More connected, more active: galaxy clusters and groups at z~1 and the
  connection between their quiescent galaxy fractions and large-scale
  environments
  High-redshift galaxy clusters, unlike local counterparts, show diverse star
formation activities. However, it is still unclear what keeps some of the
high-redshift clusters active in star formation. To address this issue, we
performed a multi-object spectroscopic (MOS) observation of 226 high-redshift
(0.8 < z < 1.3) galaxies in galaxy cluster candidates and the areas surrounding
them. Our spectroscopic observation reveals six to eight clusters/groups at z ~
0.9 and z ~ 1.3. The redshift measurements demonstrate the reliability of our
photometric redshift measurements, which in turn gives credibility for using
photometric redshift members for the analysis of large-scale structures (LSSs).
Our investigation of the large-scale environment (~10 Mpc) surrounding each
galaxy cluster reveals LSSs --- structures up to ~10 Mpc scale --- around many
of, but not all, the confirmed overdensities and the cluster candidates. We
investigate the correlation between quiescent galaxy fraction of galaxy
overdensities and their surrounding LSSs, with a larger sample of ~ 20
overdensities including photometrically selected overdensities at 0.6 < z <
0.9. Interestingly, galaxy overdensities embedded within these extended LSSs
show a lower fraction of quiescent galaxies (~ 20 %) than isolated ones at
similar redshifts (with a quiescent galaxy fraction of ~ 50 %). Furthermore, we
find a possible indication that clusters/groups with a high quiescent galaxy
fraction are more centrally concentrated. Based on these results, we suggest
that LSSs are the main reservoirs of gas and star-forming galaxies to keep
galaxy clusters fresh and extended in size at z ~ 1.
