Electron Density Distribution in H ii Regions in IC 10

Abstract We present the [O III ] λ 52 μ m map of the dwarf galaxy IC 10 obtained with the Field-Imaging Far-Infrared Line Spectrometer on board the Stratospheric Observatory for Infrared Astronomy. We combine the [O III ] λ 52 μ m map with Herschel and Spitzer observations to estimate the electron density distribution of the brightest H ii regions of IC 10. We find that the line ratio [O III ] λ 88 μ m/[O III ] λ 52 μ m gives electron density ( n e ) values ( n e [O III] ) that cover a broad range, while the n e values obtained using the line ratio [S III ] λ 33 μ m/[S III ] λ 18 μ m ( n e [S III] ) are all similar within the uncertainties. n e [O III] is similar to n e [S III] for the M1, M2, and A1 regions, and it is higher than n e [S III] for the two regions, A2 and M1b, which are the brightest in the 24 μ m continuum emission. These results suggest that for these regions, the two ions, O ++ and S ++ , trace two different ionized gas components and that the properties of the ionized gas component traced by the O ++ ion are more sensitive to the local physical conditions. In fact, while the gas layer traced by [S III ] does not keep track of the characteristics of the radiation field, the n e [O III] correlates with the star formation rate, the dust temperature, and the 24 μ m. Therefore, n e [O III] is an indicator of the evolutionary stage of the H ii region and the radiation field, with higher n e [O III] found in younger star-forming regions and in more energetic environments.