_IR ratio lies close to the relation measured in local starbursts. Therefore, L_PAH or L_MIR may be used to estimate L_IR and thus, star formation rate, of LBGs, whose fluxes at longer wavelengths are typically below current confusion limits. We also report the highest redshift detection of the 3.3 micron PAH emission feature. The PAH ratio, L_6.2/L_3.3=5.1+- 2.7, and the PAH-to-L_IR ratio, L_3.3/L_IR = 8.5 +- 4.7 x10^-4, are both in agreement with measurements in local starbursts and ULIRGs, suggesting that this line may serve as a good proxy for L_PAH or L_IR at z > 3 with the James Webb Space Telescope.

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arXiv2009

Multi-scale cluster lens mass mapping I. Strong Lensing modelling

We propose a novel technique to refine the modelling of galaxy clusters mass distribution using gravitational lensing. The idea is to combine the strengths of both "parametric" and "non-parametric" methods to improve the quality of the fit. We develop a multi-scale model that allows sharper contrast in regions of higher density where the number of constraints is generally higher. Our model consists of (i) a multi-scale grid of radial basis functions with physically motivated profiles and (ii) a list of galaxy-scale potentials at the location of the cluster member galaxies. This arrangement of potentials of different sizes allows to reach a high resolution for the model with a minimum number of parameters. We apply our model to the well studied cluster Abell 1689. We estimate the quality of our mass reconstruction with a Bayesian MCMC sampler. For a selected subset of multiple images, we manage to halve the errors between the predicted and observed image positions compared to previous studies. This owes to the flexibility of multi-scale models at intermediate scale between cluster and galaxy scale. The software developed for this paper is part of the public lenstool package which can be found at www.oamp.fr/cosmology/lenstool.

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arXiv2009

Strong gravitational lensing probes of the particle nature of dark matter

There is a vast menagerie of plausible candidates for the constituents of dark matter, both within and beyond extensions of the Standard Model of particle physics. Each of these candidates may have scattering (and other) cross section properties that are consistent with the dark matter abundance, BBN, and the most scales in the matter power spectrum; but which may have vastly different behavior at sub-galactic "cutoff" scales, below which dark matter density fluctuations are smoothed out. The only way to quantitatively measure the power spectrum behavior at sub-galactic scales at distances beyond the local universe, and indeed over cosmic time, is through probes available in multiply imaged strong gravitational lenses. Gravitational potential perturbations by dark matter substructure encode information in the observed relative magnifications, positions, and time delays in a strong lens. Each of these is sensitive to a different moment of the substructure mass function and to different effective mass ranges of the substructure. The time delay perturbations, in particular, are proving to be largely immune to the degeneracies and systematic uncertainties that have impacted exploitation of strong lenses for such studies. There is great potential for a coordinated theoretical and observational effort to enable a sophisticated exploitation of strong gravitational lenses as direct probes of dark matter properties. This opportunity motivates this white paper, and drives the need for: a) strong support of the theoretical work necessary to understand all astrophysical consequences for different dark matter candidates; and b) tailored observational campaigns, and even a fully dedicated mission, to obtain the requisite data.

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arXiv2009

Keck spectroscopic survey of strongly lensed galaxies in Abell 1703: further evidence for a relaxed, unimodal cluster

Strong gravitational lensing is a unique tool to model with great accuracy the inner mass distribution of massive galaxy clusters. In particular, clusters with large Einstein radii provide a wealth of multiply imaged systems in the cluster core allowing to determine precisely the shape of the central dark matter profile. This paper presents a spectroscopic survey in the massive cluster Abell 1703, displaying a large Einstein radius (28" at z=2.8) and a high number of strongly-lensed systems including a central ring-like configuration. We used LRIS on Keck to target multiple images and lensed galaxy candidates, and use the measured redshifts to constrain the mass distribution of the cluster using a parametric model. The data enable us to measure accurate redshifts in good agreement with their photometric redshifts, and to update the identification of multiply imaged systems by discovering 3 new systems and a radial counter image. We also report the discovery of a remarkably bright ~3.6 L* i-band dropout at z=5.827 in our mask which is only moderately magnified by the cluster (~3.0+/-0.08). The improved parametric mass model, including 16 multiple systems with 10 spectroscopic redshifts, further constrains the cluster-scale mass distribution with a generalized NFW profile of best-fit logarithmic slope alpha=0.92+/-0.04, concentration c200=4.72+/-0.40 and scale radius rs=476+/-45 kpc. Our strong-lensing model predicts a large scale shear signal consistent with Subaru weak-lensing measurements out to 4 Mpc h^-1. Together with the fact that the strong-lensing modeling requires a single dark matter clump, this suggests that Abell 1703 is a relaxed, unimodal cluster. This unique cluster could be probed further using deep X-ray, SZ and dynamics analysis, for a detailed study of the physics in a relaxed cluster. (abridged)

