3 BCG sample

The BCG sample for this study is compiled from the BCG sample in the zone of the Second Byurakan Survey (SBS) (RA $=7^{\rm h}40^{\rm m}{-}17^{\rm h}20^{\rm m}$, Dec $\,=49^{\circ}$-61$^{\circ}$). The latter is partly described by Izotov et al. (1993), Thuan et al. (1994), Pustilnik et al. (1995). The same BCG sample was in particular the subject of an HI-study by Thuan et al. (1999). Their subsample of 88 BCGs is reduced slightly here to match the strict criterion of the strength of the [O III]-line.

Our sample includes 86 BCGs with a sufficiently strong [O III]-line (EW([O III] $_{\rm\lambda5007}) > 45$ Å) and $V_{\rm hel}<6000$ km s$^{\rm -1}$. The first criterion corresponds to the line strength at which the loss of candidate emission-line galaxies (ELGs) from objective prism plates during the primary selection is moderate, while the second criterion is to limit the volume where search for non-massive neighbours can be efficiently carried out. While the question of completeness analysis of SBS BCG sample is beyond the scope of this paper, it is useful to describe the bias inherent to the BCG sample. As demonstrated by Salzer (1989), the selection in emission-line surveys is determined by the total flux in line+continuum. Thus, for apparent magnitudes close to the threshold of photographic plates, only emission lines with large enough equivalent widths (EWs) will be detected. From the carrying out of such selection on the SBS plates and the preliminary comparison in the small overlapping region of SBS detected BCGs with BCGs detected in the Hamburg/SAO survey (Ugryumov et al. 1999), for the limiting m_B=18, which is near the completness limit of the SBS sample (Pustilnik et al. 1995), galaxies having H II-regions with EW([O III] $_{\lambda5007})>(45{-}50)$ Å are selected with modest loss, and thus can be considered as representative of the whole population of galaxies with that level of SF activity. This level corresponds, for typical extragalactic H II-regions, to EW(H $\beta)>(10{-}20)$ Å, which in turn in the model of instantaneous SF burst corresponds to the burst age <(7-10) Myr (Leitherer et al. 1999). Therefore, the main bias introduced by the applied criteria to the entire BCG/H II-galaxy population is the selection of relatively young SF bursts, which can more easily be found in objective prism spectral plates. Some characteristics of the sample are: the range of apparent blue magnitudes $B = 12\hbox{$.\!\!^{\rm m}$ }5 \div 18\hbox{$.\!\!^{\rm m}$ }5$, the range of absolute blue magnitudes $M_B = -11\hbox{$.\!\!^{\rm m}$ }0 \div -19\hbox{$.\!\!^{\rm m}$ }4$. For many of these galaxies, the accuracy of radial velocities was significantly improved after HI-line measurments by Thuan et al. (1999) and additional optical spectroscopy (Izotov et al. 1994; Thuan et al. 1995). To illustrate the properties of this BCG sample, we show in Fig. 1 some of its parameter distributions (m_B, M_B, $V_{\rm hel}$).

Figure 1: Distributions of the BCG sample on mB a), MB b) and $V_{\rm hel}$ c). The hatched and unhatched parts of the MB-distribution separate BCGs from the general field group (60 galaxies) and the Local Supercluster volume (26 galaxies) respectively. See Sect. 5.3 for details.

One can see that the sample is more or less typical, with some excess of the less luminous BCGs in comparison to e.g., the BCG sample in the same sky region studied by Pustilnik et al. (1995). This is connected to the different velocity boundaries adopted for the latter paper (2000 km s$^{\rm -1}$ $< V_{\rm hel}< 10\,000$ km s$^{\rm -1}$) and ours ( $V_{\rm hel}< 6\,000$ km s$^{\rm -1}$). This sample includes a significant number of BCGs located inside the Local Supercluster that could bias the conclusions relative to the situation in the general field. This question will be addressed in the discussion section. B-magnitudes of many BCGs in the considered sample are significantly corrected based on the results of CCD-photometry (Lipovetsky et al. 2001; Kniazev et al. 2001a).

The list of the studied sample of 86 BCGs is presented in Table 2, containing the following information:
Column 1: The object's IAU-type name. For those without an alternative name the prefix SBS is applied in the last column.
Column 2: Right ascension (RA) for equinox B1950. The coordinates of BCGs in this table sometimes differ from those presented in NED, but are the most up-to-date, according to our own checks on the Digitized Sky Survey (DSS).
Column 3: Declination for equinox B1950.
Column 4: Heliocentric velocity in km s$^{\rm -1}$. For more than 60 BCGs the r.m.s. uncertainty is less than 30 km s$^{\rm -1}$ (mainly from HI data). For seven BCGs it can reach ${\sim}100$ km s$^{\rm -1}$, and for the remaining galaxies it is ${\sim}50$-60 km s$^{\rm -1}$.
Column 5: Reference to the source of velocity accepted, coded by 1 - Thuan et al. (1999): 2 - NED; 3 - Pustilnik et al. (1995); 4 - new measurments with the SAO 6m telescope (this paper, Appendix); 5 - UZC: Falco et al. (1999); 6 - Carrasco et al. (1998). Some of our BCG velocities in NED, taken from HI-measurments by Thuan et al. (1999), are not reliable, since they ignore the information on real or possible confusion. We used instead for these galaxies the velocities from Pustilnik et al. (1995).
Column 6: Distance in Mpc, accounting for Virgo infall correction from Kraan-Korteweg (1986) with $V_{\rm infall} = 220$ km s$^{\rm -1}$. For one nearby galaxy ( $V_{\rm hel} = 289$ km s$^{\rm -1}$), marked by an asterisk, the distance is accepted from Georgiev et al. (1997). For another similar galaxy (Mkn 178, $V_{\rm hel} = 249$ km s$^{\rm -1}$), the distance is accepted to be equal to that of its neighbours on the sky with close radial velocities and having photometrical determinations.
Column 7: Total B-magnitude from the unpublished CCD data (Lipovetsky et al. 2001; Kniazev et al. 2001a). They are marked by a "plus" before the value in the corresponding column. For several galaxies, B-magnitudes are based on the APM database values (Irwin 1998). They were recalculated to the standard CCD B-magnitudes using the relation between $B_{\rm CCD}$ and $B_{\rm APM}$ derived on more than 100 galaxies, as described by Kniazev et al. (2001b). Its rms uncertainty is found to be 0 $.\!\!^{\rm m}$45. The B-magnitudes obtained in this way are marked by an "asterisk" before the value. For a couple of objects, photometrical B-magnitudes are taken from NED. They are marked by a "n'' before the value.
Column 8: Absolute B-magnitude, calculated from the apparent magnitude and the distance in Col. 5, with no correction for the Galaxy extinction, since it is small for considered sky region.
Column 9: A flag to distinguish the galaxies belonging to the groups of the two types of environment (LS - Local Supercluster, GF - general field). See Sect. (5.3) for details.
Column 10: One or more alternative names, according to the information from NED.