Gas-rich dwarf galaxy multiples in the Apertif H I survey

¹ ASTRON, the Netherlands Institute for Radio Astronomy, Oude Hoogeveenseweg 4, 7991 PD Dwingeloo, The Netherlands
² Kapteyn Astronomical Institute, University of Groningen, PO Box 800 9700 AV Groningen, The Netherlands
³ Department of Space, Earth and Environment, Chalmers University of Technology, Onsala Space Observatory, 43992 Onsala, Sweden
⁴ INAF – Padova Astronomical Observatory, Vicolo dell’Osservatorio 5, I-35122 Padova, Italy
⁵ School of Physical Sciences and Nanotechnology, Yachay Tech University, Hacienda San José S/N, 100119 Urcuquí, Ecuador
⁶ Instituto de Astrofísica de Andalucía (CSIC), Glorieta de la Astronomía s/n, 18008 Granada, Spain
⁷ Department of Physics, Virginia Polytechnic and State University, Blacksburg, VA 24061-0435, USA

^⋆ Corresponding author: This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 28 March 2025
Accepted: 5 September 2025

Abstract

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.