MEDUSA I. Tracing magnetic field structures in tidal arms of the dwarf-dwarf merger NGC 1487

Journal article, 2026

Context. Radio continuum observations of dwarf-dwarf galaxy mergers, such as NGC 1487, provide crucial insights into magnetic field amplification and cosmic ray (CR) propagation during galactic assembly. Dwarf galaxies are important laboratories for studying cosmic magnetism because they can maintain strong magnetic fields via the action of turbulent dynamo despite their low mass and weak gravitational potential. Aims. The Magnetic-field Evolution in Dwarf galaxies from Ultra-deep SKA Analysis (MEDUSA) survey is the first SKA-pathfinder programme designed to obtain deep continuum, polarisation, and H I data for dwarf galaxies, enabling a comprehensive study of their radio spectra, magnetic fields, and gas kinematics across a representative population. By analysing the radio continuum spectra and polarisation of the dwarf-dwarf galaxy merger NGC 1487 from the MEDUSA sample, our aim was to determine its magnetic field strength and to characterise the large-scale and turbulent components of its magnetic field. Methods. We utilised highly sensitive multi-band radio continuum data from MeerKAT L-band (1.28 GHz) and Australia Telescope Compact Array (ATCA) L/S (2.1 GHz), C (5.5 GHz), and X-bands (9 GHz). We analysed the magnetic field configuration using polarisation and rotation measure (RM) synthesis. Results. The integrated spectral energy distribution has a non-thermal spectral index of alpha(nth) = -0.678 +/- 0.085, indicating a significant synchrotron contribution, consistent with a CR electron injection index of gamma = 2.36 (N(E)proportional to E-gamma) typical of supernova remnants. Synchrotron and inverse Compton losses cause a spectral break at nu(b) = 6.2 +/- 1.3 GHz. In star-forming regions, the magnetic field exhibits strong small-scale fluctuations in RM, suggesting the action of a small-scale dynamo. Conversely, the field becomes more ordered, aligning with the tidal arms towards the galaxy's outskirts, showing a large-scale magnetic field over approximate to 3 kpc. The CR cooling timescale of approximately 11 Myr at 1.28 GHz is similar to the escape timescale.Conclusions. Observations of the dwarf-dwarf merger NGC 1487 show that even low-mass galaxy mergers, likely the building blocks of larger galaxies in the early Universe, can rapidly amplify and produce coherent large-scale magnetic field structures, highlighting their contribution in the early magnetisation of galaxies.