Exploring the effects of diffuse ionised gas in two local analogues of high-redshift star-forming galaxies

Aims. We investigate the impact of diffuse ionised gas (DIG) on the determination of emission line ratios and gas-phase metallicities in two local analogues of high-redshift star-forming galaxies: UM 462 and IIZw 40. Understanding how DIG affects these quantities is essential for interpreting unresolved observations of distant galaxies, where integrated spectra are often used to trace their chemical evolution. Methods. Using archival Very Large Telescope, Multi-Unit Spectroscopic Explorer (MUSE) data, we spatially resolved the warm ionised medium of both galaxies. We derived oxygen abundances through the direct method and several HII-based strong-line calibrators, and we used the H α surface brightness (Σ(H α )) to distinguish regions dominated by HII or DIG emission. Results. Oxygen abundances derived from the N2 and O3N2 indices show an inverse correlation with Σ(H α ), ionisation parameters, and H α equivalent width (EW(H α )), with DIG-dominated regions exhibiting 12 + log(O/H) values higher than the mean for their galaxy by ∼0.2 dex in UM 462 and ∼0.1 dex in IIZw40. The metallicity differences derived from these strong-line calibrators reach about 0.4 dex and 0.3 dex between the highest (HII-dominated) and lowest (DIG-dominated) Σ(H α ) bins in UM 462 and IIZw40, respectively. We found a linear correlation between Δ(O/H) (O/H deviation from the mean interstellar medium value) and EW(H α ). Trends with Σ(H α ), metallicity, EW(H α ), and ionisation parameter suggest smoothly evolving ionisation conditions in the interstellar medium in our galaxies. Such trends and metallicty variations derived from HII-based calibrators reflect different ionisation sources and levels rather than true abundance changes. In particular, the use of these calibrators can lead to spurious metallicity gradients in galaxies with extended DIG tails, such as tadpole or cometary-like galaxies, which can be misinterpreted as evidence of the infall of metal-poor gas. The most likely mechanism for ionising the DIG in our sample of HII or BCD galaxies is the leakage of photons from HII regions, with shocks induced by stellar feedback processes also contributing significantly. Consequently, such contamination may affect the reliability of the derived oxygen abundances. Our results highlight the importance of accounting for DIG in galaxy metallicity estimates since it potentially biases metallicity gradient measurements. This is particularly relevant for large surveys at high redshift that rely on integrated galaxy spectra.