Observations Strong gravitational lensing
a light source passes behind gravitational lens (point mass placed in center of image). aqua circle source seen if there no lens, white spots multiple images of source.
the foreground lens galaxy. when background source quasar or resolved jet, strong lensed images point-like multiple images; when background source galaxy or extended jet emission, strong lensed images can arcs or rings. of 2017, several hundred galaxy-galaxy (g-g) strong lenses have been observed. upcoming large synoptic survey telescope , euclid surveys expected discover more 100,000 such objects.
cluster lensing
the foreground lens galaxy cluster. in case, lens powerful enough produce noticeable both strong lensing (multiple images, arcs or rings) , weak lensing effects (ellipticity distortions).
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