Extended photoionization and photodissociation in Sgr B2

We present large scale 9′ × 27′ (25 pc × 70 pc) far–IR observations of the Sgr B2 complex using the spectrometers on board the Infrared Space Observatory1 (ISO). The far–IR spectra are dominated by the strong continuum emission of dust and by the fine structure lines of high excitation potential ions (N II , N III and O III ) and those of neutral or weakly ionized atoms (O I and C II ). The line emission has revealed a very extended component of ionized gas. The study of the N III 57 μm/N II 122 μm and OIII 52μm/88μm line intensity ratios show that the ionized gas has a density of ne ≃ 102−3 cm−3 while the ionizing radiation can be characterized by a diluted but hard continuum, with effective temperatures of ∼35,000 K. Photoionization models show that the total number of Lyman photons needed to explain such an extended component is approximately equal to that of the H II regions in Sgr B2(N) and (M) condensations. We propose that the inhomogeneous and clumpy structure of the cloud allows the radiation to reach large distances through the envelope. Therefore, photodissociation regions (PDRs) can be numerous at the interface of the ionized and the neutral gas. The analysis of the O I (63 and 145 μm) and C II (158 μm) lines indicates an incident far–UV field (G0, in units of the local interstellar radiation field) of 103−4 and a H density of 103−4 cm−3 in such PDRs. We conclude that extended photoionization and photodissociation are also taking place in Sgr B2 in addition to more established phenomena such as widespread low–velocity shocks.