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2. The B3-VLA quasar sample

2.1. The radio sample

The B3 radiosource catalogue (Ficarra et al. 1985), lists 13354 sources brighter than 100 mJy at 408 MHz and covers 0.78 steradians in a sky strip from tex2html_wrap_inline1346 to tex2html_wrap_inline1348, all hour angles.

From the B3, down to the catalogue limit of 0.1 Jy, Vigotti et al. (1989) selected five complete subsamples separated by equal increments in logarithmic flux density and containing approximately the same number of sources, through the choice of different declination limits for each subsample. Furthermore, the right ascention range was restricted to exclude sky areas at low galactic latitude (R.A. tex2html_wrap_inline1350 and tex2html_wrap_inline1352).

From VLA maps in C configuration at 1465 MHz for the 1103 sources in the sample, Vigotti et al. (1989) obtained radio positions (accurate to 0.5 arcsec rms) which allowed the optical identification on POSS-I prints based on positional coincidence. The analysis of these maps has also led to the exclusion of 53 sources from the sample because they were either lobes or because some sources joined to form a single source.

These sources were reobserved at the same frequency with the VLA in A configuration allowing a better resolution, i.e. HPBW 1.4 arcsec to be compared with 14 arcsecs of C configuration. A-configuration VLA maps will be presented elsewhere (Vigotti et al. 1996, in preparation).

The final radio sample contains 1050 radiosources. Table 1 (click here) shows the sample definitions, the number of sources and the solid angle covered in each subsample. All the sky area limits are at 1950.0 epoch, except the declination limits for sample 4, which correspond to the whole B3 catalogue, and are referred to 1978.0 epoch. The solid angles are corrected for the incompleteness regions of the B3 survey.

  table229
Table 1: Definition of the B3-VLA sample

2.2. Identifications

Out of the original 352 identified objects, described in Vigotti et al. (1989), 183 were quasars candidates, defining a quasar candidate on the basis of its star-like appearance, regardless of the color, to avoid color biases.

These were examined using the positional coincidence between the optical and radio positions and the structural informations coming both from the A-configuration and C-configuration VLA maps. In order to build a complete sample of quasar candidates, we have accepted also identifications which are formally at low probability. For the unresolved sources the radio-optical rms displacement is tex2html_wrap_inline1390 and the tex2html_wrap_inline1392 radius is tex2html_wrap_inline1394, but we decided to extend the search up to 6''. Infact, examining the radio-optical displacement histogram (see Fig. 1 (click here)) for the 57 unresolved (in C-configuration) spectroscopically confirmed quasars, we see that the tex2html_wrap_inline1398 choice would have implied a loss of tex2html_wrap_inline1400 of the unresolved sources (4/57) with a displacement greater than tex2html_wrap_inline1402. A posteriori looking the A-configuration maps we understand why the distribution is not gaussian. In all the four cases the quasar was coincident with the faint component of a double radiosource of 4-5 arcseconds diameter, and with a very high flux ratio between the two radio components.

Furthermore, we obtained CCD images with the 1.5 meters Loiano Telescope of Bologna Observatory or with 3.5 meters telescope at Calar Alto of all the identified objects with uncertain optical classification from plates in the original sample, in order to be sure not to have missed any star-like object.

From the 183 quasar candidates in Vigotti et al. (1989) we excluded: (a)five candidates with tex2html_wrap_inline1408 (0019+391, 0800+399, 0800+472, 0805+406 and 1357+392). (b)three candidates which from the A-configuration VLA maps turned into empty fields (0209+390, 1012+389 and 1317+389). (c)three candidates which revealed to be plate-flaws from CCD images (1033+408, 1258+395 and 1301+393).

  figure243
Figure 1: Distribution of the radio-optical displacements for 57 unresolved quasars

2.3. Magnitudes

The candidates magnitudes in Vigotti et al. (1989) were estimates on POSS-I enlargements obtained by comparison with reference star images in the Selected Area 57. The accuracy of those estimated R magnitudes is 0.5 m. As machine measured R magnitudes are available from the APM Catalogue (Irwin 1992) which have an rms error of tex2html_wrap_inline1460, we list these in Table 4 (click here). There is a good agreement between the two magnitude scales (see Fig. 2 (click here)) with a 0.024m zero point. In two cases (0143+446b, 1342+389a) we adopted the estimated magnitudes because APM was clearly wrong (they were giving the magnitude of the quasar plus a star few seconds apart). The sample is complete down to POSS-I limit in red, nominally 20.0; however a plate to plate variation of 0.25 magnitudes could affect the sample.

  figure253
Figure 2: B3 magnitudes versus APM magnitudes


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