Surveys of ultraviolet-excess quasar candidates in large fields
Gosset, Eric[Université de Liège - ULg > Département d'astrophys., géophysique et océanographie (AGO) > Groupe d'astrophysique des hautes énergies (GAPHE) >]
Moreau, O.[Centre d'Analyse des Images et DEMIRM, Observatoire de Paris, 61 avenue de l'Observatoire, F-75014 Paris, France and Institut d'Astrophysique, Universite de Liege, Avenue de Cointe 5, B-4000 Liege, Belgium]
Surdej, Jean[Université de Liège - ULg > Département d'astrophys., géophysique et océanographie (AGO) > Astroph. extragalactique et observations spatiales (AEOS) >]
Swings, J.-P.[Institut d'Astrophysique, Universite de Liege, Avenue de Cointe 5, B-4000 Liege, Belgium]
Arp, H. C.[In illo tempore, Mount Wilson and Las Campanas Observatories of the Carnegie Institution of Washington, U.S.A. and Max-Planck-Institut fur Astrophysik, Karl-Schwarzschild-Strasse 1, D-85740 Garching bei Munchen, Germany]
[en] We describe the results of a survey for moderately bright quasars performed in a 20.6-square-degree field around the galaxy NGC 450. The quasar candidates were selected on the basis of their ultraviolet excess: by comparative visual inspection of the double image of each single object on a U/B dual-exposure Schmidt photographic plate, 95 primary and 45 secondary quasar candidates were selected on the basis of their $U$ image being too bright. The spectroscopic identification of the primary candidates led to the discovery of 59 bona fide quasars (out of which 6 were previously known). The Palomar Schmidt plate was digitised using the MAMA measuring machine and the outcoming data reduced using ad hoc procedures. A photometric calibration allowed us to derive values for the limiting magnitudes and for the U-B index selection threshold of the survey. A catalogue containing 60 quasars is presented with accurate positions, magnitudes and additional information such as redshifts. We studied the spatial distribution of the objects and detected, for the quasars of our sample, a significant deviation from randomness in the form of a propensity to cluster in pairs on the celestial sphere with a typical scale of about 10 arcmin. We also formally detected a tendency towards a 3-D clustering, but this result is induced by a single pair of quasars. A forthcoming paper will deal with a similar work performed in a field around NGC 520; the latter field is located directly to the North of the present one and slightly overlaps it. Based on observations acquired at the Mount Palomar and Las Campanas Observatories as well as at the European Southern Observatory. Also based on Schmidt-plate digitisations performed with the MAMA measuring machine of C.A.I. (I.N.S.U., Paris).