THE ENVIRONMENT OF STAR-FORMING DWARF GALAXIES The role of dwarf companion objects K.G. NOESKE ⋆ Universitäts–Sternwarte Göttingen, Germany E-mail: knoeske@uni-sw.gwdg.de J. IGLESIAS-PÁRAMO and J.M. VÍLCHEZ Instituto de Astrofísica de Canarias, E-38200 La Laguna, Tenerife, Spain K.J. FRICKE Universitäts–Sternwarte Göttingen, Germany Abstract. We are currently performing a new analysis of the close environment of star-forming dwarf galaxies and its effects on their star-forming activity. For a sample of 142 thoroughly studied objects, we search the NASA Extragalactic Database for possible companion galaxies of any apparent luminosity and angular size. From a first analysis of a part of our sample, we find only about 50% of our objects to be isolated in the sense that no companion object was detected within a redshift difference of 500 km s -1 and an angular separation corresponding to a projected distance of 1 Mpc, assuming H 0 = 75 km s -1 Mpc -1 . By splitting the sample into different redshift intervals, we demonstrate an observational bias against low-luminosity companion objects which implies that a large fraction of star-forming dwarf galaxies, although lacking bright companions, might possess dwarf companion objects. Similarly to other authors, we find no evidence for a difference in star- forming activity due to the presence or absence of possible companions. 1. Introduction Violent star formation (SF) in dwarf galaxies is among the physical processes still waiting to be explained by a commonly accepted model. Dwarf galaxies undergo- ing SF at high rates are blue, gas-rich low-mass objects, usually classified as HII galaxies (Terlevich et al., 1991), or blue compact dwarf galaxies (BCDs, Thuan and Martin, 1981). To investigate the phenomenon as a whole rather than restricting ourselves to one classification differing from another by slightly different criteria, we will consider the entire class of objects, referring to them as star-forming dwarf galaxies (SFDGs; cf. Vílchez, 1995). As the principal driver of galaxy evolution, SF in dwarf galaxies has been the topic of much preceding work, focusing on the unknown mechanisms of its trigger- ing and regulation, as well as its impact on the hosting dwarf galaxy. Among differ- ent suggestions, interaction with the SFDG’s environment has been put forward to explain the massive SF. Consequently, samples of SFDGs were surveyed for pos- ⋆ Summer Research Student, Instituto de Astrof´ ısica de Canarias. Astrophysics and Space Science 276: 577–583, 2001. © 2001 Kluwer Academic Publishers. Printed in the Netherlands.