arXiv subjects

B. Fort

Publications and source records attributed to B. Fort.

At least 19 recordsLinked to original sources

arXiv2011

Abell 611. II. X-ray and strong lensing analyses

We present the results of our analyses of the X-ray emission and of the strong lensing systems in the relaxed galaxy cluster Abell 611 (z=0.288). We infer the X-ray mass estimate deriving the density and temperature profiles of the intra-cluster medium within the radius r ~ 700 kpc through a non-parametric approach; assuming that the cluster is in hydrostatic equilibrium and adopting a matter density profile, we can recover the total mass distribution of Abell 611 via the X-ray data. We derive the total projected mass in the central region of Abell 611 performing a parametric analysis of its strong lensing features through the publicly available analysis software Lenstool. As a final step we compare the results obtained with both methods. We derive a good agreement between the X-ray and strong lensing total mass estimates in the central region (i.e. within the radius r ~100 kpc), while a marginal disagreement is found between the two mass estimates when extrapolating the strong lensing results in the outer spatial range. We suggest that in this case the X-ray/strong lensing mass disagreement can be explained by an incorrect estimate of the relative contributions of the baryonic component and of the dark matter, caused by the intrinsic degeneracy between the different mass components in the strong lensing analysis. We discuss the effect of some possible systematic errors that influence both mass estimates. We find a slight dependence of the measurements of the X-ray temperatures (and therefore of the X-ray total masses) on the background adopted in the spectral analysis, with total deviations on the value of M_200 of the order of the 1-sigma statistical error. The strong lensing mass results are instead sensitive to the parameterisation of the galactic halo mass in the central regions, in particular to the modelling of the Brightest Cluster Galaxy (BCG) baryonic component.

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arXiv2009

Photometric redshifts and cluster tomography in the ESO Distant Cluster Survey

This paper reports the results obtained on the photometric redshifts measurement and accuracy, and cluster tomography in the ESO Distant Cluster Survey (EDisCS) fields. Photometric redshifts were computed using two independent codes (Hyperz and G. Rudnick's code). The accuracy of photometric redshifts was assessed by comparing our estimates with the spectroscopic redshifts of ~1400 galaxies in the 0.3<z<1.0 domain. The accuracy for galaxies fainter than the spectroscopic control sample was estimated using a degraded version of the photometric catalog for the spectroscopic sample. The accuracy of photometric redshifts is typically sigma(Delta z/(1+z)) ~ 0.05+/-0.01, depending on the field, the filter set, and the spectral type of the galaxies. The quality of the photometric redshifts degrades by a factor of two in sigma(Delta z/(1+z)) between the brightest (I~22) and the faintest (I~24-24.5) galaxies in the EDisCS sample. The photometric determination of cluster redshifts in the EDisCS fields using a simple algorithm based on photoz is in excellent agreement with the spectroscopic values (delta(z) ~0.03-0.04 in the high-z sample and ~0.05 in the low-z sample). We also developed a method that uses both photz codes jointly to reject interlopers at magnitudes fainter than the spectroscopic limit. When applied to the spectroscopic sample, this method rejects ~50-90% of all spectroscopically confirmed non-members, while retaining at least 90% of all confirmed members. Photometric redshifts are found to be particularly useful for the identification and study of clusters of galaxies in large surveys. They enable efficient and complete pre-selection of cluster members for spectroscopy, allow accurate determinations of the cluster redshifts, and provide a means of determining cluster membership. (Abridged)

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arXiv2009

The mass profile of early-type galaxies in overdense environments: the case of the double source plane gravitational lens SL2SJ02176-0513

SL2SJ02176-0513 is a remarkable lens for the presence of two multiply-imaged systems at different redshifts lensed by a foreground massive galaxy at $z_{\rm lens}=0.656$: a bright cusp arc at $z_{\rm arc}=1.847$ and an additional double-image system at an estimated redshift of $z_{\rm dbl}\sim2.9$ based on photometry and lensing geometry. The system is located about 400 kpc away from the center of a massive group of galaxies. Mass estimates for the group are available from X-ray observations and satellite kinematics. Multicolor photometry provides an estimate of the stellar mass of the main lens galaxy. The lensing galaxy is modeled with two components (stars and dark matter), and we include the perturbing effect of the group environment, and all available constraints. We find that classic lensing degeneracies, e.g. between external convergence and mass density slope, are significantly reduced with respect to standard systems and infer tight constraints on the mass density profile: (i) the dark matter content of the main lens galaxy is in line with that of typical galaxies $f_{\rm dm}(<R_{\rm e})=0.41^{+0.09}_{-0.06}$; (ii) the required mass associated with the dark matter halo of the nearby group is consistent with X-ray and weak-lensing estimates ($\sigma_{\rm grp}=550^{+130}_{-240}$); (iii) accounting for the group contribution in the form of an external convergence, the slope of the mass density profile of the main lens galaxy alone is found to be $\alpha=-1.03^{+0.22}_{-0.16}$, consistent with the isothermal ($\alpha=-1$) slope. We demonstrate that multiple source plane systems together with good ancillary dataset can be used to disentangle local and environmental effects.

