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R. G. Clowes, C. P. Haines, I. K. Machura (Univ. Central Lancashire), L. E. Campusano (Univ. de Chile)
(1.) The Clowes & Campusano (MNRAS 249, 218 [1991]) LQG is a group of at least 18 quasars at z ~1.3, with size ~200h-1 Mpc. In 2.25' \times 2.25' centred on the LQG quasar J1046.9+0541, we have obtained photometric redshift estimates of galaxies from VIK imaging to V ~ 27, I ~26, K ~20. We find 12--15 galaxies with the colours (V-K ~6.9, I-K ~4.3) and magnitudes expected for massive ellipticals at the quasar redshift that formed early (z > 3) and then evolved passively. These galaxies are concentrated in two groups, 50'' north and east of the quasar. Within 30'' of the quasar is a concentration of blue (V-I < 1) galaxies in a band of enhanced star-formation, bisecting the two groups of elliptical galaxies. We interpret these distributions of blue and elliptical galaxies as resulting from the early stages of a cluster merger that has triggered both the quasar and the star-formation through gas-galaxy interactions. See Haines et al. (MNRAS, submitted) for full details.
(2.) We have looked at the environment of 16 low-redshift quasars (z < 0.4) across 25 deg2 in the context of the LSS in galaxy clusters. The LSS is recognised as the high density peaks (\ge 3\sigma) in the faintest objects from digitised UKST BJ plates. We find the following (in 2D). (i) Quasars trace the LSS in the clusters. (ii) Quasars lie close to the clusters but not centrally within them --- they lie on the peripheries. See Machura et al. (in preparation) for full details.
We suggest therefore that there are two major mechanisms involved in quasar formation: (i) standard galaxy-galaxy interactions; and (ii) gas-galaxy interactions at the confluence of merging clusters. The net effects should be: (a) at intermediate and high z quasar formation can occur by both mechanisms with some quasars being centrally located in clusters and others being on the peripheries of merging clusters; (b) at low z quasar formation can still occur by both mechanisms, but now infrequently, with most quasars being located on the peripheries of clusters.
The author(s) of this abstract have provided an email address for comments about the abstract: r.g.clowes@uclan.ac.uk