Bidimensional Exploration of the warm-Temperature Ionised gaS (BETIS)

I. Showcase sample and first results

¹ Institute of Space Sciences (ICE-CSIC), Campus UAB, Carrer de Can Magrans, s/n, 08193 Barcelona, Spain
e-mail: Raul.GonzalezD@autonoma.cat
² Instituto Nacional de Astrofísica, Óptica y Electrónica (INAOE-CONAHCyT), Luis E. Erro 1, 72840 Tonantzintla, Puebla, Mexico
³ Institut d’Estudis Espacials de Catalunya (IEEC), 08034 Barcelona, Spain
⁴ European Southern Observatory, Alonso de Córdova 3107, Vitacura, Casilla, 19001 Santiago, Chile
⁵ Millennium Institute of Astrophysics MAS, Nuncio Monseñor Sotero Sanz 100, Off. 104, Providencia, Santiago, Chile
⁶ Department of Physics, University of Warwick, Coventry CV4 7AL, UK
⁷ Instituto de Astronomía, Universidad Nacional Autónoma de México, A. P. 70-264, C.P. 04510, Ciudad de México, Mexico
⁸ Finnish Centre for Astronomy with ESO (FINCA), 20014 University of Turku, Finland
⁹ Tuorla Observatory, Department of Physics and Astronomy, 20014 University of Turku, Finland

Received: 1 November 2023
Accepted: 20 March 2024

Abstract

We present the Bidimensional Exploration of the warm-Temperature Ionised gaS (BETIS) project, designed for the spatial and spectral study of the diffuse ionised gas (DIG) in a selection of nearby spiral galaxies observed with the MUSE integral-field spectrograph. Our primary objective is to investigate the various ionisation mechanisms at play within the DIG. We analysed the distribution of high- and low-ionisation species in the optical spectra of the sample on a spatially resolved basis. We introduced a new methodology for spectroscopically defining the DIG, optimised for galaxies of different resolutions. Firstly, we employed an innovative adaptive binning technique on the observed datacube based on the spectroscopic signal-to-noise ratio (S/N) of the collisional [S II] line to increase the S/N of the rest of the lines including [O III], [O I], and He I. Subsequently, we created a DIG mask by eliminating the emissions associated with both bright and faint H II regions. We also examined the suitability of using Hα equivalent width (EW_Hα) as a proxy for defining the DIG and its associated ionisation regime. Notably, for EW_Hα < 3 Å – the expected emission from hot low-mass evolved stars (HOLMES) – the measured value is contingent on the chosen population synthesis technique performed. Our analysis of the showcase sample reveals a consistent cumulative DIG fraction across all galaxies in the sample, averaging around 40%–70%. The average radial distribution of the [N II]/Hα, [S II]/Hα, [O I]/Hα, and [O III]/Hβ ratios are enhanced in the DIG regimes (up to 0.2 dex). It follows similar trends between the DIG regime and the H II regions, as well as the Hα surface brightness (Σ_Hα), indicating a correlation between the ionisation of these species in both the DIG and the H II regions. The DIG loci in typical diagnostic diagrams are found, in general, within the line ratios that correspond to photoionisation due to the star formation. There is a noticeable offset correspondent to ionisation due to fast shocks. However, an individual diagnosis performed for each galaxy reveals that all the DIG in these galaxies can be attributed to photoionisation from star formation. The offset is primarily due to the contribution of Seyfert galaxies in our sample, which is closely aligned with models of ionisation from fast shocks and galactic outflows, thus mimicking the DIG emission. Our results indicate that galaxies exhibiting active galactic nucleus (AGN) activity should be considered separately when conducting a general analysis of the DIG ionisation mechanisms, since this emission is indistinguishable from high-excitation DIG.