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arXiv2008

A New Window of Exploration in the Mass Spectrum: Strong Lensing by Galaxy Groups in the SL2S

The existence of strong lensing systems with Einstein radii (Re) covering the full mass spectrum, from ~1-2" (produced by galaxy scale dark matter haloes) to >10" (produced by galaxy cluster scale haloes) have long been predicted. Many lenses with Re around 1-2" and above 10" have been reported but very few in between. In this article, we present a sample of 13 strong lensing systems with Re in the range 3"- 8", i.e. systems produced by galaxy group scale dark matter haloes, spanning a redshift range from 0.3 to 0.8. This opens a new window of exploration in the mass spectrum, around 10^{13}- 10^{14} M_{sun}, which is a crucial range for understanding the transition between galaxies and galaxy clusters. Our analysis is based on multi-colour CFHTLS images complemented with HST imaging and ground based spectroscopy. Large scale properties are derived from both the light distribution of the elliptical galaxies group members and weak lensing of the faint background galaxy population. On small scales, the strong lensing analysis yields Einstein radii between 2.5" and 8". On larger scales, the strong lenses coincide with the peak of the light distribution, suggesting that mass is traced by light. Most of the luminosity maps have complicated shapes, indicating that these intermediate mass structures are dynamically young. Fitting the reduced shear with a Singular Isothermal Sphere, we find sigma ~ 500 km/s and an upper limit of ~900 km/s for the whole sample. The mass to light ratio for the sample is found to be M/L_i ~ 250 (solar units, corrected for evolution), with an upper limit of 500. This can be compared to mass to light ratios of small groups (with sigma ~ 300 km/s and galaxy clusters with sigma > 1000 km/s, thus bridging the gap between these mass scales.

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arXiv2008

Probing the Slope of Cluster Mass Profile with Gravitational Einstein Rings: Application to Abell 1689

The strong lensing modelling of gravitational ``rings'' formed around massive galaxies is sensitive to the amplitude of the external shear and convergence produced by nearby mass condensations. In current wide field surveys, it is now possible to find out a large number of rings, typically 10 gravitational rings per square degree. We propose here, to systematically study gravitational rings around galaxy clusters to probe the cluster mass profile beyond the cluster strong lensing regions. For cluster of galaxies with multiple arc systems, we show that rings found at various distances from the cluster centre can improve the modelling by constraining the slope of the cluster mass profile. We outline the principle of the method with simple numerical simulations and we apply it to 3 rings discovered recently in Abell~1689. In particular, the lens modelling of the 3 rings confirms that the cluster is bimodal, and favours a slope of the mass profile steeper than isothermal at a cluster radius $\sim 300 \kpc$. These results are compared with previous lens modelling of Abell~1689 including weak lensing analysis. Because of the difficulty arising from the complex mass distribution in Abell~1689, we argue that the ring method will be better implemented on simpler and relaxed clusters.

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arXiv2007

Combining Strong and Weak Gravitational Lensing in Abell 1689

We present a reconstruction of the mass distribution of galaxy cluster Abell 1689 at z = 0.18 using detected strong lensing features from deep HST/ACS observations and extensive ground based spectroscopy. Earlier analyses have reported up to 32 multiply imaged systems in this cluster, of which only 3 were spectroscopically confirmed. In this work, we present a parametric strong lensing mass reconstruction using 24 multiply imaged systems with newly determined spectroscopic redshifts, which is a major step forward in building a robust mass model. In turn, the new spectroscopic data allows a more secure identification of multiply imaged systems. The resultant mass model enables us to reliably predict the redshifts of additional multiply imaged systems for which no spectra are currently available, and to use the location of these systems to further constrain the mass model. In particular, we have detected 5 strong galaxy-galaxy lensing systems just outside the Einstein ring region, further constraining the mass profile. Our strong lensing mass model is consistent with that inferred from our large scale weak lensing analysis derived using CFH12k wide field images. Thanks to a new method for reliably selecting a well defined background lensed galaxy population, we resolve the discrepancy found between the strong and weak lensing mass models reported in earlier work. [ABRIDGED]