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arXiv2008

The survival of dark matter halos in the cluster Cl0024+16

Theories of structure formation in a cold dark matter dominated Universe predict that massive clusters of galaxies assemble from the hierarchical merging of lower mass subhalos. Exploiting strong and weak gravitational lensing signals inferred from panoramic HST imaging data, we present a high resolution reconstruction of the mass distribution in the massive, lensing cluster Cl0024+16. Applying galaxy-galaxy lensing techniques we track the fate of dark matter subhalos as a function of projected cluster-centric radius out to 5 Mpc, well beyond the virial radius. We report the first detection of the statistical lensing signal of dark matter subhalos associated with late-type galaxies in clusters. The mass of a typical dark matter subhalo that hosts an L* galaxy increases with projected cluster-centric radius in line with expectations from the tidal stripping hypothesis. Early-type galaxies appear to be hosted on average in more massive dark matter subhalos compared to late-type galaxies. We interpret our findings as evidence for the active assembly of mass via tidal stripping in galaxy clusters. The mass function of dark matter subhalos as a function of projected cluster-centric radius, is compared with an equivalent mass function derived from clusters in the Millenium Run simulation populated with galaxies using semi-analytic models. The shape of the lensing determined mass functions are in very good agreement with the subhalo mass functions derived from the Millenium Run simulation. However, simulated subhalos appear to be more efficiently stripped than lensing observations suggest. (Abridged)

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arXiv2008

Optical-mechanical operation of the F2T2 Filter: a tunable filter designed to search for First Light

The Flamingos-2 Tandem Tunable filter is a tunable, narrow-band filter, consisting of two Fabry-Perot etalons in series, capable of scanning to any wavelength from 0.95 to 1.35 microns with a spectral resolution of R~800. It is an accessory mode instrument for the near-IR Flamingos-2 imaging-spectrograph designed for the Gemini South 8m Observatory and will be fed through the upcoming Multi-Conjugate Adaptive Optics feed. The primary science goal of the F2T2 filter is to perform a ground-based search for the first star forming regions in the universe at redshifts of 7 < z < 11. The construction of the F2T2 filter is complete and it is currently in its calibration and commissioning phases. In this proceeding, we describe the calibration and performance of the instrument.

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arXiv2008

A Hubble & Spitzer Space Telescope Survey for Gravitationally-Lensed Galaxies: Further Evidence for a Significant Population of Low Luminosity Galaxies beyond Redshift Seven

We present the results of a systematic search for gravitationally-lensed continuum Lyman break `drop-outs' beyond a redshift 7 conducted via very deep imaging through six foreground clusters undertaken with the Hubble and Spitzer Space Telescopes. The survey has yielded 10 z-band and 2 J-band drop-out candidates to photometric limits of J_110~=26.2 AB (5sigma). Taking into account the magnifications afforded by our clusters (1-4 magnitudes), we probe the presence of z>7 sources to unlensed limits of J_{110}~=30 AB, fainter than those charted in the Hubble Ultradeep Field. To verify the fidelity of our candidates we conduct a number of tests for instrumental effects which would lead to spurious detections, and carefully evaluate the likelihood of foreground contamination by considering photometric uncertainties in the drop-out signature, the upper limits from stacked IRAC data and the statistics of multiply-imaged sources. Overall, we conclude that we can expect about half of our sample of z-band drop-outs are likely to be at high redshift. An ambitious infrared spectroscopic campaign undertaken with the NIRSPEC spectrograph at the WM Keck Observatory for seven of the most promising candidates failed to detect any Lyman-alpha emission highlighting the challenge of making further progress in this field. While the volume density of high redshift sources will likely remain uncertain until more powerful facilities are available, our data provides the first potentially interesting constraints on the UV luminosity function at z~=7.5 at intrinsically faint limits. We discuss the implications of our results in the context of the hypothesis that the bulk of the reionizing photons in the era 7<z<12 arise in low luminosity galaxies undetected by conventional surveys.

