One of the important questions concerning the nature of the "nest'' and "chains'' of VV-galaxies is their evolution status. Current study as well as several previous publications demonstrate that many of the low-luminosity VV-galaxies are relatively nearby irregular galaxies with several bright knots of enhanced SF. Their position in two-colour diagram indicates a presence of SF burst in many of them (Zasov & Arkhipova [2000]). They roughly follow so called luminosity - metallicity relation: the least luminous galaxies show in general smaller values of O/H.
VV 432 has a very low heavy-element abundance. Its O/H is among the lowest ten values of the most metal-deficient BCGs out of more than one thousand BCGs/H II-galaxies known up-to-now. It may be considered as an example of non-evolved galaxy. Analysis of the empirical correlations suggests that dwarf galaxies with 12+log(O/H) < 7.6 can currently experience only the first in their history episode of SF (Izotov & Thuan [1999]). Therefore VV 432 with 12+log(O/H) = 7.58 is very good candidate for more detailed study. If it is situated in Virgo cluster, it will be the most metal deficient galaxy of this aggregate, being even less chemically evolved than another well-known metal-poor H II-galaxy in the direction of Virgo cluster H I 1225+01 with 12+log(O/H) = 7.66 (Salzer et al. [1991]; Chengalur et al. [1995]). One of the possible ways to resolve the dilemma of radial distance to VV 432 is a detection of brightest stars and construction of their color-magnitude diagram.
The galaxies we discuss have rather close neighbours. Icke ([1985]) first has drawn attention to the importance of relatively weak interactions to trigger gravitational instability in gas disks via generation of shocks. Many observational evidences for the important role of weak interactions to trigger SF were obtained since that time, including the detection of low mass H I-companions of nearby H II-galaxies (Chengalur et al. [1995]; Taylor et al. [1993], [1995]; Taylor [1997]) and optical faint companions of BCGs (Pustilnik et al. [1997]). Recent results on late spirals by Reshetnikov & Combes ([1997]) and Rudnick & Rix ([1998]) also suggest the importance of weak interactions to modulate SF history in these galaxies.
From the observational data discussed above some preliminary conclusions can be drawn:
Spectrophotometry of VV-galaxies shows that low luminosity representatives of this sample are in general metal-deficient objects, and in this aspect they are similar to dwarf irregular galaxies;
The extremely metal-deficient galaxy VV 432 (12+log(O/H) = 7.58) is probably the least evolved known member of Virgo cluster;
The system VV 543 radial velocity cited in the RC3 catalog and other databases is wrong. This object consists of two galaxies with discordant redshifts (a unique example among the galaxies of this type!), and probably presents an optical pair. VV 543W is an H II-galaxy with the radial velocity 1620 km s-1 higher than that of absorption-line E-type galaxy VV 543E;
VV 747 is probably a single dwarf galaxy rich of neutral hydrogen;
The presence of massive companion galaxies at the distances of few hundred kpc from the studied VV-objects with enhanced SF rate is probably indicative of the important role of weak interactions to trigger SF activity at least in some fraction of low mass VV galaxies.
Acknowledgements
We thank D.Makarov (SAO) for providing us with his MIDAS package to derive velocity curve on long-slit spectra. SAO authors appreciate the partial financial support from the RFBR grant No. 96-02-16398 One of the authors (A.Z.) thanks RFBR (grant 98-02-17102) and Federal program "Astronomy'' for financial support. We have made use of the NASA/IPAC Extragalactic Database (NED), which is operated by the Jet Propulsion Laboratory, California Institute of Technology, under contract with the National Aeronautics and Space Administration, and of the Lyon-Meudon Extragalactic Database, http://leda.univ-lyon1.fr.
Copyright The European Southern Observatory (ESO)