Surveys of ultraviolet-excess quasar candidates in large fields^*

The area around NGC 450

¹ Institut d'Astrophysique, Université de Liège, Avenue de Cointe 5, B-4000 Liège, Belgium
² Centre d'Analyse des Images et DEMIRM, Observatoire de Paris, 61 avenue de l'Observatoire, F-75014 Paris, France
³ Space Telescope Science Institute, Homewood Campus, 3700 San Martin Drive, Baltimore MD 21218, U.S.A.
⁴ In illo tempore, Mount Wilson and Las Campanas Observatories of the Carnegie Institution of Washington, U.S.A.
⁵ Max-Planck-Institut für Astrophysik, Karl-Schwarzschild-Stra$\beta$e 1, D-85740 Garching bei München, Germany

Send offprint request to: E. Gosset

Received: 1 December 1995
Accepted: 18 October 1996

Abstract

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 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.