338 research outputs found
Properties of high-z galaxies as seen through lensing clusters
We discuss the first results obtained on the study of a sample of high-z
galaxies (2 < z < 7), using the gravitational amplification effect in the core
of lensing clusters. Sources are located close to the critical lines in
clusters with well constrained mass distributions, and selected through
photometric redshifts, computed on a large wavelength domain, and lens
inversion techniques.Comment: 5 pages, 3 figures, Conference Proceedings of the "Clustering at High
Redshift" Conference, June 29 to July 2, 1999, Marseille (France
Spectroscopic follow up of arclets in AC114 with the VLT
We present the first results on the VLT/FORS-1 spectroscopic survey of
amplified sources and multiple images in the lensing cluster AC114. Background
sources were selected in the cluster core, close to the critical lines, using
photometric redshifts combined with lensing inversion criteria. Spectroscopic
results are given, together with a brief summary of the properties of some of
these high-z galaxies.Comment: 4pages. To appear in the Proceedings of the XXth Moriond Astrophysics
Meeting "Cosmological Physics with Gravitational Lensing", eds. J.-P. Kneib,
Y. Mellier, M. Moniez and J. Tran Thanh Van, Les Arcs, France, March
11th-18th 200
Oblique Convergence in the Himalayas of Western Nepal Deduced from Preliminary Results of GPS Measurements
A GPS network consisting of 29 sites was installed in central and western Nepal, with measurements taken in 1995 and partial remeasurements in 1997. Data suggest 15 +/â5 mm/yr of N180° convergence between the Higher Himalayas and India, a result that is consistent with NâS shortening across the arcuate shape of the Nepalese Himalayas and an oblique underthrusting of the Indian crust below the High Himalayas of western Nepal. A 4 +/â3 mm/year EâW extension and deviation of the principal shortening axes are inferred east of 83°E, where Quaternary faults (DarmaâBari Gad fault system and Thakkhola graben) delineate a crustal wedge. This wedge is located on the SE projection of the Karakorum fault and may segment the Himalayan thrust belt. The convergence between the outer belt of western Nepal and India is less than 3 mm/yr, an attenuation consistent with creep on a dislocation locked beneath the Lesser Himalayas. A preliminary model suggests that this N 120°E striking dislocation is affected by a 19 mm/yr thrust component and a 7 mm/yr right lateral component
Detection of correlated galaxy ellipticities on CFHT data: first evidence for gravitational lensing by large-scale structures
We report the detection of a significant (5.5 sigma) excess of correlations
between galaxy ellipticities at scales ranging from 0.5 to 3.5 arc-minutes.
This detection of a gravitational lensing signal by large-scale structure was
made using a composite high quality imaging survey of 6300 arcmin^2 obtained at
the Canada France Hawaii Telescope (CFHT) with the UH8K and CFH12K panoramic
CCD cameras. The amplitude of the excess correlation is 2.2\pm 0.2 % at 1
arcmin scale, in agreement with theoretical predictions of the lensing effect
induced by large-scale structure.We provide a quantitative analysis of
systematics which could contribute to the signal and show that the net effect
is small and can be corrected for. We show that the measured ellipticity
correlations behave as expected for a gravitational shear signal. The
relatively small size of our survey precludes tight constraints on cosmological
models. However the data are in favor of cluster normalized cosmological
models, and marginally reject Cold Dark Matter models with (Omega=0.3,
sigma_8<0.6) or (Omega=1, sigma_8=1). The detection of cosmic shear
demonstrates the technical feasibility of using weak lensing surveys to measure
dark matter clustering and the potential for cosmological parameter
measurements, in particular with upcoming wide field CCD cameras.Comment: 19 pages. 19 Figures. Revised version accepted in A&
Weak lensing analysis of MS 1008-1224 with the VLT
We present a gravitational lensing analysis of the cluster of galaxies MS
1008-1224 (z=0.30), based on very deep observations obtained using the VLT with
FORS and ISAAC during the science verification phase. We reconstructed the
projected mass distribution from B,V,R,I bands using two different methods
independently. The mass maps are remarkably similar, which confirm that the PSF
correction worked well. The ISAAC and FORS data were combined to measure the
photometric redshifts and constrain the redshift distribution of the lensed
sources. The total mass inferred from weak shear is 2.3 10^{14} h^{-1} Mo on
large scales, in agreement with the X-ray mass. The measured mass profile is
well fit by both Navarro, Frenk and White and isothermal sphere with core
radius models although the NFW is slightly better. In the inner regions, the
