Bidimensional Exploration of the warm-Temperature Ionised gaS (BETIS) II. Revisiting the ionisation mechanism of the extraplanar diffuse ionised gas

Abstract: The extraplanar diffuse ionised gas is a key component for understanding the feedback processes that connect galactic discs and their halos. In this paper, we present the second study of the BETIS project, which aims to explore the ionisation mechanisms of the eDIG. We use a sample of eight edge-on galaxies observed with MUSE and apply the methodology developed in the first paper of the BETIS project. We found that the vertical and radial profiles of the [NII]/Ha, [SII]/Ha, [OIII]/Hb, and [OI]/Ha ratios depict a complex ionisation structure within galactic halos, influenced by the spatial distribution of HII regions across the galactic plane as observed from our line of sigh, with photon leakage from OB associations constituting the main ionisation source. Our analysis excludes low-mass, hot, and evolved stars as viable candidates for secondary ionisation sources to elucidate the unusual behaviour of the line ratios at greater distances from the galactic midplane. In contrast, we ascertain that shocks induced in the interstellar medium by star formation related feedback mechanisms represent a promising secondary ionisation source of the eDIG. We present a suite of models integrating ionisation mechanisms arising from fast shocks and photoionisation associated with star formation. When applied to the classical BPT diagrams, these models reveal that the ionisation budget of the eDIG ranges from 20% to 50% across our sample, with local variations of up to 20% within individual galaxy halos. This correlates with the presence of filaments and other structural components observed within galaxy halos. The presence of shocks is additionally supported by the observation of high-density, high [OI]/Ha ratios, characteristic of shock-compressed ionised gas, likely induced by feedback from regions of intense SF within the disk. These results are consistent across all galaxies analysed in this sample.