IDCS J1426.5+3508: Cosmological implications of a massive, strong
  lensing cluster at Z = 1.75
  The galaxy cluster IDCS J1426.5+3508 at z = 1.75 is the most massive galaxy
cluster yet discovered at z > 1.4 and the first cluster at this epoch for which
the Sunyaev-Zel'Dovich effect has been observed. In this paper we report on the
discovery with HST imaging of a giant arc associated with this cluster. The
curvature of the arc suggests that the lensing mass is nearly coincident with
the brightest cluster galaxy, and the color is consistent with the arc being a
star-forming galaxy. We compare the constraint on M200 based upon strong
lensing with Sunyaev-Zel'Dovich results, finding that the two are consistent if
the redshift of the arc is z > 3. Finally, we explore the cosmological
implications of this system, considering the likelihood of the existence of a
strongly lensing galaxy cluster at this epoch in an LCDM universe. While the
existence of the cluster itself can potentially be accomodated if one considers
the entire volume covered at this redshift by all current high-redshift cluster
surveys, the existence of this strongly lensed galaxy greatly exacerbates the
long-standing giant arc problem. For standard LCDM structure formation and
observed background field galaxy counts this lens system should not exist.
Specifically, there should be no giant arcs in the entire sky as bright in
F814W as the observed arc for clusters at z \geq 1.75, and only \sim 0.3 as
bright in F160W as the observed arc. If we relax the redshift constraint to
consider 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 F814W
remains zero. These arc statistic results are independent of the mass of IDCS
J1426.5+3508. We consider possible explanations for this discrepancy.