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arXiv2008

First catalog of strong lens candidates in the COSMOS field

We present the first catalog of 67 strong galaxy-galaxy lens candidates discovered in the 1.64 square degree Hubble Space Telescope COSMOS survey. Twenty of these systems display multiple images or strongly curved large arcs. Our initial search is performed by visual inspection of the data and is restricted, for practical considerations, to massive early-type lens galaxies with arcs found at radii smaller than ~5''. Simple mass models are constructed for the best lens candidates and our results are compared to the strong lensing catalogs of the SLACS survey and the CASTLES database. These new strong galaxy-galaxy lensing systems constitute a valuable sample to study the mass distribution of early-type galaxies and their associated dark matter halos. We further expect this sample to play an important role in the testing of software algorithms designed to automatically search for strong gravitational lenses. From our analysis a robust lower limit is derived for the expected occurrence of strong galaxy-galaxy systems in current and future space-based wide-field imaging surveys. We expect that such surveys should uncover a large number of strong lensing systems (more than 10 systems per square degree), which will allow for a detailed statistical analysis of galaxy properties and will likely lead to constraints on models of gravitational structure formation and cosmology. The sample of strong lenses is available here: http://cosmosstronglensing.uni-hd.de/

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arXiv2008

Dark Matter and Baryons in the Most X-ray Luminous and Merging Galaxy Cluster RX J1347.5-1145

The galaxy cluster RX J1347-1145 is one of the most X-ray luminous and most massive clusters known. Its extreme mass makes it a prime target for studying issues addressing cluster formation and cosmology. In this paper we present new high-resolution HST/ACS and Chandra X-ray data. The high resolution and sensitivity of ACS enabled us to detect and quantify several new multiply imaged sources, we now use a total of eight for the strong lensing analysis. Combining this information with shape measurements of weak lensing sources in the central regions of the cluster, we derive a high-resolution, absolutely-calibrated mass map. This map provides the best available quantification of the total mass of the central part of the cluster to date. We compare the reconstructed mass with that inferred from the new Chandra X-ray data, and conclude that both mass estimates agree extremely well in the observed region, namely within 400 / h_70 kpc of the cluster center. In addition we study the major baryonic components (gas and stars) and hence derive the dark matter distribution in the center of the cluster. We find that the dark matter and baryons are both centered on the BCG within the uncertainties (alignment is better than <10 kpc). We measure the corresponding 1-D profiles and find that dark matter distribution is consistent with both NFW and cored profiles, indicating that a more extended radial analysis is needed to pinpoint the concentration parameter, and hence the inner slope of the dark matter profile.

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arXiv2007

A Bayesian approach to strong lensing modelling of galaxy clusters

In this paper, we describe a procedure for modelling strong lensing galaxy clusters with parametric methods, and to rank models quantitatively using the Bayesian evidence. We use a publicly available Markov chain Monte-Carlo (MCMC) sampler ('Bayesys'), allowing us to avoid local minima in the likelihood functions. To illustrate the power of the MCMC technique, we simulate three clusters of galaxies, each composed of a cluster-scale halo and a set of perturbing galaxy-scale subhalos. We ray-trace three light beams through each model to produce a catalogue of multiple images, and then use the MCMC sampler to recover the model parameters in the three different lensing configurations. We find that, for typical Hubble Space Telescope (HST)-quality imaging data, the total mass in the Einstein radius is recovered with ~1-5% error according to the considered lensing configuration. However, we find that the mass of the galaxies is strongly degenerated with the cluster mass when no multiple images appear in the cluster centre. The mass of the galaxies is generally recovered with a 20% error, largely due to the poorly constrained cut-off radius. Finally, we describe how to rank models quantitatively using the Bayesian evidence. We confirm the ability of strong lensing to constrain the mass profile in the central region of galaxy clusters in this way. Ultimately, such a method applied to strong lensing clusters with a very large number of multiple images may provide unique geometrical constraints on cosmology. The implementation of the MCMC sampler used in this paper has been done within the framework of the Lenstool software package, which is publicly available.

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arXiv2007

Where is the matter in the Merging Cluster Abell 2218?

