Black hole mass scaling relations suggest that extremely massive black holes (EMBHs) with $M_mathrm{BH}ge10^{9.4},M_{odot}$ are found in the most massive galaxies with $M_mathrm{star}ge10^{11.6},M_{odot}$, which are commonly found in dense environments, like galaxy clusters. Therefore, one can expect that there is a close connection between active EMBHs and dense environments. Here, we study the environments of 9461 galaxies and 2943 quasars at $0.24 le z le 0.40$, among which 52 are extremely massive quasars with $log(M_mathrm{BH}/M_{odot}) ge 9.4$, using Sloan Digital Sky Survey and MMT Hectospec data. We find that, on average, both massive quasars and massive galaxies reside in environments more than $sim2$ times as dense as those of their less massive counterparts with $log(M_mathrm{BH}/M_{odot}) le 9.0$. However, massive quasars reside in environments about half as dense as inactive galaxies with $log(M_mathrm{BH}/M_{odot}) ge 9.4$, and only about one third of massive quasars are found in galaxy clusters, while about two thirds of massive galaxies reside in such clusters. This indicates that massive galaxies are a much better signpost for galaxy clusters than massive quasars. The prevalence of massive quasars in moderate to low density environments is puzzling, considering that several simulation results show that these quasars appear to prefer dense environments. Several possible reasons for this discrepancy are discussed, although further investigation is needed to obtain a definite explanation.
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