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

Free access article

Issue		A&A Volume 377, Number 2, October II 2001


Page(s)		631 - 643
Section		Diffuse matter in space
DOI		http://dx.doi.org/10.1051/0004-6361:20010981

A&A 377, 631-643 (2001)
DOI: 10.1051/0004-6361:20010981

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

N. J. Rodríguez-Fernández, J. Martín-Pintado and P. de Vicente

Observatorio Astronómico Nacional, Instituto Geográfico Nacional, Apdo. 1143, 28800 Alcalá de Henares, Madrid, Spain

(Received 11 May 2001 / Accepted 5 July 2001 )

Abstract
We present an analysis of selected fine structure lines (NeII 12.8 $\mu$ m, NeIII 15.6 $\mu$ m, SIII 18.7 and 33.5 $\mu$ m, OIII 52 and 88 $\mu$ m, NII 122 $\mu$ m and NIII 57 $\mu$ m) 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 ~ $30\times30$ pc² by means of the OIII 52 $\mu$ m/88 $\mu$ m, SIII 18.7 $\mu$ m/33.5 $\mu$ m, NeIII 15.6 $\mu$ m/NeII 12.8 $\mu$ m and NIII 57 $\mu$ m/NII 122 $\mu$ m line ratios. The electron densities ( n_e) derived from the OIII lines ratio indicate the presence of diffuse ionized material with n_e of ~ 10^1.8-2.6 cm^-3. The NeIII 15.6 $\mu$ m/NeII 12.8 $\mu$ m and NIII 57 $\mu$ m/NII 122 $\mu$ m 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 $\mu$ m 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 6.4 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.

Key words: Galaxy: center -- ISM: HII regions -- ISM: dust, extinction -- ISM: individual objects: Radio Arc -- infrared: ISM: lines and bands -- X-rays: ISM: lines and bands

Offprint request: N. J. Rodríguez-Fernández, nemesio@oan.es

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