2 Observations and digitization

The observational strategies, digitization technique and methods used to retrieve the redshifts, magnitudes, colors and diameters on an homogeneous system are discussed in Paper I. Let us briefly remind that we used the ESO 1 meter Schmidt telescope with and without objective prism (O.P.), taking 3 plates for each field: two O.P. plates on Kodak IIIaJ emulsion, and one bicolor direct plate (U and R exposures separated by 30 arcsec on the same Kodak IIIaF plate). This allows a limiting magnitude on point sources of B=17.5 on O.P. plates and R = 19.5 for bicolor plates. The selection of candidate active galaxies is based on the visual evaluation of the ultraviolet excess of the object on the bicolor plate and/or the presence of at least one emission line in its O.P. spectrum. The IIIaJ emulsion allows to study the spectra of the objects in the range 3600 Å - 5330 Å. Thus, the spectral features used to select the active objects are [OIII]$\lambda\lambda 4959,5007$, H$_\beta$ and [OII]$\lambda 3728$ emission lines up to a limiting reshift $z\,=\,0.065$. Meanwhile some objetcs can be detected at further redshift using only the [OII] line. After eye selection of candidate targets using binocular microscope, parts of the plates ($10' \times 10'$ or $20' \times 20'$) centered on the objects are digitized by the MAMA (Machine A Mesurer pour l'Astronomie) machine of the Observatoire de Paris. (For a description of the MAMA machine see Guibert et al. 1984; Berger et al. 1991; Guibert & Moreau 1991; Soubiran 1992) O.P. plates were doubled in order to avoid false detections and coadded to improve the signal to noise ratio (cf. Paper I).

11 fields covering a total sky area of some 200 square degrees have been observed in the years 1989-1991 (Fig. 1). This paper deals with the emission line extragalactic objects (hereafter ELGs) found in two adjacent fields in the direction of the South extension of the Virgo Cluster:

$\alpha~=~ 13^{\rm h}12^{\rm mn}00^{\rm s}$, $\delta~=~-20\hbox{$^\circ$}30'00''$, and

$\alpha~=~13^{\rm h}12^{\rm mn}53^{\rm s}$, $\delta~=~-25\hbox{$^\circ$}10'05''$ (equinox 1950).

  

Figure 1: Configuration of the observed Schmidt plates. The fields used in this study are identified by a central star