We present a parametric strong lensing model of the cluster Abell 2218 based on HST ACS data. We constrain the lens model using 8 previously known multiply imaged systems, of which 7 have spectroscopically confirmed redshifts. In addition, we propose five candidate multiply imaged systems and estimate their redshifts using our mass model. The model parameters are optimized in the source plane by a bayesian Monte Carlo Markov Chain as implemented in the the publicly available software Lenstool. We find rms=0."12 for the scatter of the sources in the source plane, which translates into rms=1."49 between the predicted and measured image positions in the image plane. We find that the projected mass distribution of Abell 2218 is bimodal, which is supported by an analysis of the light distribution. We also find evidence for two structures in velocity space, separated by ~1000 km/s, corresponding to the two large scale dark matter clumps. We find that the lensing constraints can not be well reproduced using only dark matter halos associated with the cluster galaxies, but that the dark matter is required to be smoothly distributed in large scale halos. At 100" the enclosed projected mass is 3.8e14 solar masses. At that radius, the large scale halos contribute ~85% of the mass. We find that the model is not very sensitive to the fainter galaxy sized halos, unless they locally perturb a given multiply imaged system. Therefore, dark galaxy sized substructure can be reliably constrained only if it locally perturbs one of the systems. In an appendix we give a self-contained description of the parametric profile we use, the dual pseudo isothermal elliptical mass distribution (dPIE). This profile is a two component pseudo isothermal mass distribution (PIEMD) with both a core radii and a scale radii. (Abridged)

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arXiv2007

SuperNova Acceleration Probe (SNAP): Investigating Photometric Redshift Optimization

The aim of this paper is to investigate ways to optimize the accuracy of photometric redshifts for a SNAP like mission. We focus on how the accuracy of the photometric redshifts depends on the magnitude limit and signal-to-noise ratio, wave-length coverage, number of filters and their shapes and observed galaxy type. We use simulated galaxy catalogs constructed to reproduce observed galaxy luminosity functions from GOODS, and derive photometric redshifts using a template fitting method. By using a catalog that resembles real data, we can estimate the expected number density of galaxies for which photometric redshifts can be derived. We find that the accuracy of photometric redshifts is strongly dependent on the signal-to-noise (S/N) (i.e., S/N>10 is needed for accurate photometric redshifts). The accuracy of the photometric redshifts is also dependent on galaxy type, with smaller scatter for earlier type galaxies. Comparing results using different filter sets, we find that including the U-band is important for decreasing the fraction of outliers, i.e., ``catastrophic failures''. Using broad overlapping filters with resolution ~4gives better photometric redshifts compared to narrower filters (resolution >~5) with the same integration time. We find that filters with square response curves result in a slightly higher scatter, mainly due to a higher fraction of outliers at faint magnitudes. We also compare a 9-filter set to a 17-filter set, where we assume that the available exposure time per filter in the latter set is half that of the first set. We find that the 9-filter set gives more accurate redshifts for a larger number of objects and reaches higher redshift, while the 17-filter set is gives better results at bright magnitudes.

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arXiv2007

A Telescope Search for Decaying Relic Axions

A search for optical line emission from the two-photon decay of relic axions was conducted in the galaxy clusters Abell 2667 and 2390, using spectra from the VIMOS (Visible Multi-Object Spectrograph) integral field unit at the Very Large Telescope. New upper limits to the two-photon coupling of the axion are derived, and are at least a factor of 3 more stringent than previous upper limits in this mass window. The improvement follows from larger collecting area, integration time, and spatial resolution, as well as from improvements in signal to noise and sky subtraction made possible by strong-lensing mass models of these clusters. The new limits either require that the two-photon coupling of the axion be extremely weak or that the axion mass window between 4.5 eV and 7.7 eV be closed. Implications for sterile-neutrino dark matter are discussed briefly also.

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arXiv2007

A statistical study of multiply-imaged systems in the lensing cluster Abell 68

We have carried out an extensive spectroscopic survey with the Keck and VLT telescopes, targeting lensed galaxies in the background of the massive cluster Abell 68. Spectroscopic measurements are obtained for 26 lensed images, including a distant galaxy at z=5.4 . Redshifts have been determined for 5 out of 7 multiply-image systems. Through a careful modeling of the mass distribution in the strongly-lensed regime, we derive a mass estimate of 5.3 x 10^14 Msun within 500 kpc. Our mass model is then used to constrain the redshift distribution of the remaining multiply-imaged and singly-imaged sources. This enables us to examine the physical properties for a subsample of 7 Lyman-alpha emitters at 1.7 < z < 5.5, whose unlensed luminosities of ~ 10^41 ergs/s are fainter than similar objects found in blank fields. Of particular interest is an extended Lyman-alpha emission region surrounding a highly magnified source at z=2.6, detected in VIMOS Integral Field Spectroscopy data. The physical scale of the most distant lensed source at z=5.4 is very small (<300 pc), similar to the lensed z ~ 5.6 emitter reported by Ellis et al. (2001) in Abell 2218. New photometric data available for Abell 2218 allow for a direct comparison between these two unique objects. Our survey illustrates the practicality of using lensing clusters to probe the faint end of the z ~ 2-5 Lyman-alpha luminosity function in a manner that is complementary to blank field narrow-band surveys.

