The US survey and the incidence of bright quasars.
USHER P.D. and MITCHELL K.J.
Abstract (from CDS):
The bright quasar sample from the US survey (UBQS) comprises 19 quasars with B<16.95 mag selected within the areal completeness boundaries of seven US survey fields. A subset of 17 quasars is statistically complete for objects judged to be starlike on images taken with cameras of the 1.2 m Schmidt class that satisfy the morphological criterion B<21-[3/(4z)], where B<18.5 mag, at redshift z. Twelve objects compose a statistically complete subset that satisfies criteria on absolute magnitude MB←23.0 (where H0=50) and z>0.3. These subsets and others defined by other morphological criteria currently in use confirm the high incidence of bright US survey quasars previously established with the MB criterion alone. The number count data and the redshift distribution for the bright quasars of the UBQS are found to agree with those of the Hamburg ESO (HES) and Edinburgh quasar surveys (EQS). The number counts in the range 15.5<B<16.95 from these three quasar samples are consistently higher than those produced from other published quasar samples in the same magnitude range. These results suggest that the UBQS, HES, and EQS approach statistical completeness within the limits of their respective selection criteria. Surface densities from these three quasar samples favor the semiempirical fit of luminosity-dependent luminosity evolution (LDLE) of LaFranca and Cristiani. An observed excess of quasars in the range 0.5<z≤0.6 is discussed. The apparent overabundance does not appear to be a result of selection effects. The present data will be useful in recalculating the LDLE fit parameters.