Calibration of mid- to far-infrared spectral lines in galaxies

Context. Mid- to far-infrared (IR) lines are suitable in the study of dust-obscured regions in galaxies because dust extinction strongly decreases with wavelength, and therefore IR spectroscopy allows us to explore the most hidden regions of galaxies, where heavily obscured star formation as well as accretion onto supermassive black holes at the nuclei of galaxies occur. This is mostly important for the so-called cosmic noon (i.e. at redshifts of 1z<3), at which point most of the baryonic mass in galaxies has been assembled. Aims. Our goal is to provide reliable calibrations of the mid- to far-IR ionic fine-structure lines, the brightest H-2 pure rotational lines, and the polycyclic aromatic hydrocarbon (PAH) features, which we used to analyse current and future observations in the mm-submm range from the ground, as well as mid-IR spectroscopy from the upcoming James Webb Space Telescope. Methods. We used three samples of galaxies observed in the local Universe: star-forming galaxies (SFGs, 196), active galactic nuclei (AGN; 90-150 for various observables), and low-metallicity dwarf galaxies (40). For each population, we derive different calibrations of the observed line luminosities versus the total IR luminosities. Results. Through the resulting calibrations, we derive spectroscopic measurements of the star formation rate (SFR) and of the black hole accretion rate (BHAR) in galaxies using mid- and far-IR fine-structure lines, H-2 pure rotational lines and PAH features. In particular, we derive robust star formation tracers based on the following: the [CII]158 mu m line; the sum of the two far-IR oxygen lines, the [OI]63 mu m line, and the [OIII]88 mu m line; a combination of the neon and sulfur mid-IR lines; the bright PAH features at 6.2 and 11.3 mu m; as well as - for the first time - the H-2 rotational lines at 9.7, 12.3, and 17 mu m. We propose the [CII]158 mu m line, the combination of the two neon lines ([NeII]12.8 mu m and [NeIII]15.5 mu m), and, for solar-like metallicity galaxies that may harbour an AGN, the PAH 11.3 mu m feature as the best SFR tracers. On the other hand, a reliable measure of the BHAR can be obtained using the [OIV]25.9 mu m and the [NeV]14.3 and 24.3 mu m lines. For the most commonly observed fine-structure lines in the far-IR, we compare our calibration with the existing ALMA observations of high-redshift galaxies. We find an overall good agreement for the [CII]158 mu m line for both AGN and SFGs, while the [OIII]88 mu m line in high-z galaxies is in better agreement with the low-metallicity local galaxies (dwarf galaxy sample) than with the SFGs, suggesting that high-z galaxies might have strong radiation fields due to low metal abundances, as expected.