On faint companions in the close environment of star-forming dwarf galaxies - Possible external star formation triggers?

We have searched for companion galaxies in the close environment of 98 star-forming dwarf galaxies (SFDGs) from field and low density environments, using the NASA Extragalactic Database. Most of the companions are dwarf galaxies which due to observational selection effects were previously disregarded in environmental studies of SFDGs. A subsample at low redshift, cz < 2000 km s(-1), was chosen to partially eliminate the observational bias against distant dwarf companions. We find companion candidates for approximately 30% of the objects within a projected linear separation s(p) < 100 kpc and a redshift difference Delta cz < 500 km s(-1). The limited completeness of the available data sets, together with the non-negligible frequency of Hi clouds in the vicinity of SFDGs indicated by recent radio surveys, suggest that a considerably larger fraction of these galaxies may be accompanied by low{mass systems. This casts doubt on the hypothesis that the majority of them can be considered truly isolated. The velocity differences between companion candidates and sample SFDGs amount typically to <less than or similar to> 250 km s(-1), and show a rising distribution towards lower Delta cz. This is similarly found for dwarf satellites of spiral galaxies, suggesting a physical association between the companion candidates and the sample SFDGs. SFDGs with a close companion do not show significant differences in their H beta equivalent widths and B-V colours as compared to isolated ones. However, the available data do not allow us to rule out that interactions with close dwarf companions can influence the star formation activity in SFDGs.