Physical conditions in the dwarf local irregular galaxy IC 10 - I. Diffuse ionized gas

A detailed analysis of the physical conditions of the Interstellar Medium in the main body of the Local Group dwarf irregular galaxy IC 10 is carried out using long-slit spectroscopy. Maps of the excitation, the Ha flux, [NII]/Hα and [SII]/Hα are presented. The Diffuse Ionized Gas inside the galaxy is studied. We found that the spectral characteristics are not similar to those of spiral galaxies: the values of the ratios [NI]/Hα and [SII]/Hα are not as large as in spiral galaxies but the excitation is much larger. These values, especially the large excitation, can be explained by proposing an extra source of ionization. Different sources are studied but any of the models at hand could be really fitted except a combination of photons leaking from the H II regions in addition to the ionization provided by the WR stars present in this galaxy. Shocks are not needed to explain the line intensity ratios except in a small part in the west side of the galaxy.