Bottom panel: Average VLT/MUSE spectra of inner (orange) and outer (blue) regions in the MRG-0138 arc image, covering the 1800 − 3150 Å rest-frame wavelength range and smoothed with a ten-pixel kernel for clarity. Significant near-ultraviolet features are indicated by dotted lines. Middle panel: Signal-to-noise ratio of the difference spectrum (inner − outer) as a function of wavelength. Top panel: Mask of the MRG-0138 arc showing inner (orange) and outer (blue) regions, with the corresponding ALMA band 6 beams shown as black ellipses.