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arXiv2006

An ISOCAM survey through gravitationally lensing galaxy clusters. IV. Luminous infrared galaxies in Cl 0024+1654 and the dynamical status of clusters

Observations of the core of the massive cluster Cl 0024+1654, at a redshift z=0.39, were obtained with the Infrared Space Observatory using ISOCAM at 6.7 mum and 14.3 mum (hereafter 15 mum). Thirty five sources were detected at 15 mum and thirteen of them are spectroscopically identified with cluster galaxies. The remaining sources consist of four stars, one quasar, one foreground galaxy, three background galaxies and thirteen sources with unknown redshift. The ISOCAM sources have best-fit SEDs typical of spiral or starburst models observed 1 Gyr after the main starburst event. The median infrared luminosity of the twelve cluster galaxies is 1.0x10^11 Lsun, with 10 having infrared luminosity above 9x10^10 Lsun, and so lying near or above the 1x10^11 Lsun threshold for identification as a luminous infrared galaxy (LIRG). The [OII] star formation rates obtained for 3 cluster galaxies are one to two orders of magnitude lower than the infrared values, implying that most of the star formation is missed in the optical because it is enshrouded by dust in the starburst galaxy. The counterparts of about half of the 15 mum cluster sources are blue, luminous, star-forming systems and the type of galaxy that is usually associated with the Butcher-Oemler effect. Dust obscuration may be a major cause of the 15 mum sources appearing on the cluster main sequence. The majority of the ISOCAM sources in the Butcher-Oemler region of the colour-magnitude diagram are best fit by spiral-type SEDs whereas post-starburst models are preferred on the main sequence, with the starburst event probably triggered by interaction with one or more galaxies. Finally, the mid-infrared results on Cl 0024+1654 are compared with four other clusters observed with ISOCAM.

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arXiv2006

The CFHTLS Strong Lensing Legacy Survey: I. Survey overview and T0002 release sample

AIMS: We present data from the CFHTLS Strong Lensing Legacy Survey (SL2S). Due to the unsurpassed combined depth, area and image quality of the Canada-France-Hawaii Legacy Survey it is becoming possible to uncover a large, statistically well-defined sample of strong gravitational lenses which spans the dark halo mass spectrum predicted by the concordance model from galaxy to cluster haloes. METHODS: We describe the development of several automated procedures to find strong lenses of various mass regimes in CFHTLS images. RESULTS: The preliminary sample of about 40 strong lensing candidates discovered in the CFHTLS T0002 release, covering an effective field of view of 28 deg$^2$ is presented. These strong lensing systems were discovered using an automated search and consist mainly of gravitational arc systems with splitting angles between 2 and 15 arcsec. This sample shows for the first time that it is possible to uncover a large population of strong lenses from galaxy groups with typical halo masses of about $10^{13}h^{-1}M_\odot$. We discuss the future evolution of the SL2S project and its main scientific aims for the next 3 years, in particular our observational strategy to extract the hundreds of gravitational rings also present in these fields.

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arXiv2005

EDisCS -- the ESO Distant Cluster Survey -- Sample Definition and Optical Photometry

We present the ESO Distant Cluster Survey (EDisCS) a survey of 20 fields containing distant galaxy clusters with redshifts ranging from 0.4 to almost 1.0. Candidate clusters were chosen from among the brightest objects identified in the Las Campanas Distant Cluster Survey, half with estimated redshift z_est~0.5 and half with z_est~0.8. They were confirmed by identifying red sequences in moderately deep two colour data from VLT/FORS2. For confirmed candidates we have assembled deep three-band optical photometry using VLT/FORS2, deep near-infrared photometry in one or two bands using NTT/SOFI, deep optical spectroscopy using VLT/FORS2, wide field imaging in two or three bands using the ESO Wide Field Imager, and HST/ACS mosaic images for 10 of the most distant clusters. This first paper presents our sample and the VLT photometry we have obtained. We present images, colour-magnitude diagrams and richness estimates for our clusters, as well as giving redshifts and positions for the brightest cluster members. Subsequent papers will present our infrared photometry, spectroscopy, HST and wide-field imaging, as well as a wealth of further analysis and science results. Our reduced data become publicly available as these papers are accepted.

