Choirs, H I galaxy groups: catalogue and detection of star-forming dwarf group members

Abstract

Hα observations centred on galaxies selected from the H i Parkes All-Sky Survey (HiPASS) typically show one and sometimes two star-forming galaxies within the ∼15 arcmin beam of the Parkes 64 m H i detections. In our Survey for Ionization in Neutral Gas Galaxies (SINGG) we found 15 cases of HiPASS sources containing four or more emission line galaxies (ELGs). We name these fields Choir groups. In the most extreme case, we found a field with at least nine ELGs. In this paper, we present a catalogue of Choir group members in the context of the wider SINGG sample. The dwarf galaxies in the Choir groups would not be individually detectable in HiPASS at the observed distances if they were isolated, but are detected in SINGG narrow-band imaging due to their membership of groups with sufficiently large total H i mass. The ELGs in these groups are similar to the wider SINGG sample in terms of size, Hα equivalent width and surface brightness. Eight of these groups have two large spiral galaxies with several dwarf galaxies and may be thought of as morphological analogues of the Local Group. However, on average our groups are not significantly H i deficient, suggesting that they are at an early stage of assembly, and more like the M81 group. The Choir groups are very compact at typically only 190 kpc in projected distance between the two brightest members. They are very similar to SINGG fields in terms of star formation efficiency (SFE; the ratio of star formation rate to H i mass), showing an increasing trend in SFE with stellar mass.