ALMA studies of galaxies at the epoch of reionization revealed that the far-infrared line emission [OIII]λ88µm is often very strong, a result which suggests a diffuse hard radiation field and a porous interstellar medium (ISM), through which the ionizing photons escape. While it is not possible to study in the detail the ISM of these high-redshift galaxies, we can gain insights into the internal structure of these galaxies by studying local analogues of the ‘chemically young’ environments that characterize those high-redshift galaxies: the nearby low-metallicity dwarf galaxies.
Although electron density tracers can be found at various wavelengths, infrared fine-structure line ratios are less affected than optical tracers by temperature fluctuations and extinction. There are several infrared density tracers, each of them covering a different and complementary range of electron densities: from 1 cm−3 up to 105 cm−3. Combining data from SOFIA/FIFI-LS, Herschel/PACS, and Spitzer/IRS, we estimated the electron density for various star-forming regions of IC 10, our pilot galaxy. Our study shows that each line ratio traces a different gas component of the ionized gas. In particular, the gas with the highest density is traced by the lines emitted by the ion with the highest ionization parameter, suggesting that the denser gas is closer to the ionizing source.
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