2001A&A...377..631R

We present an analysis of selected fine structure lines (NeII 12.8µm, NeIII 15.6µm, SIII 18.7 and 33.5µm, OIII 52 and 88µm, NII 122 µm and NIII 57µm) observed with the Infrared Space Observatory (ISO) toward the Radio Arc in the Galactic center region (GCR). Most of the data were retrieved from the ISO Data Archive. We study the density of the ionized gas and the large scale ionization structure in a region of ∼30x30pc² by means of the OIII 52µm/88µm, SIII 18.7µm/33.5µm, NeIII 15.6µm/NeII12.8µm and NIII 57µm/NII 122µm line ratios. The electron densities (ne) derived from the OIII lines ratio indicate the presence of diffuse ionized material with ne of ∼10^1.8–2.6cm^–3. The NeIII 15.6µm/NeII 12.8µm and NIII 57µm/NII 122µm line ratios vary from source to source from 0.05 to 0.30 and from 0.3 to 2.5, respectively. The NIII 57/NII 122 µm ratio show two clear gradients, one pointing toward the Quintuplet cluster and the other pointing toward the Arches cluster. We have used a simple model to explain the ionization structure observed in the NIII/NII and NeIII/NeII lines ratios. The model shows that the large scale ionization of the whole region can be accounted for by the UV radiation produced by the Quintuplet and the Arches cluster. Any other ionization mechanism should play a minor role. We also investigate the influence of the clusters on the bubble of warm dust (hereafter Radio Arc Bubble, RAB) seen in the Midcourse Space Experiment (MSX) infrared images. We find that the warm dust is well correlated with the ionized gas indicating that the dust is also heated by the radiation from both clusters. Furthermore, the elliptical rather than circular symmetry of some structures like the Thermal Filaments can also be explained when one considers the combined effects of both the Arches and the Quintuplet clusters. We have also found that the RAB is filled with continuum emission of hard X-rays and with emission from the 6.4keV line of neutral or low ionized Fe. We briefly discuss the implications of these findings on the structure and morphology of the GCR interstellar medium and the possible origin of the RAB.