Are massive clusters always X-ray luminousquestionMark

Studies of galaxy clusters are important for both cosmology and the formation ofbaryonic structures. An important relation in cluster science is the f_gas Mrelation. However, the cluster samples used to constrain this relation are Xrayselected and the current XMM and Chandra archives are also heavily biased toXray selected clusters. To address this important bias, we construct a sampleof 11 most massive clusters from the SDSSC4 cluster catalog. Six of them havebeen observed by XMM or Chandra but they are all Xray luminous, while the fiveunobserved clusters are Xray faint. We propose to observe these five clustersto search for Xray faint, massive cluster, with potentially importantimplications on the cluster mass proxy relations. truncated!, Please see actual data for full text [truncated!, Please see actual data for full text]