Abstract Disentangling SF and AGN emission is essential for understanding galaxy evolution, yet remains challenging in merging systems where both processes are enhanced and spatially intertwined. Galaxy mergers drive gas inflows that simultaneously fuel nuclear starbursts and black hole accretion, shaping morphology from nuclear (≲ 250 pc) to large-scale (≳ 500 pc) regions. Radio interferometry provides an unobscured view, but separating compact nuclear starbursts, AGN, and diffuse star formation requires multiscale, multi-frequency observations. We present a systematic method to characterise multiscale radio properties in 15 local (z ≲ 0. 1) Luminous and Ultra-Luminous Infrared Galaxies (U/LIRGs) (LIR 1011L⊙). Using e-MERLIN and VLA at 1. 4, 6. 0 and 33. 0 GHz, we probe physical scales from ~10–250 pc to ~0. 5–3. 0 kpc. We decompose radio emission into nuclear (compact cores and nuclear extended) and large-scale (total and diffuse) components, comparing morphological properties (emission fractions, sizes, luminosities, surface densities) and investigating correlations with source classes, merger stages, and infrared luminosities. We find: i) nuclear emission contributes ~50% of total radio emission on average; ii) total multiscale diffuse emission (SF-related) contributes ~80% to total power; iii) nuclear emission components act together to correlate with total radio and infrared luminosities, which increase with merger stage, whilst diffuse emission at larger scales shows no clear dependence on nuclear processes; iv) sources with radio excess (lower qIR) show lower nuclear luminosity ratios Lₑ, ₃₃^N/Lₑ, ₆^N, indicating a deficit of high-frequency radio emission; since 33. 0 GHz traces recent star formation, this suggests the radio excess is dominated by non-thermal emission at lower frequencies, likely AGN-related, rather than enhanced star formation.
Lucatelli et al. (2026) studied this question.