GAMA 526784: The progenitor of a globular cluster-rich ultra-diffuse galaxy?

II. Molecular gas, neutral gas, and environment

¹ European Southern Observatory, Karl-Schwarzschild-Strasse 2, 85748 Garching bei München, Germany
² Centre for Astrophysics and Supercomputing, Swinburne University, John Street, Hawthorn VIC 3122, Australia
³ INAF – Osservatorio di Astrofisica e Scienza dello Spazio di Bologna, Via Gobetti 93/3, I-40129 Bologna, Italy
⁴ Department of Physics, Engineering Physics, and Astronomy, Queen’s University, Kingston, ON K7L 3N6, Canada
⁵ Department of Physics and Astronomy, Dartmouth College, Hanover, NH 03755, USA

^⋆ Corresponding author: luisa.buzzo@gmail.com

Received: 21 May 2025
Accepted: 24 June 2025

Abstract

Aims. We investigated the gas reservoirs, star formation properties, and environment of the ultra-diffuse galaxy GAMA 526784 to understand its formation history, the efficiency of molecular gas conversion into stars, and the possible role of an interacting companion in shaping its current morphology.

Methods. We analysed low- and high-resolution CO observations to place constraints on the molecular gas content of the galaxy, compared them with HI data, and examined the star formation efficiency of GAMA 526784. The potential influence of a newly identified nearby dwarf galaxy was assessed using photometric and spatial information.

Results. GAMA 526784 exhibits a regular HI reservoir (M_HI/M_⋆ = 2.88), but we are only able to place upper limits on its molecular gas mass (M_H2(5σ)/M_⋆ < 0.23). The galaxy’s HI reservoir and CO non-detection can be explained by several mechanisms: (1) the predominance of CO-dark H₂, which remains invisible to CO observations but contributes to star formation; (2) a time delay in HI-to-H₂ conversion following a recent interaction; or (3) elevated turbulence inhibiting gas collapse. An identified companion, optically found at a projected distance of ∼48 kpc, shows similar colours and lies in the direction of the young star clusters in GAMA 526784, indicating a possible association. We hypothesise that this companion may have triggered the formation of the star clusters in GAMA 526784 through a high-velocity encounter.

Conclusions. Our findings suggest that GAMA 526784 may have undergone a dwarf–dwarf interaction that significantly influenced its gas reservoirs and star formation activity. The presence of a nearby companion galaxy is consistent with predictions of a high-speed encounter, potentially offering a rare observational example of such an interaction in progress. We hypothesise that this encounter may have played a key role in shaping the system’s recent evolution. Future observations, particularly targeting molecular gas tracers beyond CO and resolved HI observations, will be crucial in determining the true extent of GAMA 526784’s cold gas reservoir and the nature of its recent star formation activity.