Bidimensional study of the ionisation mechanisms of the diffuse gas in galaxies of the local universe with MUSE

Abstract

The Bidimensional Exploration of the warm-Temperature Ionised gaS (BETIS) project is focused on studying the diffuse ionised gas (DIG) in nearby spiral galaxies using the MUSE integral-field spectrograph. The main goal is to understand the various ionisation mechanisms within the DIG. This involves a spatially resolved analysis of high- and low-ionisation species in optical spectra. A new method for spectroscopically defining the DIG, adaptable for different galaxy resolutions, has been introduced. Initially, an adaptive binning technique based on the signal-to-noise ratio (S/N) of the [S II] line was used to enhance the S/N of other lines like [O III], [O I], and He I. A DIG mask was then created by removing emissions from bright and faint H II regions. The suitability of using \hii equivalent width (\ha (\EWha) to define the DIG and its ionisation regime was also assessed. It was found that for \EWha < 3\r{A},, expected from hot low-mass evolved stars (HOLMES), the value depends on the population synthesis technique used. Analysis of the sample reveals a consistent DIG fraction of 40%-70% across all galaxies. The average radial distribution of the [\nii/\ha, \sii/\ha, \oi/\ha and \oiii/\hb ratios is enhanced in the DIG regimes by up to 0.2 dex. These trends indicate a correlation between ionisation in the DIG and H II regions, linked to the \hii surface brightness (\ha (\SBha)). Diagnostic diagrams show that the DIG generally falls within line ratios corresponding to photoionisation from star formation, with an offset suggesting ionisation due to fast shocks. However, individual galaxy analysis indicates that DIG ionisation is primarily from star formation, with the offset attributed to Seyfert galaxies in the sample. This suggests that galaxies with active galactic nucleus (AGN) activity should be separately analyzed, as their emission is indistinguishable from high-excitation DIG. In a second study within the BETIS project, the focus is on extraplanar diffuse ionised gas (eDIG), a key component for understanding feedback processes between galactic discs and halos. Using eight edge-on galaxies observed with MUSE, the study applies the binning methodology from the first BETIS paper. The \nii/\ha, \sii/\ha, \oi/\ha and \oiii/\hb ratios show a complex ionisation structure in galactic halos, influenced by the spatial distribution of H II regions. Lyman continuum photon leakage from OB associations is identified as the main ionisation source, with electron temperature and S+/S ionisation ratio depending on H II region distribution within galactic discs. The analysis excludes HOLMES as significant ionisation sources for explaining unusual line ratios at greater distances from the galactic midplane. Instead, shocks from star formation-related feedback mechanisms are identified as promising secondary ionisation sources for the eDIG. Models incorporating ionisation from fast shocks and star formation photoionisation suggest that the ionisation budget of the eDIG ranges from 20% to 50% across the sample, with local variations up to 20% within individual galaxy halos. This correlates with the presence of filaments and structural components in galaxy halos. High-density, high \oi/\ha ratios, characteristic of shock-compressed ionised gas, support the presence of shocks, likely from feedback in regions of intense star formation within the galactic disk.

Date of Award	10 Sept 2024
Original language	English
Supervisor	Fernando Fabián Rosales Ortega (Director) & Lluis Galbany Gonzalez (Director)