© ASTRON/Barbara Siljeg

Dwarf-dwarf galaxy encounters are a key aspect of galaxy evolution as they can ignite or temporarily suppress star formation in dwarfs and can lead to dwarf mergers. However, the frequency and impact of dwarf encounters remain poorly constrained due to limitations of spectroscopic studies, e.g. surface-brightness incompleteness of optical studies and poor spatial resolution of single-dish neutral hydrogen (HI) surveys.

In a recently accepted paper, using the Apertif HI survey, we were able to both detect and resolve dwarf galaxy multiples, enabling us to characterise their frequency in the field and study their impact on star formation rates of interacting galaxies. By identifying close companions based on projected sky separation and systemic velocity difference, we find that the average number of companions per dwarf in Apertif is 0.13, meaning that (on average) every ~8th dwarf would have a companion. When compared to previous optical spectroscopic surveys in the overlapping stellar mass range (right panel), we find a three times higher frequency (~11.6%) of dwarf companions than previously determined from optical spectroscopic studies. This result highlights the power of HI for finding dwarf multiples. Furthermore, we find evidence for an increase in star formation rates of close dwarf galaxy pairs of galaxies with similar stellar masses.

The paper in question is titled “Gas-rich dwarf galaxy multiples in the Apertif HI survey” and will soon be published in A&A. The left panel shows a close-by pair of dwarf galaxies detected by their HI emission (white contours). The right panel shows the mean number of companions per dwarf in logarithmic stellar mass bins, normalized by the number of dwarfs per bin. Apertif results are shown before and after applying robust isolation criteria (which shrink our sample and increase uncertainty), and are compared to results based on optical spectroscopy of SDSS from Besla et al. 2018.