lensing mass is about 2 times higher than the X-ray mass, which supports the
view that complex physical processes in the innermost parts of clusters are
responsible for the X-ray/lensing mass discrepancy. The central part of the
cluster is composed of two mass peaks whose the center of mass is 15 arcsecond
north of the cD galaxy. This provides an explanation for the 15 arcsecond
offset between the cD and the center of the X-ray map reported elsewhere. The
optical, X-ray and the mass distributions show that MS 1008-1224 is composed of
many subsystems which are probably undergoing a merger. MS 1008-1224 shows a
remarkable case of cluster-cluster lensing. The photometric redshifts show an
excess of galaxies located 30 arcseconds south-west of the cD galaxy at a
redshift of about 0.9 which is lensed by MS 1008-1224. These results show the
importance of getting BVRIJK images silmultenously. The VLT is a unique tool to
provide such datasets.Comment: 22 pages, submitted to A&A, paper with `big' figures available at
ftp://ftp.cita.utoronto.ca/pub/waerbeke/ms1008paper
Orogen-parallel deformation of the Himalayan mid-crust: Insights from structural and magnetic fabric analyses of the Greater Himalayan Sequence, Annapurna-Dhaulagiri Himalaya, central Nepal
The metamorphic core of the Himalaya (Greater Himalayan Sequence, GHS), in the Annapurna-Dhaulagiri region, central Nepal, recorded orogen-parallel stretching during midcrustal evolution. Anisotropy of magnetic susceptibility and field-based structural analyses suggest that midcrustal deformation of the amphibolite facies core of the GHS occurred under an oblate/suboblate strain regime with associated formation of low-angle northward dipping foliation. Magnetic and mineral stretching lineations lying within this foliation from the top of the GHS record right-lateral orogen-parallel stretching. We propose that oblate strain within a midcrustal flow accommodated oblique convergence between India and the arcuate orogenic front without the need for strain partitioning in the upper crust. Oblate flattening may have also promoted orogen-parallel melt migration and development of melt-depleted regions between km3 scale leucogranite culminations at ~50â100âkm intervals along orogen strike. Following the cessation of flow, continued oblique convergence led to upper crustal strain partitioning between orogen-perpendicular convergence on thrust faults and orogen-parallel extension on normal and strike-slip faults. In the Annapurna-Dhaulagiri Himalaya, orogen-parallel stretching lineations are interpreted as a record of transition from midcrustal orogen-perpendicular extrusion to upper crustal orogen-parallel stretching. Our findings suggest that midcrustal flow and upper crustal extension could not be maintained simultaneously and support other studies from across the Himalaya, which propose an orogen-wide transition from midcrustal orogen-perpendicular extrusion to upper crustal orogen-parallel extension during the mid-Miocene. The 3-D nature of oblate strain and orogen-parallel stretching cannot be replicated by 2-D numerical simulations of the Himalayan orogen
Thermokinematic evolution of the Annapurna-Dhaulagiri Himalaya, central Nepal: The composite orogenic system
The Himalayan orogen represents a ââComposite Orogenic Systemââ in which channel flow, wedge extrusion, and thrust stacking operate in separate ââOrogenic Domainsââ with distinct rheologies and crustal positions. We analyze 104 samples from the metamorphic core (Greater Himalayan Sequence, GHS) and bounding units of the Annapurna-Dhaulagiri Himalaya, central Nepal. Optical microscopy and electron backscatter diffraction (EBSD) analyses provide a record of deformation microstructures and an indication of active crystal slip systems, strain geometries, and deformation temperatures. These data, combined with existing thermobarometry and geochronology data are used to construct detailed deformation temperature profiles for the GHS. The profiles define a three-stage thermokinematic evolution from midcrustal channel flow (Stage 1, >7008C to 550â6508C), to rigid wedge extrusion (Stage 2, 400â6008C) and duplexing (Stage 3, <280â4008C). These tectonic processes are not mutually exclusive, but are confined to separate rheologically distinct Orogenic Domains that form the modular components of a Composite Orogenic System. These Orogenic Domains may be active at the same time at different depths/positions within the orogen. The thermokinematic evolution of the Annapurna-Dhaulagiri Himalaya describes the migration of the GHS through these Orogenic Domains and reflects the spatial and temporal variability in rheological boundary conditions that govern orogenic systems
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 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 . 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.Comment: 11 pages, 5 figures, A&A in pres
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