Electron Density Distribution in H II Regions in IC 10

Abstract

We present the [O III] λ52 μm 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] λ52 μm 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] λ88 μm/[O III] λ52 μm 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] λ33 μm/[S III] λ18 μm (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 μm 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 μm. 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.