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arXiv2007

The Stability of the Point Spread Function of the Advanced Camera for Surveys on the Hubble Space Telescope and Implications for Weak Gravitational Lensing

(abridged) We examine the spatial and temporal stability of the HST ACS Wide Field Camera (WFC) point spread function (PSF) using the two square degree COSMOS survey. We show that stochastic aliasing of the PSF necessarily occurs during `drizzling'. This aliasing is maximal if the output pixel scale is equal to the input pixel scale of 0.05''. We show that this source of PSF variation can be significantly reduced by choosing a Gaussian drizzle kernel and by setting the output pixel size to 0.03''. We show that the PSF is temporally unstable, most likely due to thermal fluctuations in the telescope's focus. We find that the primary manifestation of this thermal drift in COSMOS images is an overall slow periodic focus change. Using a modified version of TinyTim, we create undistorted stars in a 30x30 grid across the ACS WFC CCDs. These PSF models are created for telescope focus values in the range -10 microns to +5 microns, thus spanning the allowed range of telescope focus values. We then use the approximately ten well measured stars in each COSMOS field to pick the best-fit focus value for each field. The TinyTim model stars are then used to perform PSF corrections for weak lensing allowing systematics due to incorrectly modeled PSFs to be greatly reduced. We have made the software for PSF modeling using our modified version of TinyTim available to the astronomical community. We show the effects of Charge Transfer Efficiency (CTE) degradation, which distorts the object in the readout direction, mimicking a weak lensing signal. We derive a parametric correction for the effect of CTE on the shapes of objects in the COSMOS field as a function of observation date, position within the ACS WFC field, and object flux.

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arXiv2007

COSMOS: 3D weak lensing and the growth of structure

We present a three dimensional cosmic shear analysis of the Hubble Space Telescope COSMOS survey, the largest ever optical imaging program performed in space. We have measured the shapes of galaxies for the tell-tale distortions caused by weak gravitational lensing, and traced the growth of that signal as a function of redshift. Using both 2D and 3D analyses, we measure cosmological parameters Omega_m, the density of matter in the universe, and sigma_8, the normalization of the matter power spectrum. The introduction of redshift information tightens the constraints by a factor of three, and also reduces the relative sampling (or "cosmic") variance compared to recent surveys that may be larger but are only two dimensional. From the 3D analysis, we find sigma_8*(Omega_m/0.3)^0.44=0.866+^0.085_-0.068 at 68% confidence limits, including both statistical and potential systematic sources of error in the total budget. Indeed, the absolute calibration of shear measurement methods is now the dominant source of uncertainty. Assuming instead a baseline cosmology to fix the geometry of the universe, we have measured the growth of structure on both linear and non-linear physical scales. Our results thus demonstrate a proof of concept for tomographic analysis techniques that have been proposed for future weak lensing surveys by a dedicated wide-field telescope in space.

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arXiv2007

Dark matter maps reveal cosmic scaffolding

Ordinary baryonic particles (such as protons and neutrons) account for only one-sixth of the total matter in the Universe. The remainder is a mysterious "dark matter" component, which does not interact via electromagnetism and thus neither emits nor reflects light. As dark matter cannot be seen directly using traditional observations, very little is currently known about its properties. It does interact via gravity, and is most effectively probed through gravitational lensing: the deflection of light from distant galaxies by the gravitational attraction of foreground mass concentrations. This is a purely geometrical effect that is free of astrophysical assumptions and sensitive to all matter -- whether baryonic or dark. Here we show high fidelity maps of the large-scale distribution of dark matter, resolved in both angle and depth. We find a loose network of filaments, growing over time, which intersect in massive structures at the locations of clusters of galaxies. Our results are consistent with predictions of gravitationally induced structure formation, in which the initial, smooth distribution of dark matter collapses into filaments then into clusters, forming a gravitational scaffold into which gas can accumulate, and stars can be built.

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