A&A Supplement series, Vol. 123, June II 1997, 529-568
Received December 1, 1995; accepted October 18, 1996
E.
Gosset
- O.
Moreau
- J.
Surdej
- J.-P. Swings
- H.C. Arp
Send offprint request: E. Gosset
Institut d'Astrophysique, Université de Liège,
Avenue de Cointe 5, B-4000 Liège, Belgium
Centre d'Analyse des Images et DEMIRM, Observatoire de Paris,
61 avenue de l'Observatoire, F-75014 Paris, France
Space Telescope Science Institute, Homewood Campus,
3700 San Martin Drive, Baltimore MD 21218, U.S.A.
In illo tempore, Mount Wilson and Las Campanas Observatories of
the Carnegie Institution of Washington, U.S.A.
Max-Planck-Institut für Astrophysik,
Karl-Schwarzschild-Stra
e 1,
D-85740 Garching bei München, Germany
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.
keywords: quasars: general -- surveys