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

Abstract

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. 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 HI 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, we aim to determine its magnetic field strength and to characterise the large-scale and turbulent components of its magnetic field. We utilise 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. The integrated spectral energy distribution has a non-thermal spectral index of $α_{\rm nth} = -0.678\pm0.085$. Synchrotron and inverse Compton losses cause a spectral break at $ν_{\rm b} = 6.2\pm1.3$ 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 toward the galaxy's outskirts, showing a large-scale magnetic field over $\approx3$ kpc. 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 key contribution in the early magnetisation of galaxies.

Figures (13)

• Figure 1: Colour-composite images from the DESI Legacy Imaging Surveys (left panel), HIPASS H$\alpha$ image (middle panel) and GALEX FUV image (right panel) of NGC 1487 with overlaid MeerKAT radio emission contours starting at $3\,\sigma_I$ and increasing by a factor of 2 at a central frequency of 1.28 GHz ($\sigma_I=4\,\upmu$Jy/beam). The beam of $8.56^{\prime\prime} \times 7.65^{\prime\prime}$ is shown in the left corner. The scale is calculated using the distance given in Table \ref{['basics']}.
• Figure 2: Left: Total intensity map at 1.28 GHz of NGC 1487 with a noise level $\sigma_I$ of $4\,\upmu$Jy/beam observed with MeerKAT. Middle left: Total intensity map at 2.1 GHz of NGC 1487 with a noise level $\sigma_I$ of $8\,\upmu$Jy/beam observed with ATCA. Middle right: Total intensity map at 5.5 GHz of NGC 1487 with a noise level $\sigma_I$ of $5.5\,\upmu$Jy/beam observed with ATCA. Right: Total intensity map at 9 GHz of NGC 1487 with a noise level $\sigma_I$ of $5.5\,\upmu$Jy/beam observed with ATCA. The overlaid contours of the total intensity start at $3\,\sigma$ and increase by factor of 2. The resolution of the different maps can be taken from Table \ref{['obs_prop']} and is shown in the bottom left corner.
• Figure 3: Spectral energy distribution of NGC 1487. The plot displays total intensity data with different observations: GLEAM-X data gleamx, RACS-Low data racs, ASKAP EMU main survey emu_main, MeerKAT 1.28 GHz data, ATCA L/S-band, C-band, and X-band data presented in this work, and further data from the work of Grundy_2025. We show the best-fit in red with $1\sigma$ uncertainty sampled by EMCEE. The blue, orange, and green dotted lines represent the different components of the best-fit model.
• Figure 4: Inband total spectral index of the MeerKAT L-band (panel a)) and its uncertainty (panel b)) between the frequencies 0.96 and 1.59 GHz overlaid with radio emission at frequency of 1.28 GHz starting at $3\sigma$ and increasing by a factor of 2 ($\sigma=9\,\upmu$Jy/beam). The resolution of $12^{\prime\prime}$ is shown in the bottom left corner. Two-point total spectral index (panel c)) and its uncertainty (panel d)) between ATCA L/S-band (2.1 GHz) and C-band (5.5 GHz) overlaid with radio emission at frequency of 2.1 GHz starting at $3\sigma$ and increasing by a factor of 2 ($\sigma=11.3\,\upmu$Jy/beam). The resolution of $7^{\prime\prime}$ is shown in the bottom left corner.
• Figure 5: Region dependent spectral energy distribution, showing total and thermal corrected flux density of MeerKAT L-band, ATCA L/S-, C- and X-band data. The bottom right corner panel shows the H$\alpha$ map with the individual regions overlaid in different colors.