next previous
Up: The B3-VLA quasar

3. The sample completeness

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 tex2html_wrap_inline1476 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, (tex2html_wrap_inline1482 spectroscopically confirmed) down to tex2html_wrap_inline1484. 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 tex2html_wrap_inline1490 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 tex2html_wrap_inline1494: 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 tex2html_wrap_inline1496 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)).

  table278
Table 2: Estimate of Quasars in EF

  table288
Table 3: Identifications of the B3-VLA Quasar candidates sample

  figure297
Figure 3: The magnitudes distributions for different B3-VLA flux intervals and the 3CR sample. a) 3CR; b) tex2html_wrap_inline1518 Jy; c) 0.4 Jy tex2html_wrap_inline1524 Jy; d) 0.1 Jy tex2html_wrap_inline1530 Jy

3.1. The final sample

The final sample now contains 172 quasar candidates brighter than tex2html_wrap_inline1534. 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 tex2html_wrap_inline1544) 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.


next previous
Up: The B3-VLA quasar

Copyright by the European Southern Observatory (ESO)
web@ed-phys.fr