arXiv subjects

Eric Jullo

Publications and source records attributed to Eric Jullo.

87 records · Page 5Linked to original sources

arXiv2011

Measuring the Geometry of the Universe from Weak Gravitational Lensing behind Galaxy Groups in the HST COSMOS survey

Gravitational lensing can provide pure geometric tests of the structure of space-time, for instance by determining empirically the angular diameter distance-redshift relation. This geometric test has been demonstrated several times using massive clusters which produce a large lensing signal. In this case, matter at a single redshift dominates the lensing signal, so the analysis is straightforward. It is less clear how weaker signals from multiple sources at different redshifts can be stacked to demonstrate the geometric dependence. We introduce a simple measure of relative shear which for flat cosmologies separates the effect of lens and source positions into multiplicative terms, allowing signals from many different source-lens pairs to be combined. Applying this technique to a sample of groups and low-mass clusters in the COSMOS survey, we detect a clear variation of shear with distance behind the lens. This represents the first detection of the geometric effect using weak lensing by multiple, low-mass systems. The variation of distance with redshift is measured with sufficient precision to constrain the equation of state of the universe under the assumption of flatness, equivalent to a detection of a dark energy component Omega_X at greater than 99% confidence for an equation-of-state parameter -2.5 < w < -0.1. For the case w = -1, we find a value for the cosmological constant density parameter Omega_Lambda = 0.85+0.044-0.19 (68% C.L.), and detect cosmic acceleration (q_0 < 0) at the 98% C.L.. We consider the systematic uncertainties associated with this technique and discuss the prospects for applying it in forthcoming weak-lensing surveys.

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arXiv2010

Measurement of a Metallicity Gradient in a z=2 Galaxy: Implications for Inside-Out Assembly Histories

We present near-infrared imaging spectroscopy of the strongly-lensed z=2.00 galaxy SDSS J120601.69+514227.8 (`the Clone arc'). Using OSIRIS on the Keck 2 telescope with laser guide star adaptive optics, we achieve resolved spectroscopy with 0.20 arcsecond FWHM resolution in the diagnostic emission lines [O III], Halpha, and [N II]. The lensing magnification allows us to map the velocity and star formation from Halpha emission at a physical resolution of ~300 pc in the galaxy source plane. With an integrated star formation rate of ~50 Msun/yr, the galaxy is typical of sources similarly studied at this epoch. It is dispersion-dominated with a velocity gradient of +/- 80 km/s and average dispersion sigma = 85 km/s; the dynamical mass is 2.4 \times 10^{10} Msun within a half-light radius of 2.9 kpc. Robust detection of [N II] emission across the entire OSIRIS field of view enables us to trace the gas-phase metallicity distribution with 500 pc resolution. We find a strong radial gradient in both the [N II]/Halpha and [O III]/Halpha ratios indicating a metallicity gradient of -0.27 +/- 0.05 dex/kpc with central metallicity close to solar. We demonstrate that the gradient is seen independently in two multiple images. While the physical gradient is considerably steeper than that observed in local galaxies, in terms of the effective radius at that epoch, the gradient is similar. This suggests that subsequent growth occurs in an inside-out manner with the inner metallicity gradient diminished over time due to radial mixing and enrichment from star formation.

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arXiv2010

Strong Lensing as a Probe of the Mass Distribution Beyond the Einstein Radius. Mass & Light in SL2S J08544-0121, a Galaxy Group at z=0.35

Precise modelling of strong lensing systems can be affected by external mass distributions, e.g. the group or cluster within which the lens is embedded. In this article, we propose to turn this limitation to our advantage and to use precise strong lensing modelling to probe external mass distributions surrounding the lens. We consider SL2S J08544-0121, a galaxy group at z=0.35 that contains a strong lensing system. A simple elliptical isothermal potential cannot reproduce satisfactorily the strong lensing constraints. We include an external mass perturbation corresponding to the group within which the lens is embedded. The lensing properties of this perturbation are parametrised by its total mass M and a smoothing scale s that quantifies the characteristic scale over which M is distributed. For a range of these parameters, we are able to reproduce accurately the observations. This suggests that light is a good tracer of mass. Interestingly, this also shows that a localised strong lensing analysis (on scales of ~10") allows us to constrain global properties of the group as a whole (on scales of ~100). Indeed, we constrain the group mass-to-light ratio to be M/L=98+-27 (i band, solar units, not corrected for evolution) and s=20" +- 9 (2sigma confidence level). We demonstrate that these strong lensing only constraints are due to the perturbed strong lensing configuration, where the main arc is located at ~5" from the galaxy, whereas its counter-image is found at ~8". To test independently our resulting strong lensing model, we pursue an independent weak lensing analysis of the group and find a mass-to-light ratio in the range 66-146 (1sigma confidence level).

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arXiv2010

Cosmological Constraints from Strong Gravitational Lensing in Galaxy Clusters

Current efforts in observational cosmology are focused on characterizing the mass-energy content of the Universe. We present results from a geometric test based on strong lensing in galaxy clusters. Based on Hubble Space Telescope images and extensive ground-based spectroscopic follow-up of the massive galaxy cluster Abell 1689, we used a parametric model to simultaneously constrain the cluster mass distribution and dark energy equation of state. Combining our cosmological constraints with those from X-ray clusters and the Wilkinson Microwave Anisotropy Probe 5-year data gives Ωm = 0.25 +/- 0.05 and wx = -0.97 +/- 0.07 which are consistent with results from other methods. Inclusion of our method with all other techniques available brings down the current 2σ contours on the dark energy equation of state parameter wx by about 30%.

