A Spectroscopically Confirmed X-ray Cluster at z = 1.62 with a Possible Companion in the Subaru/XMM-Newton Deep Field

We report on a confirmed galaxy cluster at z = 1.62. We discovered two concentrations of galaxies at z ~ 1.6 in the Subaru/XMM-Newton deep field based on deep multi-band photometric data. We made a near-IR spectroscopic follow-up observation of them and confirmed several massive galaxies at z = 1.62. One of the two is associated with an extended X-ray emission at 4.5σ on a scale of 0farcm5, which is typical of high-z clusters. The X-ray detection suggests that it is a gravitationally bound system. The other one shows a hint of an X-ray signal, but only at 1.5σ, and we obtained only one secure redshift at z = 1.62. We are not yet sure if this is a collapsed system. The possible twins exhibit a clear red sequence at K < 22 and seem to host relatively few number of faint red galaxies. Massive red galaxies are likely old galaxies—they have colors consistent with the formation redshift of z_f = 3, and a spectral fit of the brightest confirmed member yields an age of 1.8^+0.1 _-0.2 Gyr with a mass of 2.5^+0.2 _-0.1 × 10¹¹ M _sun. Our results show that it is feasible to detect clusters at z > 1.5 in X-rays and also to perform detailed analysis of galaxies in them with the existing near-IR facilities on large telescopes.