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

Abstract

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$\alpha$ surface brightness ($\Sigma$(H$\alpha$)) to distinguish regions dominated by HII or DIG emission. Results. Oxygen abundances derived from the N2 and O3N2 indices show an inverse correlation with $\Sigma$(H$\alpha$), ionisation parameter, and EW(H$\alpha$), with DIG-dominated regions exhibiting higher 12+log(O/H) than the galaxy mean by $\sim$0.2 dex in UM 462 and $\sim$0.1 dex in IIZw 40. The metallicity differences between HII-dominated and DIG-dominated $\Sigma$(H$\alpha$) bins reach $\sim$0.4 dex and $\sim$0.3 dex in UM 462 and IIZw 40, respectively. The observed trends with $\Sigma$(H$\alpha$), metallicity, EW(H$\alpha$), and ionisation parameter indicate smoothly varying ionisation conditions rather than true abundance variations. These effects reflect different ionisation sources and levels, and can produce spurious metallicity gradients in galaxies with extended DIG structures, potentially mimicking signatures of metal-poor gas infall. In our sample, DIG ionisation is most likely dominated by photon leakage from H II regions, with additional contributions from feedback-driven shocks.