Large scale ionization of the Radio Arc region by the Quintuplet and the Arches clusters

We present an analysis of selected fine structure lines (Ne II 12.8 mum, Ne III 15.6 mum, S III 18.7 and 33.5 mum, O III 52 and 88 mum, N II 122 mum and N III 57 mum) 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 similar to 30 x 30 pc(2) by means of the O III 52 mum/88 mum, S iii 18.7 mum/33.5 mum, Ne III 15.6 mum/Ne II 12.8 mum and N III 57 mum/N II 122 mum line ratios. The electron densities (ne) derived from the O III lines ratio indicate the presence of diffuse ionized material with n(e) of similar to 10(1.8-2.6) cm(-3). The Ne III 15.6 mum/Ne II 12.8 mum and N III 57 mum/N II 122 mum line ratios vary from source to source from 0.05 to 0.30 and from 0.3 to 2.5, respectively. The N III 57/N II 122 mum 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 N III/N II and Ne III/Ne II 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 6.4 keV 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.