4 Search for neighbours and results

4.1 Search for neighbouring galaxies

We employed the following sequence for the search for neighbours of the sample BCGs. As a first pass we checked possible nearby galaxies in UZC, using the criterion $\vert \Delta V\vert < 500$ km s$^{\rm -1}$ and $d_{\rm proj} < 700$ kpc. For the second pass, we used NED in order to check whether some fainter galaxies are present in the BCG environment which could exert a stronger tidal action than the candidates found (if any) from UZC. The same criterion was applied as for the first pass, and then the candidates were additionally checked for correspondance of their projected distances to those for potential intruders, illustrated in Table 1. We also used some unpublished results on the redshifts of SBS galaxies, as well as our own results of dedicated spectroscopy of potential candidate companions. Since the galaxies, picked up in NED, present a mixture of the content of many catalogs as well as some small groups found in surveys of varying powers, it is difficult to address the question of completeness for the group of neighbouring galaxies found in the vicinity of BCGs. Therefore we can only emphasize that our estimates of neighbouring galaxies found this way are the lower limits of real numbers.

4.2 Results and types of neighbours

The results of the search for probable disturbing/interacting galaxies with the sufficiently strong tidal action on the sample BCGs are summarized in Table 3. We present the candidate disturbing galaxy which exerts the strongest tidal action on the target BCG. If there are other galaxies with comparable effect, we indicate them in the Notes on individual BCGs after the table. Table 3 contains the following data:
Column 1: BCG IAU-type name. The same as in Table 2.
Column 2: The name (or one of the best known names, such as NGC one) of the suggested disturbing galaxy. Prefix "S'' in the name means the object is from SBS. Prefix "A'' means a new object (anonymous), found as an associated galaxy in the spectroscopy data from the 6m telescope. In the case of a BCG supposed to be a merger, its name is repeated in this column. If neither merger morphology nor disturbing galaxy is identified, this field is left blank.
Column 3 and 4: RA and Declination of the disturbing galaxy for the equinox B1950. Not given for a suggested merger, or if no disturbing galaxy is identified.
Column 5: Heliocentric velocity of BCG, in km s$^{\rm -1}$. Same as in Table 2.
Column 6: Heliocentric velocity of the disturbing galaxy, in km s^-1. When the velocity of the disturbing galaxy, originally found in UZC, was known in NED with a better precision, we cite the NED value.
Column 7: The source of the value in the previous column, same coding as in the description for Table 2.
Column 8: B-magnitude of the disturbing galaxy. Most are taken from NED. The rest are either recalculated from the APM B-magnitudes through calibration with the CCD-based B-magnitudes, similar to those in Table 2, as described by Kniazev et al. (2001b), and marked by "*'' before the value, or are derived from the same CCD-frames on which target BCGs were measured. These are marked by "+''.
Column 9: Absolute B-magnitude of the disturbing galaxy, calculated with the same distance modulus as for the neighbouring BCG.
Column 10: Difference of the absolute magnitudes of BCG and its disturbing galaxy $\Delta M_{B} = M_{B}$(partner) - M_B(BCG). A negative value means that the disturbing galaxy is brighter.
Column 11: The projected distance between the BCG and the disturbing galaxy, in kpc.
Column 12: Suggested trigger classification: p - parent galaxy, when the disturbing galaxy is significantly brighter than the target BCG; b - binary system, in which the target BCG and its disturbing galaxy are dynamically comparable; f - fainter companion, when the disturbing galaxy is significantly less massive than the target BCG; m and m? - merger morphology with the various degrees of confidence. The latter classification was performed through the comparison of the morphology of candidate BCGs on the CCD frames from Lipovetsky et al. (2001) and Kniazev et al. (2001a), or on the DSS-2 images, with the galaxies shown by Keel & Wu (1995).

Figure 2: a) Distribution of blue absolute magnitude diffirences of BCGs and their disturbing galaxies; b) Distribution of the relative radial velocities of BCGs and their disturbing galaxies; c) Distribution of the projected distances between BCGs and their partner galaxies.

From the data in Table 3, one can see that the most representative group of 33 BCGs in this sample ( ${\sim}38\pm7\%$) have close enough brighter galaxies (conditionally classified as parent) with $\Delta M_{B}$ in the range -1.5$^{\rm m}$ to -6$^{\rm m}$. 23 BCGs from this sample ( ${\sim}27\pm 6\%$) are found to have disturbing galaxies of comparable brightness, or even significantly fainter. Of the remaining BCGs, which have no evident "neighbours'', 14 BCGs ( ${\sim}16\pm4\%$) show more or less evident merger morphology. Only 16 BCGs ( ${\sim}18.5\pm5\%$) have no clear indication of interaction with other galaxy. However, for many of these BCGs there are faint galaxies in their vicinity with still unknown redshifts. So the fraction of "non-interacting'' BCGs presented here should be rather treated as an upper limit.

Thus, our results indicate that gravitational interaction between gas-rich BCG progenitors and various mass galaxies in their environment may play a key role in the ignition of a SF burst. In particular, the conclusions by Taylor et al. (1995, 1997) on the important role of low-mass companions in triggering SF in "isolated'' H II-galaxies are consistent with our data. Similar conclusions are drawn by Noeske et al. (2001), based on the environment study of BCGs, mainly from the UM survey. The obtained results do not exclude that some internal mechanisms, which can trigger SF bursts in BCGs, probably work in the minority of them under certain conditions. However, in the majority of BCGs, the external trigger should be more likely, connected to interactions with other galaxies.