Quasar formation in the context of large scale structure
We present the results of an investigation of environments of quasars in the context of the large-scale structure in galaxy clusters. Our results are somewhat different from what was previously thought - a direct consequence of this new perspective and more powerful methods of analysis. In contrast with previous studies, which relied on the galaxy excess statistics within small radii around the quasars, we apply a robust cluster-finding algorithm, which allows boundary identification and redshift estimation. It is a semi-parametric method based on a maximum likelihood estimate applied to Voronoi tessellation and enhanced by a colour-cut approach. We find, using multiple sets of quasars, that low redshift quasars follow the large scale structure (LSS) traced by galaxy clusters. In contrast, the narrow emission line galaxies (NELGs) are found to be extremely weak tracers of the LSS when investigated through the same procedure. We find that most of the quasars reside within 3h 1 Mpc from the centre of a galaxy cluster with comparable redshift, but only about 20% of quasars are found in the cluster central regions (< 1h' Mpc). No significant difference has been noticed between the environments of radio-loud and radio-quiet quasars. Using published properties of the host galaxies of a subset of our quasars, we find that all of the RQQs that are found in galaxy clusters reside in elliptical host galaxies, whereas those on the peripheries reside in spiral hosts. About 20% of investigated quasars reside between two galaxy clusters, which are possibly at an early stage of merger. Of these quasars, those that are in the subset with host galaxy data all reside in very large disrupted elliptical hosts suggesting mergers of bright galaxies. In contrast, the host galaxies of quasars found on the peripheries of single clusters are less disrupted and often relatively compact. We discuss the results in the context of existing quasar formation models and conclude that at least two formation mechanisms co-exist. In cluster-cluster mergers quasars are most probably formed by galaxy-galaxy mergers, a formation mechanism pro-posed by Kauffmann & Haehnelt (2000). Quasar formation on the peripheries of single galaxy clusters suggests galaxy harassment (Lake, Katz & Moore 1998) as a possible formation mechanism. In the first step to extend our investigations to higher redshifts, a large and deep cluster catalogue has been produced from Faint Sky Variability Survey. Plans for future work based on this survey are outlined.
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