We detect a distant cluster of galaxies centered on the QSO lens and luminous X-ray source AXJ2019+112 , a.k.a .
“ The Dark Cluster ” ( Hattori et al 1997 ) .
Using deep V,I~ { } Keck images and wide-field K _ { s } imaging from the NTT , a tight red sequence of galaxies is identified within a radius of 0.2 h ^ { -1 } Mpc of the known z = 1.01 elliptical lensing galaxy .
The sequence , which includes the central elliptical galaxy , has a slope in good agreement with the model predictions of Kodama et al ( 1998 ) for z \sim 1 .
We estimate the integrated rest-frame luminosity of the cluster to be L _ { V } \geq 3.2 \times 10 ^ { 11 } h ^ { -2 } L _ { \sun } ( after accounting for significant extinction at the low latitude of this field ) , an order of magnitude higher than previous estimates .
The central region of the cluster is deconvolved using the technique of Magain , Courbin & Sohy ( 1998 ) , revealing a thick central arc coincident with an extended radio source .
All the observed lensing features are readily explained by differential magnification of a radio loud AGN by a shallow elliptical potential .
The QSO must lie just outside the diamond caustic , producing two images , and the arc is a highly magnified image formed from a region close to the center of the host galaxy , projecting inside the caustic .
The mass–to–light ratio within an aperture of 0.4 h ^ { -1 } Mpc is M _ { x } / L _ { V } = 224 ^ { +112 } _ { -78 } h ( M / L _ { V } ) _ { \sun } , using the X-ray temperature .
The strong lens model yields a compatible value , M / L _ { V } = 372 ^ { +94 } _ { -94 } h ( M / L _ { V } ) _ { \sun } , whereas an independent weak lensing analysis sets an upper limit of M / L _ { V } < 520 h ( M / L _ { V } ) _ { \sun } , typical of massive clusters .