Spitzer -selected z > 1.3 protocluster candidates in the LSST Deep Drilling Fields

Abstract

We have identified 189 candidate z > 1.3 protoclustersãnd clusters in the LSST Deep Drilling Fields. This sample will enable the measurement of the metal enrichmentãnd star formation history of clusters during their earlyãssembly period through the direct measurement of the rate of supernovae identified through the LSST. The protocluster sample was selected from galaxy o v erdensities inã Spitzer /IRAC colour-selected sample using criteria that were optimized for protocluster purity usingã realistic light-cone. Our tests reveal that 60 -80 per cent of the identified candidatesãre likely to be genuine protoclusters or clusters, which is corroborated byã ∼4 σstacked X-ray signal from these structures. We provide photometric redshift estimates for 47 candidates which exhibit strong peaks in the photo- z distribution of their candidate members. Ho we ver, the lack ofã photo- z peak does not meanã candidate is not genuine, since we findã stacked X-ray signal of similar significance from both the candidates that exhibit photo- z peaksãnd those that do not. Tests on the light-cone reveal that our pursuit ofã pure sample of protoclusters results in that sample being highly incomplete ( ∼4 per cent )ãnd heavily biased towards larger , richer , more massive,ãnd more centrally concentrated protoclusters than the total protocluster population. Most ( ∼75 per cent ) of the selected protoclustersãre likely to haveã maximum collapsed halo mass of between 10 13ãnd 10 14 M⊙, with only ∼25 per cent likely to be collapsed clustersãbo v e 10 14 M⊙. Ho we ver, theãforementioned bias ensures our sample is ∼50 per cent complete for structures that haveãlready collapsed into clusters more massive than 10 14 M⊙.