ISO observations of the Galactic center interstellar medium - Ionized gas

We present fine structure and recombination line observations of the ionized gas toward a sample of 18 sources located within 300 pc of the center of the Galaxy ( hereafter Galactic center, GC). The sources were selected as molecular clouds located far from thermal continuum sources. The fine structure lines from [NII] and [SIII] were detected in 16 sources. In 10 sources we even detected the [OIII] 88 mum line. Several techniques have been used to determine lower and upper limits to the extinction toward each source to correct the observed line fluxes. The derived electron densities of the ionized gas vary from similar to100 to less than or equal to30 cm(-3). For some sources we were able to derive N, S and Ne abundances. We found that they are similar to those measured in the H.. regions in the 5-kpc ring and in the nuclei of starburst galaxies. The fine structure lines ratios measured for all the sources can be explained by photo-ionization with an effective temperature of the ionizing radiation of 32 000-37 000 K and an ionization parameter, U, of -1 > log U > -3. The highest excitation is found in the Radio Arc region but it does not decrease smoothly with distance. There must be more ionizing sources distributed over the Galactic center than the known clusters of massive stars. Most of the clouds are located far (up to 45 pc for M-0.96+0.13) from the prominent continuum complexes (Sgr C, B...). However, it is possible that the clouds are ionized by escaped photons from those complexes. The comparison of the effective temperatures of the ionizing radiation to the measured Lyman continuum photon emission rate implies that the clouds are indeed ionized by distant sources. The excitation ratios, effective temperature and ionization parameter measured in the GC are similar to those found in some low excitation starburst galaxies. The [NeIII]/[NeII] line ratios measured in the GC sources are consistent with the results of the Thornely et al. ( 2000) model for a short burst of massive star formation less than 8 Myr ago. We have also found that the [NeII] 13 mum to far-infrared continuum ratio measured for the GC sources is similar to that of external galaxies, supporting the idea of Sturm et al. ( 2002) that the far-infrared continuum in Active Galaxies is dominated by dust heated by stellar radiation rather than by the AGN.