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I. K. Soechting, R. G. Clowes (University of Central Lancashire, UK), L. E. Campusano (Universidad de Chile)
We investigate how quasars trace the LSS in the form of galaxy clusters and study the relative positions and morphologies of clusters associated with quasars. Our cluster detection method is based on a maximum likelihood estimator applied to Voronoi cells and supported by colour discrimination. Here, we present the results for 60 low redshift radio-quiet quasars (z < 0.3) in four different areas of sky. By expressing our findings in the form of the distance of a quasar to the centre of the closest galaxy cluster at comparable redshift (zQSO +/- 0.05), we can conclude the following. (i) ~17% of RQQs reside within a galaxy cluster (distance <1 h-1Mpc), with 70% of these residing between two, possibly merging, clusters; in the remaining 30% the morphologies and colour distributions of the host clusters indicate subcluster/cluster mergers. (ii) ~46% of RQQs occur on the peripheries of galaxy clusters (1-3 h-1Mpc). (iii) The substantial fraction of RQQs (37%) that appear to occur in relatively isolated environments is very probably inflated because members of this class are often near bright stars which we can expect to be obscuring clusters. These results allow a better interpretation of the richness studies performed on smaller scales (usually <1 h-1Mpc) around quasars. The very rich environments indicate objects formed in the confluence of cluster mergers, possible suggesting gas-galaxy interactions as a previously unrecognised mechanism of quasar formation. During a cluster merger the relative velocities of the galaxies are expected to be too high for major galaxy mergers to be an efficient quasar formation mechanism. For the quasars found on the distant peripheries of galaxy clusters or in isolated environments we suggest that they are more likely to have formed in major galaxy mergers due to the availability of gas-rich spirals and their small relative velocities. The peripheral position of quasars with respect to galaxy clusters indicate that galaxy harassment may indeed be an important quasar formation/fuelling mechanism. IKS would like to acknowledge the research bursary of the University of Central Lancashire, UK.
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Bulletin of the American Astronomical Society, 34
© 2002. The American Astronomical Soceity.