Deep Imaging of AX J2019+112: The Luminosity of a “Dark Cluster”

We detect a distant cluster of galaxies centered on the QSO lens and luminousX-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 redsequence of galaxies is identified within a radius of 0.2 h^{-1} Mpc of theknown z=1.01 elliptical lensing galaxy. The sequence, which includes thecentral elliptical galaxy, has a slope in good agreement with the modelpredictions of Kodama et al (1998) for z~1. We estimate the integratedrest-frame luminosity of the cluster to be L_V > 3.2 x 10^{11}h^{-2}L_{\sun}(after accounting for significant extinction at the low latitude of thisfield), more than an order of magnitude higher than previous estimates. Thecentral region of the cluster is deconvolved using the technique of Magain,Courbin & Sohy (1998), revealing a thick central arc coincident with anextended radio source. All the observed lensing features are readily explainedby differential magnification of a radio loud AGN by a shallow ellipticalpotential. The QSO must lie just outside the diamond caustic, producing twoimages, and the arc is a highly magnified image formed from a region close tothe center of the host galaxy, projecting inside the caustic. Themass--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 lensmodel yields a compatible value, M/L_V= 372^{+94}_{-94}h(M/L_V)_{\sun}, whereasan independent weak lensing analysis sets an upper limit of M/L_V <520h(M/L_V)_{\sun}, typical of massive clusters.