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arXiv2004

Mass and Light in the Supercluster of galaxies : MS0302+17

We investigate the supercluster MS0302+17 (z~0.42) using weak lensing analysis and deep wide field CFH12k BVR photometry. Using (B-V) - (V-R) evolution tracks we identify supercluster early-types members. We derive a R band weak lensing background galaxies sample. We compute the correlations functions of light and mass and show that "light traces mass on supercluster scales". The zeta-statistics applied in cluster centers and global correlation analyses over the whole field converge toward the simple relation M/L_B=300+/-30. This independently confirms the earlier results obtained by Kaiser et al.(1998). We model dark matter halos around each galaxy by truncated isothermal spheres and find the linear relation M L still holds. However, their averaged halo truncation radius is s* ~< 200 kpc close to clusters cores, whereas it reaches a lower limit of ~ 300 kpc at the periphery. This change of s_* as function of radial distance gives indications on tidal stripping but the lack of informations on the late-type galaxies sample prevents us to separate contributions. Though all the data at hands are consistent with "mass is traced by light from early-type galaxies", we are not able to describe in details the contributions of late type galaxies. We however found it to be small.

astro-ph↗

arXiv2003

A radial mass profile analysis of the lensing cluster MS2137-23

We reanalyze the strong lens modeling of the cluster of galaxies MS2137-23 using a new data set obtained with the ESO VLT. We found the photometric redshifts of the two main arc systems are both at z=1.6. After subtraction of the central cD star light of the HST image we found that only one object lying underneath has the expected properties of the fifth image associated to the tangential arc. We improve the previous lens modelings of the central dark matter distribution of the cluster, using an isothermal model with a core (IS) and the NFW-like model with a cusp. Without the fifth image, the arc properties together with the shear map profile are equally well fit by the and by an IS and a sub-class of generalized-NFW mass profiles having inner slope power index in the range 0.7<alpha<1.2. Adding new constrains provided by the fifth image favors IS profiles that better predict the fifth image properties. A model including cluster galaxy perturbations or the the stellar mass distribution does not change our conclusions but imposes the M/L_I of the cD stellar component is below 10 at a 99% confidence level. Using our new detailed lensing model together with Chandra X-ray data and the cD stellar component we finally discuss intrinsic properties of the gravitational potential. Whereas X-ray and dark matter have a similar shape at various radius, the cD stellar isophotes are twisted by 13 deg. The sub- arc-second azimuthal shift we observe between the radial arc position and the predictions of elliptical models correspond to what is expected from a mass distribution twist. This shift may result from a projection effect of the cD and the cluster halos, thus revealing the triaxiality of the system.

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arXiv2001

VLT Spectroscopy of Galaxies Lensed by Abell AC114: Implication for the Mass Model and the Study of Low-Luminosity Galaxies at High-Redshift

We present the first results of a spectroscopic survey of faint lensed galaxie s in the core of the galaxy cluster AC114 (z=0.312) obtained from observations w ith the FORS1 spectrograph mounted on the VLT-Antu. The galaxies were chose n in areas close to the high-z critical lines predicted by the gravitational lens model of Natarajan et al (NKSE, 1998) for this cluster, according to both lensing and photometric redshift criteria. All the target galaxies are found to correspond to background galaxies with redshifts values in the [0.7, 3.5] interval. Our spectroscopic observations confirm the predicted lensing redshifts for 3 of the multiple-image galaxies, and together with predictions of the NKSE model led to the discovery of a new 5-image configuration at redshift z=3.347. A revised NKSE model, compatible with the redshift of this new multiple-image system, was generated and employed to calculate the gravitational amplifications of all the observed galaxies. The galaxies corresponding to the multiple-image systems are found to be intrinsically fainter, between 0.5 and 1.5 magnitudes, than the limiting magnitudes of existing blank field studies. When all the observed background galaxies are considered, the resulting intrinsic absolute magnitudes range from M_B~-22 to -19. Therefore, a large gain in sensitivity towards low luminosity high-z objects can actually be obtained, in agreement with theoretical expectations. This method can be used advantageously to probe the high redshift Universe and, in particular, its application to an ensemble of massive cluster cores could constraint the faint end of luminosity function of high redshift galaxies.

