A MeerKAT study of a neutral hydrogen rich grouping of galaxies with megaparsec-scale filamentary-like structure

¹ Wits Centre for Astrophysics, School of Physics, University of the Witwatersrand, 1 Jan Smuts Avenue, Johannesburg, 2000 South Africa
² Department of Physics, University of Pretoria, Private Bag X20, Pretoria, 0028 South Africa
³ Institute for Astronomy, University of Edinburgh, Royal Observatory, Blackford Hill, Edinburgh, EH9 3HJ UK
⁴ University of the Western Cape, Department of Physics and Astronomy, Bellville, Cape Town, 7535 South Africa
⁵ South African Astronomical Observatory, Observatory, Cape Town, 7925 South Africa

^⋆ Corresponding author.

Received: 28 May 2025
Accepted: 18 July 2025

Abstract

Context. Environmental effects within cosmological overdensities, such as galaxy groups and clusters, have been shown to impact galaxies and their cold gas reservoirs and thereby provide constraints on galaxy evolution models. Galaxy groups foster frequent galaxy-galaxy interactions, making them rich environments in which to study galaxy transformation.

Aims. In this work, we study a serendipitously discovered large overdensity of neutral hydrogen (H I) galaxies at z ∼ 0.04. The galaxies appear to lie in a filamentary-like structure of megaparsec scale. Using MeerKAT’s angular resolution and field of view, we were able to spatially resolve the H I galaxies while simultaneously probing large-scale structure.

Methods. The H I and sub-arcsec Dark Energy Survey (DES) imaging have revealed a large number of both interacting and disturbed galaxies in this collective group. MeerKAT data enabled us to derive H I masses and investigate interacting galaxies. We used DES and Wide-field Infrared Survey Explorer (WISE) data to quantify the star formation rates, stellar masses, and stellar morphologies of member galaxies and compared these with field scaling relations. To place this discovery and the environmental effects in context, we used the SIMBA cosmological hydrodynamical simulation to investigate the prevalence of qualitatively similar H I overdensities and their large-scale morphological properties. This enabled us to make a prediction of how frequently such structures might be serendipitously discovered with MeerKAT and SKA-Mid H I observations in comparable observation time.

Results. The combination of spatially resolved H I data and optical imaging revealed a group rich in interactions, suggesting environmental processes are already shaping galaxy properties within the structure.

Conclusions. More of these serendipitous discoveries are expected, and alongside ongoing targeted programmes, they will provide a rich, unbiased sample to study galaxy transformation and enable a MeerKAT H I perspective on large-scale structure, including filaments.