We can estimate the optical completeness of the sample, namely
how many quasars are not included in our sample because
their magnitude is fainter than 20.0 in red, as follows. In
the sample with Jy we can obtain a direct information:
55 EF out of 210
have been observed spectroscopically and only 1 EF turned out to be a
quasar while all the other EF are high z radiogalaxies (Djorgovski private
communication 1996). We therefore estimate that about 4 quasars are
fainter than the POSS-I limit.
There will be 68 Quasars in this radio flux bin, regardless of the optical
magnitude limit, 64 brigther than 20 magnitudes and 4 hidden among the
Empty Fields. This is
in good agreement with another direct information
obtained by Grueff & Vigotti (1975) by observing 113 Quasar candidates,
at limiting flux of 0.9 Jy at 408 MHz,
(
spectroscopically confirmed) down to
. They
found only 4% of the quasar sample fainter than POSS-I limit.
However
the question of how many quasars are in the EF has different answers depending
from the flux limit and from the frequency selection
of the sample. The magnitude histograms in Fig. 3 (click here) show that for samples
selected at low frequency
the sample incompleteness
is negligible at higher flux (3CR or B3-VLA at
Jy). It is also
evident that the median magnitude of the sample is becoming fainter in fainter
flux bins.
On the other hand lower flux samples c and d, as defined in
Fig. 3 (click here), show a clear cut
in the right side of the histogram due to the POSS-I magnitude limit.
An approximate evaluation of the number of the quasars in EF
versus the sample flux can be obtained af follows. As hinted from
Fig. 3 (click here) we can assume that the shape of the magnitudes distribution in
different flux bins does not vary with flux but the whole histogram is
shifted towards fainter magnitudes at lower fluxes. We compute
:
the difference of the median magnitude of b and c samples in
Fig. 3 (click here), we shift the magnitude distribution of sample b by
toward fainter magnitudes and finally we assume that the fraction of quasars
that are in EF in the sample c is the same as for the new sample b.
Than we recalculate the median of the sample c and repeat the procedure.
This allows to have a reasonable estimate of the number of quasars hidden in
EF for different flux intervals (see Table 2 (click here)).
Table 2: Estimate of Quasars in EF
Table 3: Identifications of the B3-VLA Quasar candidates sample
Figure 3: The magnitudes distributions for different B3-VLA flux
intervals
and the 3CR sample. a) 3CR; b) Jy;
c) 0.4 Jy
Jy; d) 0.1 Jy
Jy
The final sample now contains 172 quasar
candidates brighter than .
The quasar candidates sample is further subdivided
into 120 blue starlike objects (B) and 52 neutral color or red starlike
objects (N).
For all the candidates (except 35 with literature redshifts available) spectra were obtained at the 2.2 m and 3.5 m telescopes of Calar Alto during various observing runs. Some earlier data from these observations are described in Vigotti et al. (1990), and Lahulla et al. (1991). The remaining data are described in Sect. 3.
Table 3 (click here) presents the final assessment of the B3-VLA quasar candidates identifications after the identification revision and the spectroscopic observations.
The relatively high number of misidentifications with stars is due to
acceptance of
low-probability Identifications as described in Sect. 2.2.
The 3 sources classified
as BL Lac in Table 3 (click here) were previously known in literature.
Four spectra are classified as featurless: two are possible BL-Lac
candidates
on the basis of their radio morphology: 2322+396 and 2311+396a.
The former is unresolved (diam )
and has flat radio spectral index (0.0) , while the latter is a less probable
candidate, radio extended, diffuse and with a spectral index =-0.9.
The remaining two objects with featureless spectra could be either galaxies
or stars as they lack the typical radio characteristics of Bl-Lacs.
The final complete sample of quasars is presented in Table 4 (click here). Redshifts are available for 123 of the 125 spectroscopically confirmed quasar. The columns contain the following data: Column 1: name; Column 2: Right Ascention (1950.0) of the optical position (0.6 arcsec rms). Column 3: Declination (1950.0) of the optical position (0.6 arcsec rms). Column 4: Radio flux at 408 MHz. Column 5: APM red Magnitude. Column 6: Spectral Index. Column 7: Redshift. Column 8: Reference to published redshifts.