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Studying very high redshift Galaxies through Gravitational Telescopes

Final Report Summary - HIGHZLENS (Studying very high redshift Galaxies through Gravitational Telescopes)

The research carried out during this project covers different aspects of observational cosmology and physics of galaxies, taking benefit of massive clusters used as gravitational telescopes. Gravitational magnification allows us to probe more distant or intrinsically fainter objects, enabling us to do the kind of research that will only be achievable with the ELTs, but using 8-10 m class telescopes and their current or soon-forthcoming instruments.
The project was divided in two parts. First, it was necessary to perform a spectroscopic survey of multiple images in order to do the modelling of the mass distribution of the lensing clusters. This provided the sample of distant galaxies over which the science can be performed, and also allow to correct the results for the gravitational lensing effect. The second part of the project studies the lensed background galaxies in great details, and use this sample to infer the evolution in the physical parameters of distant objects.

1 - Modelling of lensing clusters

In order to use lensing clusters as gravitational telescopes to study background galaxies, it is necessary to model the cluster mass distribution accurately. This is done through the identification of multiple images that constrain the lensing potential, as well as their redshift measurements with spectroscopy. Indeed, gravitational lensing allows a direct measurement of the mass distribution of intervening structures over a wide range of spatial scales and, unlike X-ray studies, it does so without any additional assumptions about their baryonic content or dynamical state. During the course of the project, the researcher analyzed spectroscopic data he obtained at Keck, VLT and Magellan telescopes to measure redshifts for multiple imaged systems found in strong lensing clusters, and performed their mass modelling using the Lenstool lensing software. The results of this modelling are the following:

• Clusters showing a large (∼ 10 or more) number of multiple systems (e.g. Abell 1703: Richard et al. 2009 ; Abell 370: Richard et al. 2010a). The redshift measurements obtained allowed to set precise constraints on the location and geometrical shape of the critical line at that redshift, giving a much higher precision when deriving the magnification factors of new lensed sources. In the near future, we shall be able to perform cosmography with such clusters: using many multiply imaged systems at distinct redshift planes, strong lensing would allow to derive purely geometric constraints on cosmological parameters. The feasibility of this technique has been demonstrated in simulations (Gilmore & Natarajan 2009) and we recently applied it to HST observations of Abell 1689 (Jullo et al. 2010, Science).

• Large samples of strong lensing clusters (Richard et al. 2010b). The researcher led a large programme of multi-object spectroscopy at Keck with LRIS for two samples of 0.2 < z < 0.5 strong lensing clusters, MACS (MAssive Cluster Survey, PI: Ebeling) and LoCuSS (Local Cluster Substructure Survey, PI: Smith), selected out of snapshot images with Hubble. So far, the main publication concentrated on the statistical analysis of a sample of 20 z ∼ 0.2 LoCuSS clusters for which parameterized models of the mass distribution in the cluster cores have been produced.

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