Chemical abundances and deviations from the solar S/O ratio in the gas-phase interstellar medium of galaxies based on infrared emission lines

Abstract

The infrared (IR) range is extremely useful in the context of chemical abundance studies of the gas-phase interstellar medium (ISM) due to the large variety of ionic species traced in this regime, the negligible effects from dust attenuation or temperature stratification, and the amount of data that has been and will be released in the coming years. Taking advantage of available IR emission lines, we analysed the chemical content of the gas-phase ISM in a sample of 131 star-forming galaxies (SFGs) and 73 active galactic nuclei (AGNs). In particular, we derived the chemical content via their total oxygen abundance in combination with nitrogen and sulphur abundances, and with the ionisation parameter. We used a new version of the code HII-CHI-Mistry-IR v3.1, which allowed us to estimate log(N/O), 12+log(O/H), log(U) and, for the first time, 12+log(S/H) from IR emission lines, which can be applied to both SFGs and AGNs. We tested whether the estimates from this new version, which only considers sulphur lines for the derivation of sulphur abundances, are compatible with previous studies. While most of the SFGs and AGNs show solar log(N/O) abundances, we find a large spread in the log(S/O) relative abundances. Specifically, we find extremely low log(S/O) values (1/10 solar) in some SFGs and AGNs with solar-like oxygen abundances. This result warns against the use of optical and IR sulphur emission lines to estimate oxygen abundances when no prior estimation of log(S/O) is provided.

Exploring the hardness of the ionising radiation with the infrared softness diagram

“…1 Unlike previous versions of HCm-ir, version 3.2 does not consider S emission lines to derive oxygen abundances as these lines are used in the new version for the derivation of sulphur abundances. This improvement of the code is described and discussed in Pérez-Díaz et al (2024), where it is checked that this change does not imply results in the derived oxygen abundances deviating beyond the obtained errors compared to the values derived using previous versions of the 1 All versions of the code HCm can be retrieved in the web page http: //home.iaa.es/~epm/HII-CHI-mistry.html code, such as the values published by Fernández-Ontiveros et al (2021).…”

“…and [N iii] 57 µm far-IR lines obtained with FIFI-LS (Fischer et al 2018) on board the SOFIA airborne observatory (Temi et al 2018) were also taken from Peng et al (2021) The second source includes objects from the IDEOS catalogue (Hernán-Caballero et al 2016;Spoon et al 2022) IR database, which includes the measurements of 77 fitted mid-IR spectral features in the 5.4-36 µm range for all galaxies observed with Spitzer (a total of 3335 galaxies; Spoon et al 2022). From this initial sample, 153 star-forming objects (Pérez-Díaz et al 2024) and 66 (U)LIRGs (Pérez-Díaz et al 2023) were selected by omitting duplicate observations of the same object and entries corresponding to galaxies presented in the first catalogue, where the higher spectral resolution observation was used, and after verifying that star formation dominates the ionisation budget based on the strength of polycyclic aromatic hydrocarbon (PAH) features (EW(PAH6.2 µm) > 0.06 µm) and the small contribution from certain highly ionised species ([Ne v] λ14 µm/[Ne ii] λ12 µm < 0.15 and [O iv] λ26 µm/[Ne ii] λ12 µm < 0.4). Our final sample then comprises 228 star-forming galaxies and 75 (U)LIRGs from both sources.…”

“…We collected total oxygen abundances for all objects in this selected sample as derived by Pérez-Díaz et al (2024). These abundances were computed using version 3.2 of the code HII-CHI-mistry-ir (hereafter HCm-ir;Fernández-Ontiveros et al 2021;Pérez-Díaz et al 2022).…”