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The galaxy cluster IDCS J1426.5+3508 at z = 1.75 is the most massive galaxycluster yet discovered at z > 1.4 and the first cluster at this epoch for whichthe Sunyaev-Zel'Dovich effect has been observed. In this paper we report on thediscovery with HST imaging of a giant arc associated with this cluster. Thecurvature of the arc suggests that the lensing mass is nearly coincident withthe brightest cluster galaxy, and the color is consistent with the arc being astar-forming galaxy. We compare the constraint on M200 based upon stronglensing with Sunyaev-Zel'Dovich results, finding that the two are consistent ifthe redshift of the arc is z > 3. Finally, we explore the cosmologicalimplications of this system, considering the likelihood of the existence of astrongly lensing galaxy cluster at this epoch in an LCDM universe. While theexistence of the cluster itself can potentially be accomodated if one considersthe entire volume covered at this redshift by all current high-redshift clustersurveys, the existence of this strongly lensed galaxy greatly exacerbates thelong-standing giant arc problem. For standard LCDM structure formation andobserved background field galaxy counts this lens system should not exist.Specifically, there should be no giant arcs in the entire sky as bright inF814W as the observed arc for clusters at z \geq 1.75, and only \sim 0.3 asbright in F160W as the observed arc. If we relax the redshift constraint toconsider all clusters at z \geq 1.5, the expected number of giant arcs rises to\sim15 in F160W, but the number of giant arcs of this brightness in F814Wremains zero. These arc statistic results are independent of the mass of IDCSJ1426.5+3508. We consider possible explanations for this discrepancy.

IDCS J1426.5+3508: Cosmological implications of a massive, strong lensing cluster at Z = 1.75