On faint companions in the close environment of star-forming dwarf galaxies

Possible external star formation triggers?

¹ Universitäts-Sternwarte Göttingen, 37083 Göttingen, Germany
² Instituto de Astrofísica de Canarias, 38200 La Laguna, S/C de Tenerife, Spain
³ Instituto de Astrofísica de Andalucía (CSIC), 18080 Granada, Spain

Corresponding author: K. Noeske, knoeske@uni-sw.gwdg.de

Received: 25 September 2000
Accepted: 12 March 2001

Abstract

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, 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 kpc and a redshift difference 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 250 km s^-1, and show a rising distribution towards lower . 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 Hβ equivalent widths and 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.