Are z~5 QSOs found in the most massive high redshift over-densities?

Abstract

Luminous high-redshift QSOs are thought to exist within the most massive dark matter haloes in the young Universe. As a consequence they are likely to be markers for biased, over-dense regions where early galaxies cluster, regions that eventually grow into the groups and clusters seen in the lower redshift universe. In this paper we explore the clustering of galaxies around z ~ 5 QSOs as traced by Lyman Break Galaxies (LBGs). We target the fields of three QSOs using the same optical imaging and spectroscopy techniques used in the ESO Remote Galaxy Survey (ERGS, Douglas et al. 2009, 2010), which was successful in identifying individual clustered structures of LBGs. We use the statistics of the redshift clustering in ERGS to show that two of the three fields show significant clustering of LBGs at the QSO redshifts. Neither of these fields is obviously over-dense in LBGs from the imaging alone; a possible reason why previous imaging-only studies of high-redshift QSO environments have given ambiguous results. This result shows that luminous QSOs at z ~ 5 are typically found in over-dense regions. The richest QSO field contains at least nine spectroscopically confirmed objects at the same redshift including the QSO itself, seven LBGs and a second fainter QSO. While this is a very strong observational signal of clustering at z ~ 5, it is of similar strength to that seen in two structures identified in the 'blank sky' ERGS fields. This indicates that, while over-dense, the QSO environments are not more extreme than other structures that can be identified at these redshifts. The three richest structures discovered in this work and in ERGS have properties consistent with that expected for proto-clusters and likely represent the early stages in the build-up of massive current-day groups and clusters.