Do blue galaxy-clusters have hot intracluster gas?
  We present herein a systematic X-ray analysis of blue galaxy-clusters at
$z=0.84$ discovered by the Subaru telescope. The sample consisted of 43
clusters identified by combining red-sequence and blue-cloud surveys, covering
a wide range of emitter fractions (i.e., 0.3--0.8). The spatial extent of the
over-density region of emitter galaxies was approximately 1~Mpc in radius. The
average cluster mass was estimated as $0.6 (<1.5)\times10^{14}~{\rm M_\odot}$
from the stacked weak-lensing measurement. We analyzed the XMM-Newton archival
data, and measured the X-ray luminosity of the hot intracluster medium. As a
result, diffuse X-ray emission was marginally detected in 14 clusters, yielding
an average luminosity of $5\times 10^{42}~{\rm erg\,s^{-1}}$. On the contrary,
it was not significant in 29 clusters. The blue clusters were significantly
fainter than the red-dominated clusters, and the X-ray luminosity did not show
any meaningful correlation either with emitter fraction or richness. The X-ray
surface brightness was low, but the amount of gas mass was estimated to be
comparable to that observed in the $10^{13-14}~{\rm M_{\odot}}$ cluster. Based
on the results, we suggest that the blue clusters are at the early formation
stage, and the gas is yet to be compressed and heated up to produce appreciable
X-rays. Follow-up spectroscopic measurements are essential to clarify the
dynamical status and co-evolution of galaxies and hot gas in the blue clusters.
