Infrared view of the multiphase ISM in NGC 253 I. Observations and fundamental parameters of the ionised gas

Context. Massive star formation leads to enrichment of the interstellar medium with heavy elements. On the other hand, the abundance of heavy elements is a key parameter with which to study the star-formation history of galaxies. Furthermore, the total molecular hydrogen mass, usually determined by converting CO or [C II] 158 mu m luminosities, depends on the metallicity as well. However, the excitation of metallicity-sensitive emission lines depends on the gas density of the H II regions where they arise. Aims. We used spectroscopic observations of the nuclear region of the starburst galaxy NGC253 from SOFIA, Herschel, and Spitzer, as well as photometric observations from GALEX, 2MASS, Spitzer, and Herschel in order to derive physical properties such as the optical depth to correct for extinction, as well as the gas density and metallicity of the central region. Methods. Ratios of the integrated line fluxes of several species were utilised to derive the gas density and metallicity. The [O III] along with the [S III] and [N II] line flux ratios, for example, are sensitive to the gas density but nearly independent of the local temperature. As these line ratios trace different gas densities and ionisation states, we examined whether or not these lines could originate from di fferent regions within the observing beam. The ([Ne II] 13 mu m + [Ne III] 16 mu m)/H alpha line flux ratio on the other hand is independent of the depletion onto dust grains but sensitive to the Ne /H abundance ratio and is used as a tracer for metallicity of the gas. Results. We derived values for gas phase abundances of the most important species, as well as estimates for the optical depth and the gas density of the ionised gas in the nuclear region of NGC253. We obtained densities of at least two di fferent ionised components (<84 cm(-3) and similar to 170-212 cm(-3)) and a metallicity of solar value.