The central image of a gravitationally lensed quasar

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Abstract
A galaxy can act as a gravitational lens, producing multiple images of a background object. Theory predicts that there should be an odd number of images produced by the lens1,2, but hitherto almost all lensed objects have two or four images. The missing ‘central’ images, which should be faint and appear near the centre of the lensing galaxy, have long been sought as probes of galactic cores too distant to resolve with ordinary observations3,4,5,6,7. There are five candidates for central images, but in one case the third image is not necessarily the central one8,9,10, and in the others the putative central images might be foreground sources11,12,13,14,15. Here we report a secure identification of a central image, based on radio observations of one of the candidates14. Lens models using the central image reveal that the massive black hole at the centre of the lensing galaxy has a mass of 20,000M⊙ pc-2, which is in agreement with expections based on observations of galaxies that are much closer to the Earth.