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arXiv2010

LoCuSS: First Results from Strong-lensing Analysis of 20 Massive Galaxy Clusters at z~0.2

We present a statistical analysis of a sample of 20 strong lensing clusters drawn from the Local Cluster Substructure Survey (LoCuSS), based on high resolution Hubble Space Telescope imaging of the cluster cores and follow-up spectroscopic observations using the Keck-I telescope. We use detailed parameterized models of the mass distribution in the cluster cores, to measure the total cluster mass and fraction of that mass associated with substructures within R<250kpc.These measurements are compared with the distribution of baryons in the cores, as traced by the old stellar populations and the X-ray emitting intracluster medium. Our main results include: (i) the distribution of Einstein radii is log-normal, with a peak and 1sigma width of =1.16+/-0.28; (ii) we detect an X-ray/lensing mass discrepancy of =1.3 at 3 sigma significance -- clusters with larger substructure fractions displaying greater mass discrepancies, and thus greater departures from hydrostatic equilibrium; (iii) cluster substructure fraction is also correlated with the slope of the gas density profile on small scales, implying a connection between cluster-cluster mergers and gas cooling. Overall our results are consistent with the view that cluster-cluster mergers play a prominent role in shaping the properties of cluster cores, in particular causing departures from hydrostatic equilibrium, and possibly disturbing cool cores. Our results do not support recent claims that large Einstein radius clusters present a challenge to the CDM paradigm.

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arXiv2009

Abell 370 revisited: refurbished Hubble imaging of the first strong lensing cluster

We present a strong lensing analysis of the galaxy cluster Abell 370 (z=0.375) based on the recent multicolor ACS images obtained as part of the Early Release Observation (ERO) that followed the Hubble Service Mission #4. Back in 1987, the giant gravitational arc (z=0.725) in Abell 370 was one of the first pieces of evidence that massive clusters are dense enough to act as strong gravitational lenses. The new observations reveal in detail its disklike morphology, and we show that it can be interpreted as a complex five-image configuration, with a total magnification factor of 32+/-4. Moreover, the high resolution multicolor information allowed us to identify 10 multiply imaged background galaxies. We derive a mean Einstein radius of RE=39+/-2" for a source redshift at z=2, corresponding to a mass of M(<RE) = 2.82+/-0.15 1e14 Msol and M(<250 kpc)=3.8+/-0.2 1e14 Msol, in good agreement with Subaru weak-lensing measurements. The typical mass model error is smaller than 5%, a factor 3 of improvement compared to the previous lensing analysis. Abell 370 mass distribution is confirmed to be bi-modal with very small offset between the dark matter, the X-ray gas and the stellar mass. Combining this information with the velocity distribution reveals that Abell 370 is likely the merging of two equally massive clusters along the line of sight, explaining the very high mass density necessary to efficiently produce strong lensing. These new observations stress the importance of multicolor imaging for the identification of multiple images which is key to determining an accurate mass model. The very large Einstein radius makes Abell 370 one of the best clusters to search for high redshift galaxies through strong magnification in the central region.

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arXiv2009

Hubble Space Telescope Observations of a Spectacular New Strong-lensing Galaxy Cluster - MACSJ1149.5+2223 at z=0.544

We present Advanced Camera for Surveys observations of MACSJ1149.5+2223, an X-ray luminous galaxy cluster at z=0.544 discovered by the Massive Cluster Survey. The data reveal at least seven multiply-imaged galaxies, three of which we have confirmed spectroscopically. One of these is a spectacular face-on spiral galaxy at z=1.491, the four images of which are gravitationally magnified by ~8 =10^12 where N_halo is the number of cluster/group-scale halos. In terms of sheer number of merging cluster/group-scale components, this is the most complex strong-lensing cluster core studied to date. The total cluster mass and fraction of that mass associated with substructures within R<=500kpc, are measured to be M_tot=(6.7+/-0.4)x10^14Msol and f_sub=0.25+/-0.12 respectively. Our model also rules out recent claims of a flat density profile at >~7sigma confidence, thus highlighting the critical importance of spectroscopic redshifts of multiply-imaged galaxies when modeling strong lensing clusters. Overall our results attest to the efficiency of X-ray selection in finding the most powerful cluster lenses, including complicated merging systems.

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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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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

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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arXiv2006

First results from the VIMOS-IFU survey of gravitationally lensing clusters at z~0.2

We present the on-going observational program of a VIMOS Integral Field Unit survey of the central regions of massive, gravitational lensing galaxy clusters at redshift z~0.2. We have observed six clusters using the low-resolution blue grism (R about 200), and the spectroscopic survey is complemented by a wealth of photometric data, including Hubble Space Telescope optical data and near infrared VLT data. The principal scientific aims of this project are: the study of the high-z lensed galaxies, the transformation and evolution of galaxies in cluster cores and the use of multiple images to constrain cosmography. We briefly report here on the first results from this project on the clusters Abell 2667 and Abell 68.

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