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

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Abstract
We detect a distant cluster of galaxies centered on the QSO lens and luminous X-ray source AX J2019+112, a.k.a. the "Dark Cluster." Using deep V and I Keck images and wide-field Ks imaging from the New Technology Telescope (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. for z ~ 1. We estimate the integrated rest-frame luminosity of the cluster to be LV ≥ 3.2 × 1011 h-2 L☉ (after accounting for significant extinction at the low latitude of this field), more than an order of magnitude higher than previous estimates. The central region of the cluster is deconvolved using the technique of Magain, Courbin, & Sohy, 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 active galactic nucleus by a shallow elliptical potential. The QSO must lie just outside the diamond caustic, producing two images; 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 MX/LV = 224h(M/LV)☉, using the X-ray temperature. The strong lens model yields a compatible value, M/LV = 372h(M/LV)☉, whereas an independent weak-lensing analysis sets an upper limit of M/LV < 520h(M/LV)☉, typical of massive clusters.
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