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and Cosmology

Extragalactic Astronomy and Cosmology: An Introduction

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6.5 Clusters of Galaxies as Gravitational Lenses<br />

267<br />

Fig. 6.38. Analysis of the cluster of galaxies MS 1054−03 by<br />

the weak lensing effect. In the upper left panel, a ground-based<br />

image is shown with a field size of 7. ′ 5 × 7. ′ 5. In this image,<br />

about 2400 faint objects are detected, the majority of which<br />

are galaxies at high redshift. From the measured ellipticities of<br />

the galaxies, the tidal field of the cluster can be reconstructed,<br />

<strong>and</strong> from this the projected mass distribution Σ(θ),presented<br />

in the lower left panel; the latter is indicated by the black<br />

contours, while the white contours represent the smoothed<br />

light distribution of the cluster galaxies. A mosaic of HST<br />

images allows the ellipticity measurement of a significantly<br />

larger number of galaxies, <strong>and</strong> with better accuracy. The tidal<br />

field resulting from these measurements is displayed in the<br />

upper right panel, with the reconstructed surface mass density<br />

shown in the lower right panel. One can clearly see that the<br />

cluster is strongly structured, with three density maxima which<br />

correspond to regions with bright cluster galaxies. This cluster<br />

seems to be currently in the process of formation through<br />

a merger of smaller entities

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