Rendezvous in CAVITY: Kinematics and gas properties of an isolated dwarf-dwarf merging pair in a cosmic void region

¹ Dpto. de Física Teórica y del Cosmos, Facultad de Ciencias (Edificio Mecenas), University of Granada, E-18071 Granada, Spain
² Institut de Radioastonomie Millimétrique (IRAM), Av. Divina Pastora 7, Núcleo Central, 18012 Granada, Spain
³ Instituto Carlos I de Fí sica Teórica y Computacional, Universidad de Granada, 18071 Granada, Spain
⁴ Departamento de Física de la Tierra y Astrofísica, Universidad Complutense de Madrid, 28040 Madrid, Spain
⁵ Instituto de Astrofísica de Andalucía – CSIC, Glorieta de la Astronomía s/n, 18008 Granada, Spain
⁶ Kapteyn Astronomical Institute, University of Groningen, PO Box 800, 9700 AV Groningen, The Netherlands
⁷ Instituto de Astrofísica de Canarias, c/Vía Láctea s/n, E-38205 La Laguna, Tenerife, Spain
⁸ Departamento de Astrofísica, Universidad de La Laguna, E-38206 La Laguna, Tenerife, Spain
⁹ Département de Physique, de Génie Physique et d’Optique, Université Laval, and Centre de Recherche en Astrophysique du Québec (CRAQ), Québec, QC G1V 0A6, Canada

^⋆ Corresponding author.

Received: 20 December 2024
Accepted: 18 April 2025

Abstract

Context. Galaxy mergers are pivotal events in the evolutionary history of galaxies, with their impact believed to be particularly significant in dwarf galaxies due to their low masses. However, these events remain largely underexplored, especially in pristine environments such as voids.

Aims. In this work, we report the serendipitous identification of an isolated merging dwarf system with a total stellar mass of M_★∼10^9.7 M_⊙, located in the centre of a cosmic void. This system is one of the rare examples, and possibly the first, of merging dwarf galaxy pairs studied within the central region of a cosmic void. This system is remarkable due to its orientation relative to the line of sight and its unique local and large-scale environment.

Methods. Using CAVITY PPAK-IFU data combined with deep optical broadband imaging from the Isaac Newton Telescope, we analysed the kinematics and ionised gas properties of each dwarf galaxy in the system by employing a full spectral fitting technique.

Results. The orientation of this merging pair relative to the line of sight allowed us to determine the dynamical mass of each component, which we found to have similar dynamical masses within galactocentric distances of up to 2.9 kpc. These galaxies were likely star-forming dwarfs with rotating discs prior to the merger. While the gas-phase metallicity of both components is consistent with that of star-forming dwarf galaxies, the star formation rates observed in both components exceed those typically reported for equally massive star-forming dwarf galaxies. This indicates that the merger has presumably contributed to enhancing star formation. Our analysis shows no signs of AGN activity in this merging pair. Furthermore, we found no significant difference in the optical g-r colour of this merging pair compared to other merging dwarf pairs across different environments.

Conclusions. While most merging events occur in group-like environments with a high galaxy density and the tidal influence of a host halo, and isolated mergers typically involve galaxies with significant mass differences, the identified merging pair does not follow these patterns. We speculate that the global dynamics of the void or past three-body encounters involving components of this pair and a nearby dwarf galaxy might have triggered this merging event.