Electron density distribution in HII regions in IC10

Abstract

We present the [OIII]52μm map of the dwarf galaxy IC10, obtained with the Field-Imaging Far-Infrared Line Spectrometer (FIFI-LS) on board the Stratospheric Observatory for Infrared Astronomy (SOFIA). We combine the [OIII]52μm map with Herschel and Spitzer observations, to estimate the electron density distribution of the brightest HII regions of IC10. We find that the line ratio [OIII]88μm/[OIII]52μm gives electron density (n_e) values (n_e_OIII) that cover a broad range, while the n_e values obtained using the line ratio [SIII]33μm/[SIII]18μm (n_e_SIII) are all similar within the uncertainties. n_e_OIII is similar to n_e_SIII for the M1, M2 and A1 regions, and it is higher than n_e_SIII for the two regions, A2 and M1b, which are the brightest in the 24μm continuum emission. These results suggest that for these regions the two ions, O++ and S++, trace two different ionised gas components, and that the properties of the ionised gas component traced by the O++ ion are more sensitive to the local physical conditions. In fact, while the gas layer traced by [SIII] does not keep track of the characteristics of the radiation field, the n_e_OIII, correlates with the star formation rate (SFR), the dust temperature and the 24μm. Therefore, n_e_OIII is an indicator of the evolutionary stage of the HII region and the radiation field, with higher n_e_OIII, found in younger SF regions and in more energetic environments.

19 pages, 13 figures, 5 tables. Paper accepted for publication in ApJ