2025A&A...703A.295S

Context. 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 in spectroscopic studies, such as surface-brightness incompleteness in optical studies and poor spatial resolution in single-dish neutral hydrogen (H I) surveys.
Aims. We aim to quantify the frequency of isolated gas-rich dwarf galaxy multiples using the untargeted, interferometric Apertif H I survey and study the impact of the interaction on star formation rates of galaxies as a function of the on-sky separation.
Methods. Our parent dwarf sample consists of 2481 gas-rich galaxies with stellar masses in the range ∼10⁶ < M_⋆/M_⊙ ≲ 5 × 10⁹, for which we identified close companions based on projected separation (r_p) and systemic velocity difference (|ΔV_sys|). We explored both constant thresholds for r_p and |ΔV_sys| corresponding to 150 kpc and 150 km s^–1 on all galaxies in our sample as well as mass-dependent thresholds based on a stellar-to-halo mass relation.
Results. We find the average number of companions per dwarf in our sample to be 13% (20%) when considering mass-dependent (constant) thresholds. We find that the frequency (∼11.6%) of dwarf companions in the stellar mass regime of 2 × 10⁸ < M_⋆/M_⊙ < 5 × 10⁹ is three times higher than previously determined from optical spectroscopic studies, highlighting the power of H I for finding dwarf multiples. Furthermore, we find evidence for an increase in star formation rates (SFRs) of close dwarf galaxy pairs of galaxies with similar stellar masses.