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Some statistical trends and distributions

Our resulting sample of ELGs presented in Tables 2 and 3 is not artificially biased to any specific type of galaxies. Besides of the selection criterion based on the strength of [OIII] nebular lines, we do not apply any other selection. However, for we observed all the ELGs left after earlier spectroscopy by other groups it appeared that the fraction of the Case catalog faint galaxies in our part is higher. The effect of this is seen on the histograms in Fig. 2a (apparent magnitude) and Fig. 2b (absolute magnitude). Our subsample (solid line) is on average fainter by about one magnitude in comparison with the sample of all other Case ELGs observed by other groups (dashed line). In this respect the correlations shown in Figures 3 and 4 may be somehow biased relative to the similar correlations for the whole Case ELG sample.

Since we are looking for selection of a sample of Case BCGs, it is interesting to examine how the detected BCGs are connected with Case emission-line codes. In Fig. 3a we show the histogram with all our observed Case galaxies, distributed versus the 5 Case catalog emission-line codes, with roude separation in each column into the galaxies of 3 different types: BCGs (cross-hatched), all other ELGs (hatched), and galaxies without detectable emission lines (white). The galaxies with inconclusive spectra are not included in this histogram. It is evident, that for the emission-line codes ``vs'', ``s'' and ``m'' BCGs dominate all other types, however being only a quarter by number among the Case galaxies marked as ``w?'', and only one fifth, if ``w?'' inconclusive spectra are also taken into account. Altogether BCGs comprise about 60 % of all Case ELGs which display emission lines in our spectra, or about 44 % of all Case galaxies with some emission-line code.

Since as we have noticed our Case ELG subsample is biased to the fainter magnitudes relative to the whole sample, it is interesting to check the effect of the faintest (m_CG=18) galaxies on these distributions. We show in Fig. 3b the same histogram for 31 faintest galaxies of our subsample. Again Case galaxies with the code ``s'' and ``m'' are dominated by BCGs. Among those with the code ``w'' about 40 % are BCGs. And those, coded with ``w?'', have again a fraction of BCGs about one quarter.

However, despite this similarity, we notice that 95 % of all the faintest Case galaxies with emission lines, are classified as BCGs. So, this should result in some enhancement of BCG fraction in our subsample, in comparison with the whole Case ELG sample.

From our data we can also follow the correlation of the slit-spectroscopy parameters (EW([OIII] $\lambda$5007) with those from objective-prism spectra (the code of emission line strength). As can be seen in Fig. 4a, indeed there is some correlation of the code and the average for this code the value of EW([OIII] $\lambda$5007), but the scattering is extremely large. The brighter galaxies in each code have systematically lower EWs, what is in agreement with assigning the emission-line code in the Case catalog according to the total line flux, and not to the relative strength of the line and underlying continuum. This fact was already noticed by Salzer et al. ([1995]). The same histogram for the faintest Case catalog galaxies, observed by us (Fig. 4b), clearly demonstrates the same correlation, but the scattering is much reduced, for the most of (brighter) galaxies with very low EWs have gone. The corresponding correlation coefficients and confidence levels are given in the figure legend.