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The statistical properties of HSS BCGs

The sky distribution of the BCGs from the HSS sample is shown in Fig. 6 together with BCGs from the SBS and Case surveys (Izotov et al. [1993b]; Pustilnik et al. [1995]; Ugryumov [1997]; Ugryumov et al. [1998]). Our aim is to create a large sample (the Northern BCG sample) in this whole area of about 3000 square degrees. For this it is important to understand how the properties of the objects in these three samples compare to each other. First, we compare the apparent magnitude distributions of the three samples. For the HSS BCG sample we use the data of the 47 BCGs found in this first part of the survey. We consider it as representative (excluding the brighter part of the sample, which will be discussed later). An abrupt decrease of the number of objects in the magnitude range between $m_B = 18\fm0$ and $19\fm0$ is present in the distributions of all three samples (Fig. 7) although the fraction of galaxies in this magnitude range is different in each sample. As it was shown by Pustilnik et al. ([1995]) the BCG sample in the SBS zone can be considered as rather complete up to $m_{pg} = 18\fm0$. The similar behaviour of the faint end of the apparent magnitude distributions infers that the Northern BCG sample may have a similar completeness down to this limiting magnitude. It is evident from Fig. 7 that there is a significant deficit in the HSS of galaxies with $m_B \le 16\fm0$ caused by the adopted limits in the parameter space of the selection criteria. Additionally, several known bright emission-line galaxies are found in the HSS, but not observed in this work. These selection constraints lead to a difference between the mean apparent magnitudes of HSS BCGs and the BCGs of the two other samples. The real brightness distribution of HSS BCGs will be discussed in more detail in later papers.

Despite of the differences in the apparent magnitude distributions, the absolute photographic magnitude distributions for the same BCG samples shown in the histograms of Fig. 7 look much more similar and have similar mean luminosities. This may indicate that the contents of all three BCG samples are the same.

The radial velocity distributions shown for the same BCG samples in Fig. 8 have similar break values of the redshift (about 0.035), at which a significant fall-off takes place. This again reflects the similar limiting apparent magnitudes of all three samples. The absence of nearby HSS objects in this histogram is artificial, because the bright BCGs with known velocities were not observed here. For the sake of clarity it should be noted that galaxies were selected in SBS both through UV-excess and emission lines. The BCG sample however was exclusively selected from emission-line candidates. Therefore the redshift distribution of the SBS is similar to the other two samples. The similarities of BCG properties in the HSS, SBS and Case surveys imply that the galaxies from these surveys can be combined into a larger sample to study the spatial distribution of BCGs in this area. A first comparison of the luminosity function which can be derived from these different samples can be found in Hopp (1998).