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By Margrethe Wold, Mark Lacy, Per B. Lilje and S. Serjeant


We have made a survey of quasar environments at 0.5 ≤ z ≤ 0.8, using a sample of both radio-loud and radio-quiet quasars matched in B-band luminosity. Our observations include images of background control fields to provide a good determination of the field galaxy counts. About 10 per cent of the quasars appear to live in rich clusters, whereas approximately 45 per cent live in environments similar to that of field galaxies. The richness of galaxies within a 0.5 Mpc radius around the radioquiet quasars is found to be indistinguishable from the richness around the radio-loud quasars, corresponding on average to groups or poorer clusters of galaxies. Comparing the galaxy richness in the radio-loud quasar fields with quasar fields in the literature, we find no evidence of an evolution in the environment with epoch. Instead, a weak, but significant correlation between quasar radio luminosity and environmental richness is present. It is thus possible that the environments of quasars, at least the powerful ones, do not evolve much between the present epoch and z ≈ 0.8. Sample selection and observations We report on a survey of the galaxy environments of intermediate redshift radio-loud and radio-quiet quasars (RLQs and RQQs, hereafter), described in more detail by Wold et al. (2000a; 2000b). The quasar sample was pulled from complete catalogues with different flux limits, in order to cancel the correlation between redshift and luminosity known to exist in flux-limited surveys. It contains 20 RQQs and 21 steepspectrum RLQs within a narrow redshift range (0.5 ≤ z ≤ 0.8), but with a wide range in AGN luminosity, both optical and radio. The two 1 2 quasar samples are matched in both redshift and optical luminosity, and the distribution of sources in the redshift–optical/radio luminosity plane is shown in Fig. 1. Our assumed cosmology has H0 = 50 km s −1 Mpc −1

Year: 2001
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