4.1 − 5.2 μm portion on the spectra of Arp 220 and VV114 nuclear regions. The E and W nuclei show ¹²CO ro-vibrational lines up to J_low = 23 (i.e. within the wavelength range marked with a dot-dashed horizontal line; for completeness, ¹³CO range is also reported, similar to Fig. 2 in González-Alfonso et al. 2024). Detailed modelling of the CO transitions will be presented in Buiten et al., in prep. See B.1 for further details.