English (UK)
Using data from PMAS, the multi-fiber spectrograph on the 3.5-meter telescope at Calar Alto Observatory, researchers from the Calar Alto Void Integral-field Treasury surveY (CAVITY) project, led by the University of Granada, have discovered a rare and ongoing merger between two dwarf galaxies located deep inside a cosmic void, one of the least populated regions of the Universe.
Galaxy mergers are powerful events that are key in shaping how galaxies grow, change, and form new stars. When galaxies collide, gravitational forces cause them to exchange gas and stars, trigger bursts of star formation, and alter their structures. While mergers between massive galaxies have been widely studied, catching such an event between low-mass dwarf galaxies is extraordinarily rare, especially in such a void environment.
For the first time, this study presents a detailed analysis of an ongoing merger event near the centre of a cosmic void, involving two dwarf galaxies of nearly equal stellar mass. In terms of ordinary matter (stars, gas, and dust), each of these dwarves is less than a twentieth of the mass of the Milky Way (once more, dark matter apart), our own spiral galaxy hosting the Sun and over a hundred billion of other stars.
The two dwarves were observed for the CAVITY team with PMAS, the Potsdam Multi-Aperture Spectrograph installed on the 3.5 m telescope at Calar Alto, the largest in mainland Europe. Thanks to the large field of view of PMAS in its PPAK fibre bundle mode (see figure), both small galaxies were observed in a single pointing encompassing the central area of the merger until their very outskirts, sampled by over 300 PPAK sensitive fibres. Each of these fibres provides a spectrum covering the full visible wavelength range, allowing us to study the chemical species, age, and velocities of the matter in every one of the 300 points of the area sampled by PPAK. That is, PMAS offers a complete view of the physics at play in this one-on-one, strong interaction between small galaxies located well inside a void, thus not influenced by a third party.
Dwarf–dwarf merger observed with the PPAK instrument at the Calar Alto observatory. The instrument's field of view (orange hexagon overlayed over the SDSS image, top-left) fully captures the system, including its faint tidal features extending into the outskirts. Both merging components exhibit rotation (gas velocity map, bottom left) and an exceptionally high Star Formation Rate (SFR logarithmic map, bottom right). Deep imaging in two colours (g- and r-bands, giving the “blue minus red” color map top-right) from the Isaac Newton Telescope reveals a striking dust distribution, likely shaped by shock waves produced during the merger. Credits: B.Bidaran/University of Granada/CAHA/INT.
“This merger is unusual in several ways,” explains Bahar Bidaran, lead author of the study and postdoctoral researcher at the University of Granada. “Most mergers occur in dense group environments or involve unequal mass pairs. However, this system does not follow either of those patterns. We speculate that the global dynamics of the void, or perhaps a past three-body interaction involving another nearby dwarf galaxy, may have triggered this unique encounter”.
The merging system has a combined stellar mass of just under 10 billion solar masses. The team detected rotating gaseous disks and strong ionised gas emission, signatures of a starburst likely sparked by the merger. Thanks to the favorable alignment of the system along the line of sight, the researchers were able to measure the dynamic mass of each of the merging progenitor galaxies. In addition, deep optical imaging from the Isaac Newton Telescope revealed a striking dust distribution: two connected arc-like structures that may have formed from shock fronts produced during the merger. Based on the data, the two progenitor galaxies were likely disk-like and star-forming, typical of those found in voids.
"Thanks to an accurate reduction of the spectral cubes, we can unravel with spatial resolution how the outbursts of star formation are triggered and how the gaseous disks interact during this merger in the cosmic vacuum. This information is key to understanding how these dwarf galaxies evolve in isolation," says Rubén García-Benito, researcher at the IAA-CSIC and head of data processing for the CAVITY project.
This discovery opens a valuable window into understanding how galaxies evolve in extremely low-density environments, far from the crowded cosmic web where most galaxy interactions occur. “This is a truly exciting discovery that underscores the potential and impact of the CAVITY project. It changes the way we view galaxies in cosmic voids, and in my opinion, will significantly advance the understanding of dark halo formation in the early Universe,” explains Prof. Isabel Pérez, PI of the CAVITY survey and full professor at the Theoretical and Physical Cosmology department at Granada University.
The CAVITY team’s findings not only offer new clues about dwarf galaxy formation and interaction but also highlight the remarkable capabilities of the PPAK integral-field unit and the 3.5-meter telescope at Calar Alto Observatory. The instrument’s wide field of view and fine spatial resolution were key to detect this merging system's faint, compact signatures, features that would be nearly impossible to identify without high-resolution observations. This study highlights how powerful ground-based facilities like Calar Alto can push the boundaries of the understanding of galaxy evolution in the most elusive corners of the Universe.
Jesús Aceituno, director of the observatory adds “Calar Alto commitment with very large, long-term programs like CAVITY, one of the three ongoing on the 3.5 m telescope, allows us to tackle ambitious science projects with a real legacy value for the community. CAVITY already published its first data release in July 2024 with great success and extra-galactic astronomers worldwide are eager to download new reduced PPAK spectra in future data releases.”
The Calar Alto Observatory is one of the infrastructures that belong to the national map of Unique Scientific and Technical Infrastructures (Spanish acronym: ICTS), approved on March 11th, 2022, by the Science, Technology and Innovation Policy Council (CPCTI).
PUBLICATION
Bidaran et al. 2025, accepted and published by Astronomy & Astrophysics
DOI: 10.1051/0004-6361/202453556
CONTACTS
University of Granada - Bahar Bidaran
Calar Alto Observatory - Gilles Bergond
COMMUNICATION – CALAR ALTO OBSERVATORY