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arXiv2001

Cosmic Shear Statistics and Cosmology

We report a measurement of cosmic shear correlations using an effective area of 6.5 sq. deg. of the VIRMOS deep imaging survey in progress at the Canada-France-Hawaii Telescope. We measured various shear correlation functions, the aperture mass statistic and the top-hat smoothed variance of the shear with a detection significance exceeding 12 sigma for each of them. We present results on angular scales from 3 arc-seconds to half a degree. The consistency of different statistical measures is demonstrated and confirms the lensing origin of the signal through tests that rely on the scalar nature of the gravitational potential. For Cold Dark Matter models we find $σ_8 Ω_0^{0.6}=0.43^{+0.04}_{-0.05}$ at the 95% confidence level. The measurement over almost three decades of scale allows to discuss the effect of the shape of the power spectrum on the cosmological parameter estimation. The degeneracy on sigma_8-Omega_0 can be broken if priors on the shape of the linear power spectrum (that can be parameterized by Gamma) are assumed. For instance, with Gamma=0.21 and at the 95% confidence level, we obtain 0.6 0.65 and Omega_0<0.4 for flat (Lambda-CDM) models. From the tangential/radial modes decomposition we can set an upper limit on the intrinsic shape alignment, which was recently suggested as a possible contribution to the lensing signal. Within the error bars, there is no detection of intrinsic shape alignment for scales larger than 1'.

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arXiv2001

Cosmic shear surveys

Gravitational weak shear produced by large-scale structures of the universe induces a correlated ellipticity distribution of distant galaxies. The amplitude and evolution with angular scale of the signal depend on cosmological models and can be inverted in order to constrain the power spectrum and the cosmological parameters. We present our recent analysis of 50 uncorrelated VLT fields and the very first constrains on ($Ω_m,σ_8$) and the nature of primordial fluctuations based on the join analysis of present-day cosmic shear surveys.

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arXiv2000

Terapixel Surveys for Cosmic Shear

The recent detections of cosmic shear signal announced by several groups have demonstrated the feasibility of this challenging program and convinced astro- nomers of its potential for cosmology. Cosmic shear analysis demands to handle Gigabytes of data in order to probe several square degrees in subarcsecond deep imaging mode. The success of these surveys is sensitive to the designs of the observation strategy, the organization of the data reduction pipelines and the links of the data base with surveys like X-ray or spectroscopic follow up. We describe the cosmic shear surveys we have carried out at the VLT and at CFHT and the way we handle this huge data set in a more general context including the VIRMOS and the XMM-LSS surveys, and the future CMB surveys.

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arXiv2000

Cosmic Shear Analysis in 50 Uncorrelated VLT Fields. Implications for Omega_0, sigma_8

We observed with the camera FORS1 on the VLT (UT1, ANTU) 50 randomly selected fields and analyzed the cosmic shear inside circular apertures with diameter ranging from 0.5 to 5.0 arc-minutes. The images were obtained in optimal conditions by using the Service Observing proposed by ESO on the VLT, which enabled us to get a well-defined and homogeneous set of data. The 50 fields cover a 0.65 square-degrees area spread over more than 1000 square-degrees which provides a sample ideal for minimizing the cosmic variance. Using the same techniques as in Van Waerbeke et al. 2000, we measured the cosmic shear signal and investigated the systematics of the VLT sample. We find a significant excess of correlations between galaxy ellipticities on those angular scales. The amplitude and the shape of the correlation as function of angular scale are remarkably similar to those reported so far. Using our combined VLT and CFHT data and adding the results published by other teams we put the first joint constraints on Omega_0 and sigma_8 using cosmic shear surveys. From a deduced average of the redshift of the sources the combined data are consistent with sigma_8\simeq (0.59 \pm 0.03) Omega_0^{-0.47} (for a CDM power spectrum and $Γ=0.21$), in excellent agreement with the cosmological constraints obtained from the local cluster abundance.

astro-ph↗

arXiv2000

Cosmic Shear and Clusters of Galaxies

The first detections of cosmic shear signal reported recently by 4 independent groups cover angular scales between one and 10 arcmin. On those scales, the cosmic shear is a signature of non-linear perturbations, like groups and clusters of galaxies. We present the results obtained by our team on CFHT and on the VLT and discuss its impact for the analysis of cluster abundances and cosmology

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arXiv2000

Cosmic Shear with the CFHT

We present results of our cosmic shear survey in progress at the Canada- France-Hawaii Telescope (CFHT). We analysed 1.7 sq. degrees of data, out of which we measured a significant correlation of galaxy shape-orientation over several arcmin scale. We present measurements of the variance of the shear $<γ^2>$ and of the correlation functions $ $, $ $. The signal is consistent with gravitational lensing by large scale structures predictions. The level of residual systematics after PSF correction is small compared to our measured signal. We outline several evolutions of our work by using additional data covering 9 sq. degrees.

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